Study on Stonewall Trees: Maintenance Approach for the Six

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Study on Stonewall Trees:

Maintenance Approach for the Six Stonewall Trees

on Slope no. 11SW-A/R577, Bonham Road

C.Y. Jim

The University of Hong Kong

[email protected]

Highways Department

Government of the HKSAR

2012

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CONTENTS

LIST OF TABLES 5

LIST OF PHOTOS 6

1. INTRODUCTION 7

1.1 Preamble 7

1.2 Scope of study 7

1.2.1 Background and general concerns 7

1.2.2 Evaluation on the conditions of the stonewall trees 8

1.2.3 Recommendations on the maintenance of the stonewall trees 8

1.3 Deliverables 9

1.3.1 Draft study report 9

1.3.2 Final study report 9

1.3.3 Presentation of the report in a seminar 9

2. BACKGROUND AND GENERAL CONCERNS 10

2.1 Development of stonewall trees on retaining structures 10

2.1.1 Urban development and stone retaining walls 10

2.1.2 Tree flora on stone retaining walls 11

2.1.3 Pre-adaptation of strangler figs to wall habitat 12

2.1.4 Colonization of stone retaining walls by strangler figs 14

2.2 Effect of stonewall trees on retaining structures 17

2.2.1 Past studies of stone retaining walls 17

2.2.2 Historical cases of fatal stone wall failures 19

2.2.3 Historical cases of stone wall failures without casualties 20

2.2.4 Recent tree failures affecting stonewall trees 23

2.2.5 Stonewall tree failures with first-hand field inspection 25

2.2.6 Modes of root-wall interactions 27

2.3 Factors on stability and health of stonewall trees 31

2.3.1 Intrinsic wall factors 31

2.3.2 Quality of wall environs 34

2.3.3 Extrinsic impacts on walls and environs 36

2.3.4 Stonewall tree factor 40

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3. TREE RISK ASSESSMENT AND ARBORICULTURAL 43

RECOMMENDATIONS

3.1 Risk assessment of the stonewall trees 43

3.1.1 Site and general tree conditions 43

3.1.2 Assessment of T1 and arboricultural recommendations 45






3.2 Tree values, potential risks and preservation 91

3.2.1 Value of the stonewall trees 91

3.2.2 Risk of the stonewall trees 92

3.2.3 Potentials and limitations in tree preservation 94

4. RECOMMENDATIONS ON STONEWALL TREE MAINTENANCE 97

4.1 Short-term practical maintenance measures for 97

individual trees

4.1.1 Maintenance of T1 97






4.2 Long-term practical tree maintenance measures 101

4.2.1 Install soil strips at wall crest and toe 101

4.2.2 Remove joint seals to permit new root penetration 102

4.2.3 Prevent wedging effect on tree stability 102

4.2.4 Stop repeated removal of lower branches 103

4.2.5 Stop unprofessional and unnecessary pruning practice 103

4.3 Preventing grave impacts of excavation 105

4.4 Guidelines to inspect and monitor tree structural stability 107

and health

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5. EXECUTIVE SUMMARY 109

5.1 Preamble 109

5.2 Study objectives 109

5.3 Background and general concerns 110

5.4 Effect of stonewall trees on retaining structures 111

5.5 Factors on tree stability and health 111

5.6 Tree values, potential risks and preservation 112

5.7 Tree risk assessment and arboricultural recommendations 112

5.8 Long-term practical tree maintenance measures 113

REFERENCES 115

Appendix Resistograph micro-drilling to estimate wood density

(conducted by a contractor on 31 October 2012)

Tables 1 to 7

Photo slides 1 to 201

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LIST OF TABLES

Table 1. Stonewall tree assessment results of T1






Table 7. The microdrilling points of T2, T4 and T5 and respective photo

references.

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LIST OF PHOTOS

THE SITE Photo T0-1 to T0-9 Slides 2 to 11

TREE T1 Photo T1-1 to T1-8 Slides 12 to 20






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1. INTRODUCTION

1.1 Preamble

The Highways Department (HyD) is responsible for maintenance of

vegetation growing on registered slopes assigned to it. These registered

slopes include, among others, stone walls as retaining structures along

roadsides. Some of these stone walls have trees growing directly on the

structures, with roots anchoring on the walls or penetrating through the

walls to the soil lying behind (the ‘aft-soil”) to capture water and nutrients

and to secure anchorage.

These stonewall trees are unique landscape assets, but under some

circumstances they may create risks to nearby residents or road users.

They demand special attention or treatment in order to preserve them in a

safe and healthy condition. With reference to the six stonewall trees on

slope no. 11SW-A/R577 along Bonham Road (Photos T01 and T02), the

main aims of the study are to identify the factors affecting the health and

stability of stonewall trees, formulate suitable maintenance measures for

their sustainable growth, and advise on mitigation measures to minimize

the risk of tree failure. The maintenance responsibility of wall trees on

this SIMAR slope was assigned to HyD in 2004.

1.2 Scope of study

1.2.1 Background and general concerns

(a) Review in general the development of stonewall trees on retaining

structures in Hong Kong

This includes a brief account on the local stonewall construction that

enables stonewall tree development and an account of suitable tree

species adapted to stonewall habitat.

(b) Analyze the effect of stonewall trees on the retaining structures and their

stability on walls based on available stonewall tree failure records

This involves an analysis, with reference to available typical tree

failure records, on the physical and biological impact of stonewall

trees on the retaining structures supporting them and hence the effect

on the stability of the trees on these structures.

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(c) Identify factors affecting stability and health of stonewall trees

This involves literature review and site observation to identify and

elaborate on factors affecting the stability and health conditions of

stonewall trees.

1.2.2 Evaluation on the conditions of the stonewall trees

(a) Examine and conduct tree risk assessment for the existing six stonewall

trees

This includes site inspection (and aerial inspection, if necessary) to

examine the conditions of the trees. Observation with adequate

illustrations and coloured photos and proper annotations shall be

provided. Where necessary, testing for defective areas with

appropriate tools and analysis on the test results shall be provided.

(b) Appraise the value and assess the potential risks of these stonewall trees

to the local community

This involves the appreciation of the value of the stonewall trees to the

local community and an assessment of the potential risks to nearby

residents and other road users, as well as after taking into

consideration of the value of these trees and the potential risks, the

provision of balanced and impartial advice on the preservation of these

trees.

(c) Review the potentials and limitations in preserving these stonewall trees

This consists of identification of the limitations and site constraints in

preserving the trees. Formulation of practical proposals to enhance

their preservation with risk mitigated. Illustrations shall be provided to

clearly present the proposals.

1.2.3 Recommendations on the maintenance of the stonewall trees

(a) Formulate short and long term practical maintenance measures to

effectively enhance the sustainability of these stonewall trees

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This includes recommendations on maintenance measures to be

carried out during routine maintenance and special operation/checking

as required to enhance the sustainability of these trees and minimize

the risk of failure.

(b) Prepare guidelines to inspect and monitor the structural stability and

health conditions of these stonewall trees

This involves the preparation of maintenance guidelines to clearly list

out the aspects to inspect, monitoring procedures and maintenance

operation shall be provided. Where appropriate, methodology of

conducting the inspection and monitoring works with illustrations

shall be included.

1.3 Deliverables

1.3.1 Draft study report

The report is structured mainly according to the above scope of study.

Three coloured hard copies and 1 CD containing the draft report to be

submitted on a date as agreed by the Government Representative.

1.3.2 Final study report

Three coloured hard copies and 1 CD containing the final report to be

submitted within 14 days upon comments on the draft report are issued by

the Government Representative.

1.3.3 Presentation of the report in a seminar

The findings of the report shall be presented in a two-hour seminar in

2012 for government officials, including address enquiries as raised in the

seminar. HyD will organize the seminar and arrange a venue in Hong

Kong. The date and time of the seminar is to be mutually agreed.

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2. BACKGROUND AND GENERAL CONCERNS

2.1 Development of stonewall trees on retaining structures

2.1.1 Urban development and stone retaining walls

(a) Terrain constraints to city growth

In the course urban expansion in Hong Kong since the 1840s, the grave

shortage of easily developable land was felt right at the incipient stage.

Beginning with the City of Victoria founded at the north coast of Hong

Kong Island, the rugged topography has steep hills plunging into the

harbour to leave little flat land for buildings and roads. Strenuous efforts

were made to create land to accommodate the fast city growth. The quest

for developable land adopted two concurrent approaches, namely

reclamation from the sea and terracing the hillslopes. Both methods incur

substantial costs in terms of time and funding.

(b) Hillslope terracing and retaining structures

In the course of cutting the slopes into a series of terraces that resemble a

flight of giant steps climbing up the hill, attempts were made to maximize

the usable flat surfaces for urban development. Instead of leaving a strip

of slope along the contour between an upper and a lower terrace, a vertical

cut face was formed. A retaining structure, often a masonry retaining wall,

has to be constructed to stabilize the disturbed and oversteepened slope

and to prevent its geotechnical failure (Jim, 1998, 2010). Buildings were

often constructed close to the retaining structures at both the toe and the

crest positions. The traditional Hakka masons were widely recruited to

build the stone retaining walls based on traditional Chinese design (Lo,

1971).

(c) Distribution of masonry retaining walls

The government has recorded over 2500 retaining structure in Hong Kong,

of which about 1700 are stone retaining walls of different materials,

dimensions and designs built over the years (Chan 1996). They play a key

role in providing for and sustaining the continued growth of the city that

literally scale up the hillslopes. Concentrated mainly in the north parts of

Hong Kong Island, they are particularly common in the old districts of

Western, Central, Wanchai and Happy Valley. The walls are distributed

from a low footslope level to the Midlevels, with some found on the Peak.

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In recent years, stone walls have been replaced by reinforced concrete

walls, hence the old and fine masonry structures embedded in our city

denote an important but declining cultural heritage of the community.

(d) Stone wall design and plant life

A properly designed and constructed old stone wall has an earth core and

stone blocks on the external and internal vertical faces (Chan, 1996). The

joints between the individual masonry blocks may be mortared, but the

older ones tend to be left unfilled, in the dry stone wall tradition. The

presence of numerous gaps between the stones and soil in the wall core

and behind the wall (the “aft-soil”) offered opportunities for plant life

(Jim, 1998; Jim and Chen, 2011). Some retaining walls were constructed

without the core and interior stone face (Chan 1996). The vertical habitats

look apparently hostile or even inhospitable to plant growth. Yet a small

cohort of plants has the ability to colonize spontaneously the artificial

cliffs in the city to usher nature into the heart of the city. In a similar

manner, some trees would colonize the external walls of old buildings

(Jim and Chen, 2011).

2.1.2 Tree flora on stone retaining walls

(a) Keystone stonewall tree species

The humid subtropical climate Hong Kong permits the existence of a rich

plant biodiversity. A special group of woody plants called Banyans (genus

Ficus) of the Mulberry family (Moraceae) contains members with pre-

adapted characteristics to thrive on the seemingly rather precarious

vertical and harsh sites (Jim, 1990, 2006). The most prominent members

include trees that can attain large dimensions exceeding 20 m height and

crown spread. The botanical nomenclature adopts the Flora of Hong

Kong (Agriculture, Fisheries and Conservation Department, 2007, 2008,

2009, 2011). The English common names adopt the Check List of Hong

Kong Plants 2004 (Agriculture, Fisheries and Conservation Department,

2004). Most wall trees are dominated by native species, with few exotic

ones (* marked with an asterisk). They are represented mainly by (Jim,

1998, 2008a, 2010; Jim and Chen, 2010):

Chinese Banyan (Ficus microcarpa).

Big-leaved Fig (Ficus virens var. sublanceolata).

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(b) Secondary Ficus species

Other less common stonewall tree species in the same genus include:

Common Red-stem Fig (Ficus variegata var. chlorocarpa).

Japanese Superb Fig (Ficus subpisocarpa; previously known as Ficus

superba).

Opposite-leaved Fig (Ficus hispida).

Indian-rubber Tree (Ficus elastica)*.

Mock Bodh Tree (Ficus rumphii)*.

(c) Species related to genus Ficus

Two close relatives of Ficus trees, in the same Mulberry family can

sometimes dwell successfully as ruderals (semi-natural members of the

plants of a given place) on stone walls:

Paper Mulberry (Broussonetia papyrifera).

White Mulberry (Morus alba)*.

(d) Accidental stonewall tree species

Occasionally, some non-Ficus trees with strong rooting habit could

accidentally establish on stone walls as garden escapees, such as:

Chinese Hackberry (Celtis sinensis).

Elephant’s Ear (Macaranga tanarius).

Autumn Maple (Bischofia javanica).

Tree Cotton (Bombax ceiba)*.

2.1.3 Pre-adaptation of strangler figs to wall habitat

(a) Strangler figs in natural tropical forests

The Ficus trees that grow particularly well on stone walls tend to be

stranglers in the natural tropical forest habitat. Such trees are pre-adapted

to grow on the artificial cliffs mainly due to the vigorous and unusual root

growth habit. Their precarious life begins as tiny seeds from parent

Banyan trees that are eaten by birds. Some birds would perch on the

tropical forest trees during which droppings could be deposited. The

droppings contain Banyan seeds which can survive the vigorous digestion

process. If by chance the droppings land on the upper surface of branches

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or branch forks, a tiny proportion of the lodged seeds may germinate

successfully. The lodging sites can be situated up to 30 m above the forest

floor.

(b) Early epiphytic and precarious existence

The initial growth form from the seedling to the sapling stages is an

epiphyte, which is a plant that grows on the surface of another plant (the

host tree). The role of the host during the crucial germinal period is

providing a substrate for the attached Banyan (Figueiredo et al., 1995; Jim,

2006; GCW, 2011; IFLA, 2011). At this epiphytic stage, the young

Banyan has to overcome multiple stresses that can easily terminate its life.

Its roots are located far away from the soil on the forest floor. Day in and

day out, it has to struggle to acquire sufficient sustenance to support itself.

Nutrient supply has to come from the meagre amount of dropping and

accumulated organic debris on the branch surface or fork. Water supply

relies on rainfall or dew condensation absorbed by the organic debris and

the bark of the host tree, plus some stem flow. Anchorage depends on the

limited amount of roots sent out by the feeble plant to grip the host branch.

(c) Imperative aerial root development

The young epiphytic Banyan would soon develop a modest amount of

branches and leaves to conduct photosynthesis and manufacture its own

food. It would divert a notable amount of its hard-earned energy to form

aerial roots that hang downwards. As the epiphyte grows gradually bigger,

it can devote more energy to elongate the aerial roots and send out more.

These soft and rope-like roots in due course will extend further down

towards the forest floor. It may take some years for the aerial roots to

eventually reach the soil. At this point, the epiphyte is given literally a

new lease of life. From a struggling and weak epiphyte, it will henceforth

be transformed into a strong and rigorous strangler fig.

(d) Aerial roots reaching the forest floor soil

Upon reaching the forest floor, the aerial roots will forthwith take root in

the soil and develop an extensive normal root system in the soil. This

ground root system can tap a significantly larger amount of nutrients and

water to feed the tree. With notably enhanced sustenance, involving

multiple folds of increase, the Banyan strangler can grow quickly. Its

crown can expand and often cover up if not overwhelm that of the host

tree to capture more sunshine. More importantly, its multiple strands of

aerial roots that dangle around the trunk of the host tree can undergo a

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drastic changes. They gradually thicken and become woody through the

process of lignification.

(e) Strangulation of the host tree

Neighbouring aerial roots that establish physical contact can fuse together

in a process of self-grafting. In time, a basket-like mesh of lignified aerial

roots is formed wrapping around the trunk of the host tree. The continued

secondary thickening of the host trunk exerts a force that pushes outwards

away from the trunk centre. The continued thickening of lignified aerial

roots exerts a force that pushes towards the trunk centre. The two

opposing forces join hands to apply an exceptionally strong force to

literally strangle the host tree trunk. The vascular phloem lying below the

bark of the host trunk can be squeezed tightly and eventually cut off. The

host tree, deprived of water and nutrient supply from the roots to the

leaves, and food from the leaves to the roots, will be strangled until it dies.

(f) New lease of life for the strangler

Once dead, it will take about one to two years for the host tree, now a

victim, to decompose completely without leaving a trace. Meanwhile, the

Banyan can take advantage of the nutrients released by the decomposing

tissues of the victim tree. The niche original occupied by the host tree has

now been taken over by the Banyan. The strangler fig from then on will

continue to grow as though it has begun its life on the ground. Beginning

its growth well above the ground and growing initially downwards, it has

been transformed into a normal upward growing tree. Starting off as a

feeble epiphyte, it is now developing into a giant ground-hugging tree of

the forest.

2.1.4 Colonization of stone retaining walls by strangler figs

(a) Stone wall as surrogate host tree

The strangler fig would treat the stone wall in the city as the host tree. It

begins as a seed deposited together with bird droppings in a suitable

microsite on the stone wall, such as an unfilled crevice between stones, a

protruding pointing between stones, uneven stone surface, wall ledge, wall

coping, and wall toe. Of the many seeds that manage to land on microsites,

only a tiny proportion can escape subsequent dislodgement by natural

forces of wind, water, or gravity before they have the chance to germinate.

Many will perish by desiccation due to the scarcity of water in landing

microsites. Of the seeds that can stay and remain alive on the wall, only a

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small proportion can germinate successfully due to the paucity of nutrients

and water. Of the seedlings that can manage to grow up, only a small

proportion can reach the sapling stage, because of prolonged lack of

nutrients or water, or dislodgement due to natural forces or advertent or

inadvertent removal by humans (Jim, 1998, 2008a, 2010; Jim and Chen,

2010).

(b) Early epiphytic and perilous existence

Beginning as an epiphyte of a microsite, the early growth of a wall tree

can only be sluggish and precarious. The forces of attrition and stresses

will continue to take their toll. Of the many seedlings that manage to reach

the sapling stage, only a small proportion can soldier on to become mature

stonewall trees. Making use of its natural strangler capability, the wall tree

can gain a strong foothold on the vertical habitat. The unusual and

exceptionally vigorous growth habit of its roots plays the key role in its

successful colonization of the artificial vertical habitat. The roots can grip

the wall face and penetrate the gaps between masonry blocks to reach the

aft-soil. Once it is established, it tends to hold fast to the vertical habitat to

realize their potential biological size under the right combination of

conditions.

(c) Roots reaching aft-soil triggering new lease of life

Once the aft-soil is reached by the roots, a new lease of life for the

hitherto struggling tree can be triggered (Jim, 1998, 2008a, 2010; Jim and

Chen, 2010). A large amount of normal roots can spread out and capture

nutrients and water from a large aft-soil catchment. The spreading root

system in the soil also helps to reinforce the tree’s anchorage to allow the

tree to reach sizeable dimensions. The roots that establish physical contact

with each other can develop sturdy wood fusion by self-grafting. The

resulting root system can form a tightly knitted network on the wall face.

The wood fusion between lignified aerial roots and the tree’s own

branches and trunks can form root stands and props to reinforce the

biomass structure. Overall, the strangler fig is pre-adapted to thrive on

stone walls. Humans provide the stone walls and nature provides the

strangler figs. They form a good match to generate a unique urban

ecological feature in our city.

(d) History of stone wall colonization by trees

With an urban history that has extended over 160 years, Hong Kong has

built many stone retaining walls that offer a rather long duration for

Banyan trees to establish on them. Some stonewall trees are older than a

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century and have reached a huge size, often bigger than their ground-

growing counterparts. They provide living landmarks in our old districts,

some of which would be treeless but for the presence of the hanging trees

(Jim, 2008b). The number, dimension, distribution, condition and

ecological and cultural values of our stonewall trees are second to none in

the whole wide world. They qualify as a unique world-class ecological

gem of our city, and deserve to be protected as the natural-cum-cultural

heritage of our community.

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2.2 Effect of stonewall trees on retaining structures

The successful growth of stonewall trees demands the fulfilment of a host

of fundamental conditions. The presence of the right kind of microsites

for seeds to lodge on the wall surface constitutes a critical starting point.

Thereafter, it is the ability to overcome the negative impacts and chronic

stresses that would make or mar a wall tree’s continued growth.

In the course of growth on the stone wall, under some special

circumstances, the condition of the wall could be modified or disturbed.

Thus far, few detailed scientific studies have been conducted on the

specific impact of stonewall trees on wall structure. Such detailed studies

lie outside the purview of the present project. Previous studies mainly rely

on the examination of documentary records. This interpretation is based

on a review of the relevant literature, mainly recent government-

commissioned studies on stonewall trees, and supplemented by the

author’s past research experience on stonewall trees in Hong Kong.

2.2.1 Past studies of stone retaining walls

(a) Two main government studies

The cases of past wall and tree failures associated with stone retaining

walls have been summarized in two government publications:

Chan, Y.C. (1996) Study of Old Masonry Retaining Walls in Hong

Kong. GEO Report No. 31, Geotechnical Engineering Office, Civil

Engineering and Development Department, Government of the

HKSAR.

GEO (2011a) Study on Masonry Walls with Trees. GEO Report No.

257, CM Wong & Associates, Geotechnical Engineering Office, Civil

Engineering and Development Department, Government of the

HKSAR.

(b) Inclusion of previous study findings

The above two recent studies have embodied the key findings of three

previous government studies:

The 1977 study by Binnie & Partners under the ambit of the Public

Works Department.

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The 1980 study by the Geotechnical Control Branch (GCB) of the

Building Ordinance Office.

The 1980 Geotechnical Control Office (GCO) study on old masonry

retaining walls.

(c) Stone wall and stonewall tree failure records

The first report (Chan, 1996) focused squarely on the engineering aspect

of the masonry structure with some assessment of trees that grow on them.

The second report (GEO, 2011a) aimed at clarifying the relationship

between stone retaining structures and the associated stonewall trees. Both

studies include retrospective analysis of some failed walls based on

official file records.

These government documents denote an official source of information

containing the interpretation of incident reports pertaining to masonry

retaining walls filed by relevant departments. It is believed that no other

similar systematic information is available. They embody records and

analyses that tend to focus on wall failure cases per se. It is likely that the

tree failure cases that were not associated with wall failure, especially

those that did not involve large trees, were not reported to the GEO or its

predecessor GCO. Thus this source of official information is likely to omit

a notable number of stonewall tree failure incidents. The corollary is that

the wall failure cases in the files may not provide a representative sample

to reflect the nature of tree failure on stone retaining walls.

The other issue concerns the objectives of the incident reports which tend

to concentrate on the physical fabric of the wall and the interpretation of

the mode and possible causes of wall failure. The assessors who

conducted the field evaluations and filled the forms were often not trained

in tree science and tree failure appraisal, and hence they may not be able

to decipher the fundamental and immediate causes of the tree failure. As a

result, the incident reports contain little direct or indirect information on

the mode and causes of tree failure.

As such, these records cannot be relied upon to distil high-quality

scientific assessment of tree failures at masonry retaining wall sites. The

corollary is that it may not be advisable to use such information to

establish practices or management measures to manage stonewall trees.

Moreover, the nature and information contents of the records do not lend

themselves to retrospective interpretation of the causes of tree failure.

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Any attempt to do so would invariably incur speculative elements that are

not based on objective and unambiguous scientific evidence.

2.2.2 Historical cases of fatal stone wall failures

The following case studies of stone retaining wall failures are extracted

from the government reports. They denote the more notable incidents

with a reasonable amount of documentary records. The more prominent

wall failures involve fatalities, which were followed by court inquiries or

scientific investigations, contain more details.

(a) Case A: St Joseph Terrace 1917

The failure of old stone walls is not a recent phenomenon. Back on 16

July 1917, a stone retaining wall at St Joseph Terrace (above Caine Road

and adjacent to the Catholic Cathedral) collapsed (Chan, 1996; South

China Morning Post, 17 July 1917). The unfortunate incident destroyed

the rear part of two tenement blocks at 10 and 12 Caine Road, and

claimed six lives. The failure of the dry rubble wall, built in 1911, was

mainly explained by its inadequate thickness. The failure was preceded

by the widening of a crack at the corner of the wall. The collapse was

triggered by the repaving work at the wall-crest platform used by the St

Joseph College as a playground, causing saturation of the retained soil.

About half of the old paving at the platform was removed at the time of

the incident, exposing the underlying soil directly to the heavy antecedent

rains. The record did not mention the presence of trees on the wall.

(b) Case B: Po Hing Fong 1925

The most catastrophic wall failure happened in the early history of our

city’s development on 17 July 1925 at Po Hing Fong in Sheung Wan. It

happened after a prolonged period of heavy rains. The en masse collapse

of a wall built in 1860 destroyed seven brick tenement buildings lying in

front of the wall, incurring a loss of 75 lives. The historical record did not

mention whether trees were present on the wall (Chan, 1996; South China

Morning Post, 18 July 1925). The redevelopment of the No. 8 Police

Station at the wall-crest platform involving excavation for the foundation

of a new building could have contributed to the failure. A subsequent

court of inquiry recommended improvement in the design, workmanship

and maintenance of masonry retaining walls to avoid recurrence of similar

mishaps (South China Morning Post, 29-31 July 1925, 8 August 1925, 3

and 5 September 1925).

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(c) Case C: Kwun Lung Lau 1994

A fatal wall collapse happened on a more recently built stone retaining

wall (Morgenstern and Geotechnical Engineering Office, 2000) in 1994 at

Kwun Lung Lau, Western district. It resulted in five fatalities. As no

trees were found on the wall, they did not play a role in the wall breakage

or collapse. A subsequent scientific study was conducted with the help of

an external geotechnical expert to probe into the causes of the wall

collapse. Fundamental weaknesses in wall design and pipe leakage behind

the wall were found to be the contributory factors.

2.2.3 Historical cases of stone wall failures without casualties

Other notable cases of wall failure which did not incur casualties have

been analyzed and reported in Chan (1996):

(a) Case D: Castle Road 1970

A short section (about 9 m long) of the wall failed after days of heavy

rainfall at 10 Castle Road on 19 June 1970. It was found to be of poor

quality mass concrete and disturbed prior to collapse by recent excavation

by the gas company. No casualty and no wall tree were reported. It can

be interpreted that the failure was not related to stonewall trees.

(b) Case E: May Road 1973

Almost the entire 40 m length of the retaining wall at Thorp Manor, 1

May Road, collapsed on 2 September 1973. It was then under the

influence of Typhoon Ellen. The Thorp Manor was being demolished at

that time. Trees and shrubs were found lying on the debris, and some

vegetation was present on the small remaining parts of the wall. Whether

the vegetation contributed to the failure was not interpreted, and its role

cannot be ascertained from the limited amount and quality of visual and

other records.

(c) Case F: Caine Lane 1976

The wall at Caine Lane behind the U-Lam Terrace failed on 25 August

1976. It has been reported that the site suffered from high ground water

table, and horizontal drains were installed to lower it. Little other

information is available to interpret the fundamental and immediate

causes of the wall collapse. The role of trees in stressing the wall was not

mentioned in the report.

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(d) Case G: Circular Pathway 1977

The wall at 3-7 Circular Pathway was found to be critically unstable in

August 1977, which was associated with Typhoon Vera and a low

pressure trough bringing plenty of rains. Shortly before the incident, the

old buildings in front of the wall were demolished together with the

removal of arches between the wall and the buildings. Longitudinal cracks

and land subsidence were detected at the wall-crest platform along

Circular Pathway, and the cracks were widening and extending. A bulge

was formed on the wall which was attributed to an unknown source of

water exerting pressure on the structure.

The wall had to be stabilized by constructing a free draining embankment

and the demolition of buildings at 24A-25A Circular Pathway at the crest

platform to reduce loading on the stressed wall. These geotechnical

measures were able to stop the wall from further movements. The records

did not mention the presence or the role of trees in the destabilization of

the wall.

(e) Case H: Old Peak Road 1978

The wall at 22 Old Peak Road was found to display a series of distress

symptoms on 11 May 1978. They include bulge, cracked concrete parapet

wall at the crest, and crack parallel to the wall along the middle of the

road at wall crest. The cracked areas also suffered from land subsidence.

These symptoms were apparently associated with a recently reinstated

trench for telephone line. The old wall was subsequently stabilized by

constructing a new concrete wall in front of it. The record was silent

concerning the presence or role of stonewall trees as an agent of wall

destabilization.

(f) Case I: Fat Hing Street 1978

At 14-16 Fat Hing Street adjacent to 48-56 Queen’s Road West, a stone

wall failed on 29 July 1978. The timing was associated with Typhoon

Agnes bringing abundant rainfalls. The old buildings in front of the wall

were demolished prior to the incident. Shortly before yielding of the wall,

a 0.8 m deep trench was opened for water pipe installation at the wall

crest platform subparallel to and near the wall. The sheet piling at the wall

toe completed at least six months before the incident could have

contributed to the failure. The immediate cause of failure was attributed to

the build-up of water pressure behind the wall. Besides heavy rains, the

trench at the wall crest would have play part in increasing the water

22

pressure on the wall. Trees were not mentioned in the records as a cause

of wall destabilization.

(g) Case J: Wing Wa Terrace 1978

The subject wall was found at 1-10 Wing Wa Terrace near Hospital Road.

It lent support to the Wing Wa Terrace at its crest. It deviates from other

cases in that the wall failed rather uncommonly outside the wet season on

13 November 1978. It was not associated with particularly heavy rains or

a large amount of accumulated rainfalls prior to the collapse. The wall

presented a host of distress signals prior to the failure. They included

bulge at the failed position, and steepening of the wall batter, failure of a

steel strut, movement of masonry blocks at several positions, and broken

steps adjacent to the wall.

The owner was required by the government to implement remedial

strengthening works which were initiated in September 1978. They

included installation of horizontal drains to draw down the ground water,

concrete counterweight at the crest to prevent overturning, and sheetpiles

to stabilize the wall toe. The vibration of the sheetpile driving caused

crack development parallel to and extending for half of the wall length at

the crest platform.

The immediate causes of failure were attributed to the sheetpiling getting

too close to the wall generating vibration to damage the masonry structure,

and the sheet piles physically breaking the wall’s spread footing. The

vibration could also have broken a sewer behind the wall to induce a local

rise in groundwater level. Trees did not contribute to the wall

destabilization or failure.

(h) Case K: Jewish Recreation Club 1979

On 3 August 1979, associated with Typhoon Hope bringing plenty of

rains, a section of the retaining wall at the northern edge of the Jewish

Recreation Club near Robinson Road failed. The platform supported by

the wall, forming the main grounds of the Club, were unpaved and used

for car parking and as a tennis court. The wall had bulged for an

unspecified period before the incident. The yielded part situated at the

west end adjacent to the tennis court, however, did not bulge as seriously

as the remaining part to its east. No tree was involved in the incident.

23

(i) Case L: Peak Road 2007

A wall failure that fortunately did not result in casualties (GEO, 2010)

happened on 3 June 2007 on the hillslope adjacent to 84 Peak Road. The

wall is believed to be built in the 1920s, measuring up to 2.2 m height and

14 m long; it is of the squared rubble type of dry wall. The wall failure

involved some masonry blocks falling off, sliding down a slope and

landed on the Peak Road. Different possible causes for the failure were

explored. Uneven ground settlement associated with heavy rainfall at the

wall base could have loosened masonry blocks. The roots of a stonewall

tree near the failure zone were suspected to have contributed to

displacement of the stones probably due to root wedging action. This is

thus far the only case that included stonewall tree as one of the causes of

wall failure.

2.2.4 Recent tree failures affecting stonewall trees

The details of the case studies M to R of stonewall tree failure can be

found in the main text as well and appendices of a recent government wall

study report (GEO, 2011). They will not be repeated here. Instead, the

possible causes of these tree failures and their impacts on walls could be

summarized as follows. Information about case S is mainly obtained from

a newspaper report which contained photographs. Additional information

about this case was acquired from the web-based government slope

information system (CEDD, 2012).

(a) Case M: Hill Road 1997 (11SW-A/FR24)

A tree situated near the wall crest uprooted and fell down, and it caused

localized collateral damage to the wall by extracting two masonry blocks.

The wall did not fail and the general stability of the wall was not affected

by the tree loss.

(b) Case N: Ka Wai Man Road 1999 (11SW-A/R120)

A tree situated on a cut slope above the wall was uprooted under typhoon

influence. The subject tree was not a stonewall tree. The wall did not fail

and its structure was hardly affected by the tree collapse. Some joints

near the wall crest showed signs of having been repaired, but they might

not be related to the tree failure incident.

24

(c) Case O: Clarence Terrace 1999 (11SW-A/R751)

A tree planted on a roadside children’s playground at the wall-crest

platform was uprooted and hit the top of the adjacent stone wall. No

stonewall tree was involved in the incident, and the structure of the

subject wall was not affected.

(d) Case P: Tai Hang Road 2003 (11SE-A/C897, previously filed as 11SE-

A/R51)

This case did not involve a stone retaining wall and the subject trees was

not a stonewall tree. The feature wall in front of Lai Sing Court at Tai

Hang Road is actually stone pitching serving as a surface veneer on a cut

slope face. A tree situated on a slope above the stone pitching was

uprooted under strong wind. In the course of its fall, some stone blocks

were dislodged from the stone pitching.

(e) Case Q: Lei Cheng Uk Swimming Pool 2004 (11NW-B/C321)

A tree fell down from a cut slope adjacent to the Lei Cheng Uk Swimming

Pool. The slope was covered by stone pitching. The case did not involve

stone retaining wall or stonewall tree.

(f) Case R: Wyndham Street 2005 (11SW-B/R735)

A stonewall tree situated at the crest of the feature wall fell down

probably due to strong wind. The structure of the wall was not damaged

by the tree failure. No record of tree dimensions was available. The

photograph shown in GEO (2011: 25) implies a relatively small tree.

(g) Case S: Tai Hang Road 2011 (11SE-A/C114(2))

A wall tree failure at 50C Tai Hang Road opposite the True Light Middle

School was reported in the media. It occurred on 13 May 2011. The

Chinese Banyan was about 10 m tall with a trunk diameter around 50 cm

m grew in the middle part of the stone retaining wall which measures 9 m

tall (CEDD, 2012). The wall is inclined at 70 degree, which is well above

the lean of most local stone retaining walls. The tree developed a large

amount of surface roots forming a dense mat of self-grafted network on

the wall face.

It lost its grip on the wall on the rainy day with amber rainstorm warning.

It was found that the tree roots encountered difficulties in their attempt to

penetrate the wall which has tightly packed square masonry blocks with

25

well mortared joints. With few roots growing through the joints to spread

in the aft-soil, its biomass was unable to hold firmly against overturning.

In other words, the tree did not have sufficient roots to provide tensional

pull against toppling. The cut face of the remaining trunk base showed

healthy sound wood unaffected by decay.

2.2.5 Stonewall tree failures with first-hand field inspection

Two cases of tree collapse were subject to detailed first-hand field

evaluation by the author to interpret the causes of failure based on

evidence.

(a) Case T: Pokfulam Road 1992 (11SW-A/R346)

A Chinese Banyan (Ficus microcarpa) that grew on the wall at Pokfulam

Road near the West Gate of the University of Hong Kong collapsed on 01

February 1992. The following account provides a first-hand field

inspection of the subject tree by the author shortly after the incident. The

tall stone retaining wall reaches a height of 15 m, above which lies a

rather natural vegetated slope with open soil. The trunk base of the failed

tree rested on the wall crest, with roots spreading in two main directions.

The surface roots moved downwards to cover the wall face profusely and

to reach the wall toe. At the wall crest, another group of roots grew

backwards and spread extensively in the soil of the slope. As a result, the

tree developed an exceptionally strong anchorage that is seldom found in

stonewall trees, because the wall-crest roots provide ample tensional pull

to hold the tree against toppling or overturning. Few of the surface roots

were able to penetrate the mortared joints of this wall with tight and well

filled gaps between masonry blocks. Thus the tree stability is dependent

heavily on the wall-crest roots dwelling in the slope soil.

Unfortunately, shortly before the tree failure, a utility company opened a

trench at the wall-crest slope adjacent and parallel to the wall. As a result,

the entire bundle of critical wall-crest roots of the subject tree was

completely severed. The sudden loss of the crucial tensional pull left the

tree vulnerable to failure. On the event day, the tree literally fell down

onto the carriageway of Pokfulam Road, and displayed vividly the

wholesale cutting of all the tension roots at the wall crest. Thus this tree

failure could be attributed to an improper and massive root cutting due to

trenching at the wall crest. It has nothing to do with normal causes of tree

risks such as insecure anchorage, weak structure or wood decay. The

trunk base and a large portion of the surface roots were detached from the

26

wall face. The dropping of the tree from the wall did not damage the

masonry structure.

(b) Case U: Forbes Street 2012 (11SW-A/R838)

A stonewall tree collapse during the peak of Typhoon Vicente around

mid-night on 23 July 2012 at the historical stone wall at Forbes Street in

Kennedy Town. The site has been used by the MTRC to build the new

Kennedy Town station. The footprint of the station and the associated

tunnels have been located away from the wall to avoid disturbing its

masonry structure as well as the 20 odd stonewall trees dwelling on it.

The subject tree is a Chinese Banyan (Ficus microcarpa) that grew near

the wall crest close to the western and taller end of the long wall. The

rather large wall tree measured about 13 m tall, 16-18 m crown spread,

and it was supported by a single stout trunk of 1.25 m diameter. Its trunk

base perches at about 12 m from the road level and at about 2 m below the

wall crest. The tree was marked by a rather complete, balanced and

sprawling crown, normal foliage size, colour and density, and few missing

limbs or major branches. The wall has no visible signs of distress,

indicating that the masonry structure is in a sound condition. The surface

roots of the tree spread on the masonry face all the way down to the wall

toe, where they penetrated into the soil lying under the pavement.

After its collapse, the author’s detailed site inspection found that the tree

was supported mainly from below by the masses of surface roots that were

pressed tightly against the wall face. The tension roots that pulled the tree

towards the wall, and hence held it against toppling or overturning,

unfortunately, were grossly inadequate. Only five torn tension roots were

observed on the wall at around the trunk base position. They penetrated

the wall joints, except one that explored a weep hole. None of them were

thicker than 10 cm in diameter. The torn surfaces of the tension roots

exposed fresh and sound wood, indicating that they were unaffected by

wood decay.

Overall, the tree failed due to the lack of tension root development, which

was severely restricted by the sealing of joints between masonry blocks.

It encountered unusually strong gusts during the typhoon that exceeded

the strength of the limited tension roots in terms of number and thickness,

resulting in breakage of tension roots and tree collapse. The maximum

wind speed around the time of tree failure recorded at the nearest weather

station at Green Island reached nearly 100 km/h. The detachment of the

tree from the wall imposed no damage at all on the strong masonry

27

structure. No stone was shifted or broken, and no joint was widened as a

result of the tree’s downfall.

A clear distinction should be drawn between stone wall failure per se, and

stonewall tree failure. The stonewall trees brought down by en masse

stone wall failure, strictly speaking, is a collateral damage. Such cases do

need to be interpreted as stonewall tree failure. A bona fide stonewall tree

failure should be associated with mechanical weaknesses in the root

system or the stems, or localized disruption and destabilization of the

stone structure due to static or dynamic forces exerted by the tree.

2.2.6 Modes of root-wall interactions

(a) Four main types of interactions

The growth of stonewall trees may interact with the wall structure in the

following manners:

Surface roots growing on stonewall face.

Roots growing through the joints into the wall structure.

Roots growing in the aft-soil lying behind the wall.

Surcharge of tree mass and wind effect on wall.

(b) Surface roots

Stonewall trees often spread their roots on the wall face, and such roots

can be labelled as surface roots. Ordinary trees would develop roots in the

soil and therefore below the soil surface. Some Banyans, however, can

develop and maintain a notable amount of roots outside the soil domain.

The most common stonewall tree species, Chinese Banyan (Ficus

microcarpa) and Big Leaved Banyan (Ficus virens var. sublanceolata),

are particularly adept in generating a large amount of surface roots. This

rooting habit vividly expresses the inherent strangler fig trait (cf. Section

2.1.4). Instead of using lignified roots to wrap around the trunk of the

host tree in the tropical forest, the lignified roots of stonewall Banyans

have found a substitute host by gripping the wall.

Depending on the species and individual tree character, the vertical and

horizontal spread, diameter, density, and self-grafting wood union of

surface roots can vary significantly from tree to tree. These four surface

root traits would influence tree interaction with the wall structure. The

surface roots grow more or less parallel to the wall face. They are usually

28

pressed tightly against the masonry blocks. Surface roots characterized by

extensive spread, relatively thick diameter, high density, and high

frequency of wood union, can offer a strong living network to protect the

wall by literally holding the masonry blocks in place. Such ramified and

interconnected living roots have high tensile strength. The effect is

tantamount to installing a steel mesh on the wall face. They can help to

hold the masonry blocks in place, prevent displacement, and overall

stabilize the wall structure. They are effective in checking their

displacement especially in the perpendicular direction away from the wall

face.

Most of the surface roots are lignified and they do not play the role of

water and nutrient absorption. In the root-system hierarchy and associated

physiological functions of the tree root system, they are transport roots

rather than absorption roots. They help to transport the water and nutrients

absorbed by fine roots dwelling in the aft-soil to the leaves to support

photosynthesis. Without the potentially disturbing impacts of water and

nutrient absorption, the surface roots would not secrete chemical

substances to degrade the stones. Thus they do not induce chemical

weathering which otherwise may in the long term compromise the

mechanical strength and integrity of the wall. Additionally, the surface

roots are instrumental in gripping the wall surface to contribute to the

tree’s foothold and in supporting the tree’s biomass.

(c) Roots in joints

Roots growing through the joints may expand by secondary thickening to

induce the wedging action, which may displace masonry blocks from their

equilibrium positions. However, stone retaining walls have tightly

interlocking stones that are firmly pressed against each other to resist the

relatively low pressure exerted by root diameter growth. This action could

seldom shift stones and disrupt wall stability.

Field observations of Banyan roots that have penetrated the joints indicate

the highly adaptable growth habit. The normal circular cross-section of a

root, upon entering a gap, will assume a planar shape to fit the geometry

of the joint. After venturing through the gap, the root will grow into the

aft-soil by resuming the normal cylindrical form. Thus the chance of

Banyan roots entering a joint and pushing stones apart is rather remote.

The example of stone displacement by stonewall tree shown in Plates 8.3

and 8.4 of Chan (1996: 113) occurs at the end of the wall where the stone

interlocking effect is relatively weak. The photographs clearly indicate

the unusual growth of the trunk of a tree directly in the gap, rather than

29

penetration by tree roots. A small number of cases of stone displacement

are associated with roots exploring gaps, for instance at the Bonham Road

wall in this study, and at the western end of the Forbes Street wall. In

these cases, the impact on wall structure could be considered as local and

limited. At the wall crest, the roots may enter the gap under the coping to

lift it. Similar to the wall end, the limited interlocking effect has permitted

roots to widen the horizontal gap at the wall top.

(d) Roots in aft-soil

The Banyan roots have a natural propensity to grow away from light into

the gaps between masonry blocks, in line with the physiological response

of negative phototropism. Roots tend to ramify in the aft-soil to anchor the

tree and strengthen the soil. They offer additional reinforcement to the

wall by providing tensional pull, and raise the frictional forces between

the wall and the retained soil. For the older generation of loosely packed

and dry walls with plenty of gaps, more roots can penetrate and spread in

the aft-soil to provide notable wall stabilization. For the later tightly

packed and mortared wall, the limited amount of aft-soil exploration by

roots will correspondingly reduce this soil strengthening effect.

(e) Root decay in aft-soil

The aft-soil, like most soils, contains pores of different sizes which

depend mainly on texture, structure and bulk density (degree of

compaction). Tree roots that grow in the aft-soil could die and leave

behind cylindrical pores. Small roots tend to have shorter life span and

their continual death and replacement by new roots would only leave

small pores with limited impact on soil water movement. If the dead roots

are large, notable cylindrical pores could be formed to facilitate the

transmission of soil moisture. If such pores could concentrate subsurface

water flow, pressure could be imposed locally on the stone wall. Most

trees tend to keep their large roots and hence their death and formation of

notable cylindrical pores will be limited. The invasion by wood-decay

fungi and consumption by termites, however, could kill vulnerable large

roots to leave channels for water transmission.

(f) Surcharge of tree on stone wall

A tree hanging on a retaining wall, with a certain biomass and wind

resistance and affected by gravity and wind, denotes an additional

surcharge to the structure. A review of the relevant literature on the

analysis of the relevant forces is given in GEO (2011). The load can

increase the overturning moment and apply an extra toe pressure to the

30

wall. The size of the tree in terms of total aboveground biomass, trunk

diameter, the sail effect of the crown, crown porosity and permeability to

wind, and bending flexibility of branches (involving the changing frontal

area and drag coefficient in response to wind velocity), could determine

the magnitude of the surcharge. Part of the wind load acting on a

stonewall tree could be transmitted to the wall.

If a tree has an inherently weak biomass structure or has its mechanical

strength compromised by decay or cracking, it may break under strong

wind. If the root anchorage is inadequate, or its root system has been

damaged by natural or artificial forces, the tree may be uprooted under

strong wind. Overall, three modes of failure due to wind could be

enumerated: branch breakage, trunk breakage, and uprooting. Before

failure, the tree under the influence of strong wind would transmit part of

the energy to the wall structure. The root system per se will be strained

together with the interfaces between roots and masonry blocks, and

between roots and aft-soil. Of the few cases of stonewall tree failures, it

has been found that the masonry structure was hardly affected (GEO,

2011).

31

2.3 Factors on stability and health of stonewall trees

2.3.1 Intrinsic wall factors

(a) Gaps between masonry blocks

Dry random rubble walls with wide and plentiful gaps between masonry

blocks can permit rather free passage of air and water into and out of the

aft-soil. The same micro-niches could facilitate seed lodging and seedling

germination. The gaps offer avenues for roots to penetrate into the wall

core and aft-soil. In comparison, dry squared rubble walls have narrower

and less plentiful gaps and hence they are less amenable to colonization

by stonewall trees. Modern stone walls with squared and coursed masonry

blocks and joints completely filled by mortar offer significantly fewer

chances for roots to penetrate. They are less conducive or unfriendly to

the growth of stonewall trees.

(b) Stone size and length of joints

A wall with smaller masonry blocks contains a longer total length of joints.

Joints offer critical microsites for soil and debris accumulation, and

moisture and nutrient supply. They are conducive to seed lodging, seed

germination and growth and root penetration. More joints are therefore

favourable to stonewall tree growth and establishment. They help to

sustain their continued development and stability.

(c) Stone shape and horizontal joints

A wall composed of mainly rectangular masonry blocks with a long

horizontal axis can provide more horizontal joints in comparison with

vertical ones. Horizontally-oriented joints are more receptive to seed

lodging and seedling growth. They offer more secure microsites for seeds

and seedlings to stay on the wall and resist dislodgement by gravity, wind

or running water.

(d) Wall inclination

Most walls are not vertical. Instead, they tend to be slightly inclined up to

about 10 degrees from the plumb. A more tilted wall provides somewhat

less harsh microsites that are more receptive to the lodging of seeds,

seedling growth and tree establishment.

32

(e) Wall height

Birds that bring the seeds to the wall joints via their droppings would

avoid getting too close to humans. A tall wall offers a larger proportion of

its wall face situated away human disturbance which occurs mainly at the

wall toe. Seedlings that grow up at the wall toe and at the lower part of the

wall are more likely to be disturbed by humans. They are often removed

to provide clearance to pedestrian movement along the narrow footpaths

or lanes in front of the wall. Thus low walls are less conducive to wall

tree establishment and long-term existence. For a similar wall length, a

tall wall provides a large surface area to receive seeds and to permit tree

growth.

(f) Original joint filling

The treatment of the wall joints at the time of construction is known as

joint filling. Dry walls have their joint left un-mortared. The ashlar walls

with squared or rectangular stones and mortared joints are not too

amenable to tree establishment. They lack the necessary micro-niches for

seeds to lodge, for seedling growth, root penetration, tree establishment,

and continued tree growth.

(g) Cement sealing of dry wall joints

Originally dry walls tend to be sealed recently in the course of

overzealous and improper wall maintenance practices. Often, a cement

plus sand mortar is used to fill the open joints. Some old dry walls have

all the joints effectively seals in this manner. In some cases, the filling has

resulted in a wide band of mortar along the joints in an undesirable and

unsightly practice labelled as buttering.

The newly applied mortar often wraps around existing roots and girdles

them as they continue to grow in diameter in the process of secondary

thickening. The girdled roots will have its phloem or sapwood restricted

or cut off, thus curtailing the ability of the affected tree to acquire water

and nutrients, and to distribute the food manufactured by photosynthesis

to reach the root system. Girdled roots tend to develop wounds that could

be invaded by wood-decay fungi and other natural enemies. Existing roots

in the aft-soil will also suffer from the lack of aeration, with inadequate

exchange of soil air with outside air. Root respiration will be dampened

due to the lack of oxygen supply, and root functions could be dampened

by the accumulation of carbon dioxide due to inadequate dissipation.

33

Meanwhile, new roots will have no avenue to enter the wall gaps and

acquire new sources of sustenance from the aft-soil. Overall, tree growth

can be stifled, leading to gradual and premature decline. The weakened

tree could lose its ability to ward off natural enemies, which may lead to

decay and other growth problems. Thus joint sealing can serve as a trigger

factor leading to deleterious snow-balling effects on tree health and safety.

Wall management should refrain from sealing open joints.

Measures could be adopted to ensure that wall maintenance works can

remain conservative and appropriate. They should be friendly to the

present cohort of stonewall trees, and permit the unimpeded establishment

of the future generation of stonewall trees. For outstanding walls with

heritage value, it is important that the maintenance should adhere

faithfully to the authenticity principles.

(h) Weathering state of masonry blocks

More weathered stones can release more nutrients in dissolved form from

the chemical breakdown of their constituent minerals. The modified

materials can retain some water for use by some adventitious roots that

happen to grow on the masonry surface.

(i) Seepage on wall face

Long-term seepage on the wall face could indicate a relatively high

groundwater table, providing a ready source of water for absorption by

adventitious roots resting on the surface of masonry blocks. Moreover, it

also indicates that the aft-soil has a rather regular supply of water to feed

the roots growing there. However, if the water table stays at a high level

for a long duration in relation to the wall, it could imply the application of

hydraulic pressure on the wall.

The sudden increase in seepage should be interpreted with caution, as it

could be an ominous sign. The source of water should be traced, as it

could originate from the breakage of potable water mains, or stormwater

drain, or sewage pipe. Such unexpected and unnatural release of a large

amount of water could exert inordinate pressure on the wall and it may

induce wall failure. The excessive soil moisture could drive out air to

harm the roots. If the leakage comes from a sewage pipe, the chemical

constituents which may include pollutants could harm the trees.

34

(j) Weep holes

Not all retaining wall have built-in weep holes. For dry walls with plenty

of gaps between masonry blocks, it is not necessary to provide them. For

mortared walls, they may or may not be present. Weep holes can offer

microsites for seed lodging and seedling growth, but they alone do not

offer sufficient conditions to permit the establishment of large wall trees.

Weep holes can provide water with dissolved nutrients to nourish surface

adventitious roots, and this role is more important in the early stage of a

wall tree’s tenure when few roots have spread in the aft-soil. For well-

established large wall trees, this function may not be important.

2.3.2 Quality of wall environs

(a) Moisture supply of aft-soil

Above and behind the wall crest, the site is often occupied by a formed

platform usually with a building and paved land surface. Such site

conditions would limit the supply of water to the aft-soil via infiltration of

rainwater. The wall trees will have to depend on groundwater. If the

groundwater table behind the retaining wall is dammed and stays at a high

level, it may keep the aft-soil too wet and be unfavourable to tree growth.

The gaps between masonry blocks and weep holes, however, could

discharge the groundwater and keep the water table low relative to the

wall. If the groundwater table is occasionally high and not too low most of

the time, the tree roots can take up water from the capillary fringe of the

ground water.

(b) Volume of aft-soil

The volume of aft-soil, similar to the case of ground-growing trees, would

play a key role in supporting the growth of stonewall trees. A larger soil

volume can permit extensive spread of roots and capture more moisture

and nutrients to satisfy tree requirements. More soil could contribute to a

firm tree anchorage by the root system. The volume of aft-soil could be

restricted by various reasons. The presence of hard rock, partly weather

rock, building foundation and other subterranean installations could limit

the amount of aft-soil that can be explored by roots.

(c) Quality of aft-soil

Besides the volume of soil accessible to root growth, the quality of soil

would regulate the physical and chemical conditions for root growth and

35

functions. Soil conditions that are unfavourable to roots include: soil

compaction, high stone content, excessively coarse texture (too sandy),

excessively fine texture (too clayey), poor structure, low porosity,

insufficient or excessive moisture, restricted aeration, low organic matter

content, inadequate available nutrients, and presence of soil pollutants.

(d) Wall-crest slope with unsealed soil

At the wall crest, the presence of unsealed soil covered by vegetation is

conducive to wall tree colonization and development. Proximity to natural

vegetation has been found to facilitate stonewall tree growth and

establishment. In particular, the presence of natural woodland at the crest

slope could enhance the number and vigour of stonewall trees. Other types

of companion wall vegetation, such as herbaceous and non-vascular (moss

and lichen) are also favoured by the nearby natural vegetation cover.

(e) Wind exposure

A stone wall at an exposed and windy site is hostile to seed lodging and

seedling growth. The birds or bats that could bring seeds along with their

droppings may shun the unduly windy locations. They are less likely to

build their nests on the stonewall trees at such locations, or visit the site to

seek food or shelter, or to rest. The seeds that manage to lodge in the

crevices could be easily blown away by strong wind or washed away by

rain in conjunction with wind. Similarly, seed germination and seedling

growth could be dampened by strong wind. Under extreme weather

conditions, seedlings that are usually rather precariously attached to the

wall could be literally plucked and swept away. Even medium or large

trees could yield to exceptionally ferocious gusts, resulting in branch loss,

stem breakage or uprooting failure.

(f) Solar access

Most stonewall tree species prefer to have plenty of sunshine to allow

them to flourish. Sites that are sandwiched between tall buildings could be

heavily shaded to suppress photosynthesis and food production. The

amount of solar energy that can reach the stonewall trees could be

evaluated by the sky view factor. Most stone wall sites are situated close

to buildings or narrow roads with buildings nearby. Thus they are unable

to enjoy unimpeded sunshine. The lack of solar access is closely related

to the lack of above-ground growth space to accommodate normal crown

spread. For sites with open space at the wall crest platform, the wall trees

tend to be stronger and can reach larger dimensions.

36

(g) Air quality

Most stone retaining walls occur in the Mid-levels areas where roads are

narrow and capacity for traffic is limited. Some of the main roads,

however, can have heavy vehicular flow during the peak hours to

contribute to air pollution. The narrow roads bounded by high-rise

buildings could trap air pollutant emitted by vehicles, and dampen the

growth of stonewall trees. As they usually perch on an elevated position,

the air pollution impact could be somewhat relieved.

(h) Proximity to buildings and roads

Most stone wall sites are rather cramped with close juxtaposition of

buildings and roads. The urban form is typical of our compact mode of

development. Proximity to buildings would incur conflicts when branches

get too close to windows. Branches extending into the carriageway space

at a low level will have to be cleared. For trunks or branches that grow at

a low level, frequent removal would be required to maintain footpath

clearance. Repeated pruning especially of large branches could dampen

tree vigour and invite invasion by wood-decay fungi.

2.3.3 Extrinsic impacts on walls and environs

(a) Soil nail installation and grouting damage

Grouting can impose grave physical and chemical harms on stonewall

trees. In the course of drilling, the roots in the aft-soil and on the wall

face could be injured or severed. The drilling machine may also

accidentally hit the trunks and branches and impose injuries and breakage.

The hot exhaust emitted by the machines could stress the trees. The

introduction of grouting materials under pressure into the drill holes could

induce more serious and long-term damages. The fluid is likely to

migrate away from the borehole into the surrounding aft-soil. A fluid

with low viscosity and a soil with high porosity would facilitate wider

migration to affect a larger volume of aft-soil.

The alkaline grouting fluid in contact with living roots could harm them

directly. The soil pH could be raised to cause nutrient imbalance

especially for micronutrients in the long run. Soil particles coated by

grouting materials will not be able to release nutrients into the soil

solution for root uptake. Alteration of the soil chemical environment will

suppress the activities of soil organisms, including the microbes that are

instrumental in decomposition and nutrient cycling.

37

Soil pores could be permanently occupied by solidified grouting materials,

thus reducing the internal soil spaces necessary for moisture storage,

moisture transmission, drainage, aeration and root growth. Existing tree

roots engulfed by the grouting fluid will be deprived of the means to

acquire water, nutrient and air. They will decline and may eventually die.

Installation of the soil nail head could damage surface roots and occupy

the spaces that could otherwise be used by tree roots. The multiple

deleterious impacts of grouting are irreversible, and the affected trees will

usually decline gradually over some years. Stone walls that have been

treated with grouting often lead to degeneration of the companion

stonewall trees.

(b) Stabilization treatment of contiguous wall crest and toe slopes

Stonewall trees situated at or near the wall crest often send their roots into

the slope there. Similarly, the surface roots of stonewall trees tend to send

their roots into the slope at the wall toe. The presence of unsealed soil at

the wall crest and wall toe provides a wonderful opportunity for tree roots

to explore and capture water and nutrients. The additional supplies would

greatly improve tree health and vigour, and permit the benefited trees to

attain a bigger size.

With strong roots growing in the crest slope in the backward direction, the

stonewall tree could be firmly anchored and stabilized by the tensional

pull to resist the forces of overturning and toppling. Aerial roots that can

land on the wall toe slope could lignified and develop into new root stands

or secondary trunks to provide additional support to the stonewall tree.

The conditions that permit roots to continue to thrive at the wall crest and

wall toe slopes should be maintained to sustain the health of the stonewall

trees

Some slopes at the crest and toe positions of retaining walls in recently

years were subject to stabilization treatments. The soil is often heavily

compacted to render it too dense for root growth. Existing roots could be

injured by the strong pressure exerted on the soil in the course of

compaction. The excavation associated with the slope works would

remove, cut or injure tree roots which tend to be concentrated in the upper

soil layer. The removal of the ground vegetation would reduce the

blanketing effect on soil moisture and temperature, and the related

functions of soil organism activities, organic matter decomposition and

nutrient recycling.

38

The collapse of soil pores due to heavy compaction will render the soil

unsuitable for root growth. The significant loss of macropores (> 60 μm

diameter), in particular, would deprive the soil of the key avenues for

infiltration, percolation, aeration and root growth. The degraded soil

environment would become unsuitable to the growth of new roots. The

loss of old roots can hardly be compensated by new roots, resulting in a

net reduction in the root system in terms of both size (length and surface

area) and function. In due course, the tree would lose too many roots to

trigger an irreversible decline spiral. Root decline in the crest slope is

particularly alarming, as they may provide the bulk of the critical

tensional pull to hold the hanging tree in place. The loss of anchorage

could therefore generate hazard trees that may be prone to collapse.

If the compacted soil at the crest or toe slope is sealed by shotcrete, which

is often applied, the damaged will be further aggravated. The impermeable

seal will nullify the chance for the recovery of the groundcover vegetation

and undergrowth on the treated slope. Water and air will not be able to

infiltrate into the soil, thus roots growing there will be deprived of such

pertinent supplies. The unvegetated bare shotcrete surface tends to absorb

an appreciable amount of solar heat and transmit it downwards to make

the soil environment unwelcomed to roots and soil organisms. Under such

stifling soil conditions, old roots will gradually decline whilst new roots

will find it difficult to develop. The tree decline process triggered by the

compaction treatment would be aggravated by the shotcrete sealing,

resulting in accelerated tree degradation.

(c) Soil disturbance at contiguous paved areas at wall toe

Stonewall trees often send their roots downwards along the wall face to

reach the generally paved areas at the wall toe, such as platform or

footpath. If soil is accessible at the wall toe, such as an open soil strip or

just a narrow gap with unsealed soil, the roots tend to grow into the

available soil. Such roots tend to spread under the paving to capture water

and nutrients from a larger catchment. Their development is enhanced if

the paving is permeable, such as unit pavers. This way, the stonewall trees

could be greatly benefited by acquiring additional water, nutrients and

anchorage. They are likely to grow stronger and bigger.

The conditions that allow roots to grow into such wall toe soil should be

kept to maintain the health of the stonewall trees. The possible

disturbances that may degrade, hamper, cut or kill such wall toe roots

should be avoided. The volume and quality of the soil and the access of

the soil to moisture and air should not be curtailed. The deleterious

39

impacts include filling the soil gap with concrete or cement, opening a

trench or a pit adjacent to the soil strip, or compacting the soil.

If it is feasible, measures could be adopted to improve the soil conditions

for root growth at the wall toe. If no soil strip is present, a new one could

be created by removing a strip of paving. The width of the soil strip could

be adjusted according to the footpath width and the pedestrian traffic

volume. Existing soil strips could be improved by widening and adding

soil amendments. In treating the soil, care should be taken to avoid

damaging existing roots in the strip.

(d) Soil disturbance at contiguous paved areas at wall crest

The wall crest area could be a paved platform in the grounds of a

residential, commercial, government, institutional and community

development, or a public road. Such paved areas could be disturbed due

to excavation in the course of renovation, repaving, utility trenching, and

foundation work for building redevelopment. Roots could be removed

together with the enveloping soil in the course of digging. Existing soil

volume could be occupied by new utility lines, junction boxes and other

subterranean installations. Lowering the height of an existing platform

could remove some aft-soil. The existing tree roots will be eliminated, and

the soil volume will be permanently reduced.

Removing the old paving could increase water infiltration and increase the

soil moisture content of the aft-soil. Whereas more water could be

available for tree growth, the elevated pressure of the soil water could

have implications on wall stability. Replacing the old and cracked paving

with new paving could cut off water supply to the aft-soil and dampen the

growth of stonewall trees.

(e) Skin wall impact

Building a new reinforced concrete wall in front of an old masonry

retaining wall has been adopted as one of the geotechnical methods to

stabilize it. Existing stonewall trees would need to be carefully protected

to permit their continued growth and to maintain their health and vigour.

The main concerns are the trapping of surface roots in the gap between the

skin wall and the old stone wall, and hence the lack of space for continued

growth and thickening of the roots. The moisture trapped in the gap may

stay there for too long to induce wood decay. Inserting spongy and water-

absorbing materials in the gap may not help, as the continued thickening

of roots may soon exhaust the available space. The same materials tend to

40

retain water for extended periods to facilitate the growth of wood-decay

fungi.

The skin wall would cover up the masonry wall surface to prevent aerial

roots from reaching the joints to grow into the aft-soil. The chance for

new roots to reach the aft-soil is thus deprived, which may dampen the

further development of the stonewall trees.

(f) Trenching at wall crest or wall toe platform

Trenches opened at the wall crest platform could sever the roots of

stonewall trees. Trees situated at or near the crest are more seriously

affected as more of their roots are found near the surface of the crest

platform. Similar impacts would be incurred for roots growing into the

wall crest slope. Trenching adjacent to the wall crest behind stonewall

trees could cut most roots that may provide the crucial tensional pull to

hold the trees. The mistreated trees are very likely to collapse.

Trenching at the wall toe is usually situated close to or adjacent to the wall,

because most footpaths or lanes contiguous to the wall are narrow. If the

stonewall trees have sent their roots into the soil in the ground, most roots

will be severed or seriously damaged due to excavation. The loss of water

and nutrient supply will dampen tree health.

2.3.4 Stonewall tree factor

(a) Tree species

The bona fide stonewall trees, represented by the strangler figs (cf.

Sections 2.13 and 2.14), have strong and sprawling roots to grip the wall,

and penetrate the joints to acquire water, nutrients and anchorage from the

aft-soil catchment. Other species may accidentally grow on stone walls,

but their ability to capture resources or secure a foothold is limited in

comparison with their Banyan companions.

(b) Tree dimensions

Apparently, large stonewall trees are more liable to failure. This is due to

the intuitive thinking that they are heavy, more prone to wind damage, and

may have accumulated more decay and structural problems over time. In

reality, every tree has its individual growth habit and vicissitudes due to

both intrinsic genetic and extrinsic factors. A tree can be subject to the

whims of different and changing habitat conditions, environmental

41

impacts, interactions with natural enemies and the built-up fabric, and tree

management regime. A tree regardless of size or age could remain strong

and stable. Conversely, a tree regardless of size of age could become weak

and unstable. The failure of a large tree, however, could lead to more

serious consequences on the targets.

(c) Tree biomass structure

Like a ground-growing tree, a stonewall tree can be beset by different

biomass structural problems. Such problems could predispose a tree to

failure. Most of the problems could be identified by a thorough and

professional tree risk assessment using the advanced strand of visual tree

assessment method. The main structural weaknesses include decay, canker,

cavity or crack at trunks, limbs and main branches, compression fork (also

known as v-crotch) especially those associated with included bark and

internal decay, lack of tension wood or response wood at critical positions,

and branches that are too long, heavy or low.

(d) Tree lean

Many ground-growing trees tend to lean slightly, and for them an

inclination angle exceeding 20 degree from vertical is considered as the

critical threshold. As wall trees cling on vertical habitats, many would

incline by more than this limit, which could range from < 10 to nearly 90

degree. A heavy lean does not necessarily imply instability, and the

converse is true. The ability to anchor and support the tree biomass and

freedom from structural defects are critical in the stability of stone wall

trees.

The tree tilting angle would need to be assessed in conjunction with the

nature of the anchorage, to reckon if it is adequate to hold the weight of

the tree in both static and dynamic terms. The field assessment may not

provide all the information necessary to judge, because the amount of

roots that have penetrate into the aft-soil and provide tensional pull cannot

be ascertained with a high degree of certainty. Especially for trees with a

wide and dense development of surface roots on the wall face, the amount

and size of roots that have turned backwards to enter the aft-soil cannot be

evaluated easily and accurately.

For a trunk with a heavy lean exceeding 35 degree or so, the amount,

position and configuration of the tension wood at the base and on the

upper side (opposite to the tilting direction) should be assessed in detail.

Strong and effective tension response wood of the right size and shape and

developed at the right position could help to pull the trunk or limb against

42

failure. The structural defects that could contribute to instability should be

thoroughly assessed.

(e) Root growing habit

The development of a dense network of surface roots on the wall face may

not be equated with tree stability. On the contrary, the lack of surface root

development cannot be construed as a certain sign of instability. Each

stonewall tree has to be assessed in detail on an individual basis.

The critical concern is the presence of surface roots with the ability to turn

backwards to reach the aft-soil. A sufficient number of such backward-

growing roots of sufficient size and spread, with reference to tree size and

hence loading, could provide collectively adequate pulling force to

counteract the overturning tendency. Surface roots alone could provide

support and some water-nutrient absorption capability, but without

enough backward-growing roots the anchorage may not stand the test of

exceptionally strong wind.

(f) Quality of pruning work and tree maintenance

Many stonewall trees were subject to rather unprofessional treatments in

the past when the standard of as well as the concern for tree care was not

that high. As a result, a notable proportion of stonewall trees have suffered

widely from the legacy of improper pruning. They commonly include

branch tipping and heading cuts, which would induce decay and

development of long and heavy epicormic branches at or near the decayed

cutting positions. The risk susceptibility of some stonewall trees has been

pushed to a higher level due to such erroneous treatments and their

collateral consequences of unstable and aggravated biomass structure.

The present tree managers have inherited a host of such weakened trees

beset by structural defects which are difficult to rectify.

43

3. TREES RISK ASSESSMENT AND ARBORICULTURAL

RECOMMENDATIONS

3.1 Risk assessment of the stonewall trees

3.1.1 Site and general tree conditions

The old masonry retaining wall is located on the south side of Bonham

Road opposite numbers 29 to 35, near the junction with Park Road (Photo

T0-1). The SIMAR slope registration number is 11SW-A/R577. Its

management responsibility together with the stonewall trees has been

assigned to the Highways Department (Photo T0-2).

Situated at the wall crest and running parallel to the wall is St Stephen’s

Lane. This narrow street permits the buildings at the back of the wall to be

set back to provide rooms for the stonewall trees to expand southwards

with little obstruction by buildings. Based on HyD record, the wall runs

for about 61 m long in a roughly east-west direction. Its height varies

from the taller west end at 5.23 m to 2.13 m at the east end (Photo T0-3).

The wall is believed to be built together with Bonham Road, hence it is

reckoned to be about 150 year old. It represents the first generation of

stone retaining walls in Hong Kong. As such, the masonry structure has

intrinsic historical and heritage value.

The wall is composed of irregularly sized and shaped masonry blocks that

are not coursed (Photos T03 and T04). The original design is likely to be

a dry random rubble wall with unmortared joints. The present mortar

filling all the joints was probably added at an unknown time some decades

ago. The stones come from local volcanic rocks. The stone surfaces are

hammer finished roughly with little dressing. Some masonry blocks show

clear signs of weathering. The wall face is generally dry with no sign of

long-term seepage. It is equipped with weep holes which might be added

after its construction. At the west end, a short section of the wall displays

modern features and is composed of squared granite stones that are well

coursed (Photo T0-4). It denotes a reconstructed section installed after

WWII.

The surrounding land use is dominated by residential with ground-floor

shops. The vehicular traffic along Bonham Road remains heavy most of

the day time. The solar access, air quality and wind exposure of the site

can be rated as medium in a three-point scale.

44

Six trees are found on the wall, and they are mainly dwelling at or near the

wall crest. They are all Chinese Banyans (Ficus microcarpa). Starting

from the west, they are respectively labelled as T1 to T6 in this report

(Photos T03 to T07). T3 to T6 are clustered closely together in about one-

third of the wall length at the east side. T1 is located at the far west end.

T2 stands near the middle facing Centre Street. T1 is the smallest at 7.26

m height and 30 cm DBH, whereas T2 is the largest at 16.68 m height and

146 cm aggregate DBH. Judging from the size and slow growth rate of

stonewall trees, T2 is reckoned to have dwelt on the wall for more than a

century. It is one of the few stonewall trees in Hong Kong with such a

large size, good tree form and health. As such, it could be recognized as a

living heritage of the community. T3 to T6 are young at an estimated age

of 60 years. T1 is youngest at about 15 years old.

A bus bay occupies a large proportion of the road length in front of the

wall (Photos T0-8 ad T0-1). The pavement at the wall toe is narrow except

where it widens somewhat towards the east end at the junction with Park

Road. Thus double-decker bus could get rather close to the trees. The

footpath is covered by recently installed unit pavers to permit some

infiltration of water and air into the underlying soil. A shallow U-channel

drain has been installed along two stretches of the wall toe (Photos T0-8

and T0-9).

The field survey of the six trees was conducted on 01 to 07 June 2012.

The detailed assessment results of the six trees were recorded in dedicated

forms designed for stonewall trees in Tables 1 to 6. The annotated

photographs referenced in the forms are given in the attached PowerPoint

photo album containing 199 slides.

Three trees, namely T2, T4 and T5, were recommended for microdrilling

to ascertain the condition of internal decay or cavity. This task was

conducted on 31 October 2012 by a contractor engaged by the HyD, and

the drilling results conveyed to me on 13 November 2012 has been

included in the Appendix. The positions of the 10 drills for T2 and 5 drills

each for T4 and T5 are shown respectively in Appendix Figures T2-P0,

T4-P0 and T5-P0. Information on the corresponding photographs

indicating the proposed drilling positions are summarized in Table 7. This

report has subsequently been updated in the light of the microdrilling

findings.

45

3.1.2 Assessment of T1 and arboricultural recommendations

This is a young tree that dwells at the crest of the wall and hangs well

above Bonham Road (Photo T1-1). The compact and rounded crown has

a limited span of 8 to 9 m (Photos T1-1 and T1-2). It keeps most of its

leaves and branches, with no sign of dieback (Photo T1-3). The live

crown ratio exceeds 70%, and the tree vigour is rated as good. The leaves

are of normal density, size and colour. The small tree leans at about 15

degree towards Bonham Road, and it does not display symptoms

concerning instability. The tree does not suffer from notable trunk, crotch

or main branch defects.

T1 rests on a renovated section of the retaining wall which is composed

of squared granite stones and equipped with two concrete beams (Photo

T1-4). It is likely to be constructed after WWII. The masonry blocks at

and around the trunk base has remained intact. At the time of field survey,

part of the tree was covered by the scaffolding and protective nylon net of

the adjacent building site which was under renovation (Photos T1-4 and

T1-5). Its large neighbour T2 shields it on the east side (Photo T1-6).

A short single trunk of 1.46 m height and 30 cm diameter supports three

limbs at more or less that same height, denoting the crowded branching

phenomenon (Photo T1-7). The crotches between the three limbs are of

the stronger U-type rather than the inherently weak V-type. Measures

could be taken to remove the use of the tree as support for construction-

renovation purpose (Photo T1-8). Few surface roots have spread on the

wall face.

Tree hazard assessment has a score of 5 which is rated as low (Table 1G).

Minor trimming of a crowded upright and closely spaced competing

branch and a nearly horizontal and relatively long branch are

recommended in Table 1H and explained in Table 1I. The stone wall of

modern design has closely packed masonry blocks and well mortared

joints that could effectively ward off penetration by tree roots to reach the

aft-soil. In the longer term, the lack of access to the soil catchment behind

the wall to obtain new sources of water, nutrients and anchorage may

suppress its growth rate and reduce its ability to reinforce its foothold.

46


(a) Overall tree structure and condition

(i) Tree-wall relationship and tree dimensions

This is one of the most outstanding stonewall trees in Hong Kong by

virtue of its height, crown spread, trunk diameter, tree form, health and

vigour (Photo T2-1). It is situated in a prominent location that can be

appreciated from different angles by many residents and road users. The

dense and sprawling tree stands well above the carriageway of Bonham

Road with heavy vehicular and pedestrian traffic during the day time

(Photo T2-2). The large tree measures 16.68 m tall, and its trunk base is

elevated at 3.84 m above the road level.

(ii) Tree quality and OVT designation

The meritorious specimen tree has fine qualities that deserve special

attention. It has been designated under the government's Old and

Valuable Tree (OVT) Register. It should be kept in the Register. A well-

designed and durable information plaque could be installed at St

Stephen's Lane which offers a convenient and safe location to appreciate

the magnificent stonewall tree. The pavement on the opposite side of

Bonham Road would offer a second suitable location to put up a plaque.

(iii) Tree anchorage at wall crest

The tree growth was initiated near the wall crest with the trunk base

leaning outwards at 32 degree, but assuming a more upright posture

higher up from 9 to 13 degree (Photo T2-3). Besides hanging above

Bonham Road, the south side hangs above St Stephen’s Lane to get close

to the buildings there (Photos T2-4 and T2-5). From the top of the Centre

Street, the sprawling 27 m east-west spread of the crown can be fully

appreciated. In the north-south direction, the crown spread is somewhat

narrower but it still spans about 22 m (Photo T2-6).

(iv) Multiple and codominant trunks

Three stout codominant trunks collectively support the huge biomass

(Photos T2-7, T2-8 and T2-9). Their trunk diameters (DBH) measure

respective at 115 cm for trunk A, 51 cm for B and 74 cm for C, with an

aggregate of 146 cm. The thickest trunk A is split at a low level into two

strong limbs (A1 and A2).

47

(v) Tree hazard rating


The main contribution to tree risk is the breakage of branches that are

structurally weak or decayed.

(b) Assessment of surface roots and interface with the stone wall

(i) Limited spread of roots on stone wall face

The spread of surface roots on the wall face is relatively limited in

comparison with the huge tree size (Photo T2-10). Most roots that have

ventured into the aft-soil are likely to penetrate the joints at and around

the trunk base. Some surface roots have descended to the wall toe to

enter the soil below the pavement (Photo T2-11). However, the paving

and cement filling of the wall toe gap have restricted this mode of root

development. Besides obstructing the growth of existing roots at the wall

toe, it will be quite impossible for new roots to enter the soil below the

pavement.

(ii) Tree anchorage and wall bulge

The large tree has established a firm hold on the wall by sending its roots

into the aft-soil near the crest (Photo T2-12). The trunk base points

backwards towards the wall crest, and the trunks curve upwards (Photos

T2-13 and T2-14). At the east side of the trunk base, the wall face shows

a small and slight bulge (Photo T2-15), which could be related to root

growth behind the stones or to the pressure exerted by the retained soil on

the stones. This symptom needs to be very closely monitored by a

geotechnical engineer especially to assess whether the bulge will continue

to push outwards and enlarge.

(iii) East side surface roots and masonry

Root spread beyond the vicinity of the trunk base has been limited by the

sealing of joints by cement (Photo T2-15). A few masonry blocks on the

east side of the trunk base has been displaced mainly outwards away from

the wall face (Photo T2-16). The presence of two small holes at the same

location adjacent to a thick surface root implies the dislodgement of

masonry blocks in the past. The upper hole has been filled with cement,

whereas the lower hole remains vacant (Photo T2-16).

48

(iv) West side surface roots and masonry

Root spread on the west side beyond the trunk base has also been

restricted due to the rather thorough sealing of joints by cement

displaying the buttering phenomenon (Photo T2-17). Adjacent to the

trunk base on the west side, two masonry blocks are protruding from the

wall face indicating perpendicular displacement (Photo T2-18). Further

away from the trunk base on its west side, the upper one-third of the wall

displays an uneven surface, which may indicate the poor workmanship or

slight stone displacement by root growth behind the stone façade (Photo

T2-19).

(v) Boulder size and shape

It is notable that the boulders in the lower two-third of the wall are larger,

whereas those in the upper one-third are smaller (Photo T2-10). This

vertical variation in stone size implies that the upper part of the wall has

more gaps between stones to permit penetration by tree roots to enter the

aft-soil (cf. Section 2.3.1a). The smaller stones near the wall top are less

constrained by the confining pressure of interlocking masonry blocks.

Thus they are more amenable to be displaced by roots. This wall-tree

interaction favours trees but is detrimental to wall integrity. If the

exploitation by tree roots is not excessively, it could be tolerated.

(vi) Removing rubbish from surface roots

Measures can be implemented to improve the root growth conditions of

the tree. The rubbish that has deposited on the surface roots and the trunk

bases and crotches, together with the leaf litter, should be regularly

removed with the help of a brush to avoid moisture accumulation which

could induce decay (Photo T2-14). Thorough removal of rubbish in the

gap is also essential to avoid the wedging effect (cf. Section 4.2.3).

(vii) Removing cement sealing at masonry joints

The limitations to root growth due to the lack of open and penetrable

joints between masonry blocks, and the lack of accessible soil at the wall

toe and crest positions, could be ameliorated to enhance tree growth and

performance (Photos T2-10, T2-17, T2-19). The joints between masonry

blocks have been sealed recently by cement, thus stopping their

penetration by tree roots. The rigid cement seal also restricts expansion

of existing roots and may cause girdling injury as they continue to thicken.

Where the roots are physically obstructed, consideration could be given to

49

localized removal of the cement seal to permit some new roots to grow

into the joints and existing roots to expand (cf. Section 4.2.2).

(viii) Installing soil strip at wall toe and crest

An open soil strip filled with a good-quality soil mix could be installed at

the wall toe at Bonham Road to allow growth of roots in the soil below

the pavement (Photo T2-11). Similarly, an open soil strip filled with a

good-quality soil mix could be installed at the wall crest to allow growth

of roots in the soil below the pavement at St Stephen's Lane (Photo T2-5)

(cf. Section 4.2.1).

(c) Assessment of tree crown

(i) Crown spread and condition

The crown spread in the east-west direction is broad and rather

symmetrical and balanced (Photo T2-20). The branching and foliage

densities are reckoned as high and typical for a healthy Chinese Banyan

(Photo T2-21). At different sides of the crown, these indicators of tree

health and vigour are well expressed (Photos T2-22, T2-23 and T2-24).

At the ends of the branches, the twigs and leaves do not show signs of

dieback (Photo T2-25). The foliage density, leaf size and leaf colour fall

into the normal category. The crown is beset by some tipped branches.

Overall tree vigour is rated as good.

(ii) Excessive removal of low branches

The live crown ratio of the tree stands slightly above 70%, and the twig

dieback can be rated as low at < 5% (Photo T2-26). However, the tree has

suffered from excessive removal of lower branches (Photo T2-20). This

undesirable practice could reduce the ability of the tree to develop proper

trunk taper to compromise its stability (cf. Section 4.2.4). The reduction

in taper, meaning less wood development in the lower part of the trunks,

could jeopardize the tree’s ability to support the bulky crown load. As the

tree adds more weight to its crown, the inadequate corresponding

thickening of the trunk base could lead to tree failure. The tree care

practice henceforth will need to be revamped to help the tree to recover

from this improper treatment. The preferential and repeated removal of

lower branches must forthwith be stopped. In the future, the emergence

of new branches in the lower parts of the trunk with the potential to

develop into sizeable and safe branches should be allowed to grow.

50

(iii) Crown conflict with nearby buildings

The crown arches above Bonham Road and extends towards the

residential building on the opposite side (Photos T2-27, T2-28 and T2-29).

If the branches get too close to windows, it could block too much natural

light. The swinging of branches in strong wind could hit the windows and

create a nuisance or a hazard. Such proximal branches could be shortened

by the proper reduction cut.

(d) Assessment of trunks and branches

(i) Overall condition

A detailed assessment of the tree’s scaffold can yield telltale signs about

the integrity and stability of the biomass structure. The evaluation could

follow a logical sequence, beginning with the large members, namely the

trunks and limbs, and then move on to the main and smaller branches.

The tree has not had a thorough and high-calibre crown cleaning for some

years, and it has accumulated a fair amount of decayed and structurally

weak or defective branches. It calls for a professional and comprehensive

treatment to rectify the arboricultural problems. The following notable

issues deserve attention and follow-up actions. Details about the full

range of recommended arboricultural treatments and their explanations

have been given in Table 2 Parts H and I.

(ii) Photo T2-30: Tree scaffold

Stems A1 and C are found to be constituted by substantially expanded

and elongated epicormic branches (Photo T2-30). The contact between

the parent stem and the daughter should be monitored to see whether the

decay has set in to compromise the strength of the inherently weak

attachment. Close visual inspection assisted by a wooden mallet should

ascertain the wood condition at the critical junctures.

(iii) Photo T2-31: Stem A1 with large decayed wound (microdrill points 1

to 3)

Stem A1 has a large wound with decay and cavity development, with

notable bulgewood formation (Photo T2-31). The wood condition inside

the wound and shortly above and below it needs to be evaluated with the

help of microdrilling. The positions and directions of recommended

drillings are annotated in the photograph. If the recommended

microdrilling finds that the decay at the wound has spread to occupy more

51

than one-third of the nominal trunk diameter, the branch loads of the

affected stems have to be reduced correspondingly.

The microdrilling results of stem A1 at points 1, 2 and 3 explore the

internal decay condition at the large decayed wound (Appendix Figure

T2-P1, T2-P2 and T2-P3; Photo T2-31). At point 1 which is situated

above the large wound, a small cavity is found near the centre with a

diameter of about 4 cm (denoted by the red zone in Appendix Figure T2-

P1). This is likely to be an extension of the decay column from the main

decay locus at the large wound. From the wall thickness (t) and stem

radius (r) as shown in Graph T2-P1, a t/r ratio can be calculated to

estimate the residual strength of the stem:

First wall thickness, t1 = 13 cm

Second wall thickness, t2 = 17 cm

Average wall thickness, t = (13+17)/2 = 15 cm

Stem radius, r = 17 cm

t/r ratio = 15/17 = 0.88

The t/r ratio at point 1 is notably higher than the critical t/r ratio of 0.33,

suggesting that the wood shell around the internal cavity should have

sufficient wood thickness to hold the stem. The cavity is internal and it

does not have an opening, hence the strength of the wood shell has not

been compromised. Furthermore, the wood around the cavity shows

limited penetration of advanced decay (denoted by the yellow zone in

Appendix Figure T2-P1), suggesting that the fungal infection at this

location has been compartmentalized. The steep angle of the graph on the

two edges of the cavity signifies that wood strength has not been notably

reduced at a short distance away from the cavity. In other words, the

impact of wood decay has been largely contained within the confine of

the cavity. It should be noted that around point 1 the loss of wood

strength due to cavity formation has not triggered compensatory

development of notable response wood. However, the lignified aerial

roots that have grafted with the stem could provide some reinforcement.

At point 2 which drills through the large open wound (Photo T2-31),

Appendix Figure T2-P2 shows the changes in wood strength along the

drill path. The stem diameter has been thickened due to the development

of response wood creating a bulgewood symptom, which compensates for

the loss of strength due to wood decay. Three pockets of rather advanced

decay have been detected by microdrilling, namely centred at 16.4 cm,

25.5 cm and 29.0 cm positions on the graph, indicating more decay on the

lower half of the stem. The decay pattern suggests the development of

52

multiple decay columns. The first decay column is likely to be connected

to the cavity detected at point 1. The development of response wood

around the wound has provided compensation for the loss of strength due

to decay.

At point 3, the wood quality as shown by the microdrilling trace

(Appendix Figure T2-P3) is generally good with limited decay. No

advanced decay pocket or cavity has been found.

The overall interpretation of microdrilling results of stem A1 at and

proximal to the large decayed wound (Photo T2-31) is that there is

sufficient sound wood including response wood to support the stem

weight. No drastic pruning or stem removal is necessary. As a

precautionary measure, about 10% of the end load should be trimmed.

The pruning should selectively remove the thinner and weaker branches

with a view to leaving an even spread of remaining branches. The decay

may continue to deteriorate to a more advanced stage and may spread to

affect more wood. This position of weakness needs to be monitored

continually in future inspections.

(iv) Photo T2-32: Stem A1 with decayed wound and possible termite

attack

An open and irregular wound on the south side of stem A1 has been

formed due to breakage loss of a branch some months ago (Photo T2-32).

The suspected termite attack should be checked to see if it is still active.

If so, apply effective termite extermination using the hormone bait

method at the earliest opportunity. The wound with jagged edges and an

uneven surface should be very carefully trimmed using a sharp arborist

manual saw with the help of a carpenter’s chisel. As far as possible, well-

formed callus tissues should be left undisturbed.

(v) Photo T2-33: Stem A1 with tipped end and epicormic cluster

A tipped branch of medium size of stem A1 has developed a cluster of

sprouts at or near the wound to serve as replacement branches and

compensatory photosynthetic tissues (Photo T2-33). Detailed visual

inspection at close quarters with the help of a wooden mallet should

clarify the condition and extent of decay at the tipped end of the parent

branch. The cluster of sprouts attached to the decayed tip should be

reduced in proportion to the amount of remaining sound wood. The load

reduction could be achieved by selective trimming of relatively weak and

thin sprouts, leaving an even spacing of stronger sprouts.

53

(vi) Photo T2-34: Stem A1 with crooked section

A branch of stem A1 has a somewhat crooked section (Photo T2-34). It

should be inspected visually at close quarters and with the help of a

wooden mallet to see if it is suffering from mechanical defects such as

cracking or splitting. If so, the weight of the branch should be

correspondingly reduced.

(vii) Photo T2-35: Stem A1 with heavy epicormic branch at tipped end

A thick epicormic branch of stem A1 is curving upwards from an old

branch tipping wound (Photo T2-35). Inspecting the wound at close

quarters should verify the condition and extent of decay at the tipped end.

The weight of the branch should be proportionally trimmed according to

the amount of remaining sound wood. The rubbish that hangs on the

branches should be carefully extricated and removed in the course of

pruning with the help of a hydraulic platform.

(viii) Photo T2-36: Stem A2 with truncated branch and decay

A truncated branch wound on stem A2 has been invaded by wood-decay

fungi (Photo T2-36). The jagged edges of the wound and its uneven

surface should be very carefully trimmed using a sharp arborist manual

saw and with the help of a carpenter’s chisel. As far as possible, well-

formed callus tissues should be left undisturbed.

(ix) Photo T2-37: Stem B with decayed tipped end and epicormic cluster

A tipped and decayed branch of stem B has developed multiple sprouts at

and near the wound to serve as replacement branches and compensatory

photosynthetic tissues. (Photo T2-37). Detailed visual inspection at close

quarters with the help of a wooden mallet should confirm the condition

and extent of decay at the tipped end of the parent branch, and its

influence on wood strength. The cluster of sprouts attached to the decayed

tip should be reduced in proportion to the amount of remaining sound

wood. The load reduction could be achieved by selective trimming of

relatively weak and thin sprouts, leaving an even spacing of stronger

sprouts.

(x) Photo T2-38: Stem B with fractured and decayed branch end

A fractured and decayed branch of stem B is supporting an expanded

epicormic branch (Photo T2-38). Detailed visual inspection at close

quarters with the help of a wooden mallet should clarify the condition and

54

extent of decay at the fractured end of the parent branch. The results of

inspection should inform whether the daughter epicormic branch should

be shortened to reduce its weight. The jagged edges of the wound should

be carefully trimmed using a sharp arborist manual saw. Concerning the

three parallel branches, trimming of the above epicormic branch should

reduce the competition.

(xi) Photo T2-39: Stem B with crowded branches

On stem B, a crowded branching problem is compounded by two parallel

branches (Photo T2-39). The sum of the diameters of the three daughter

branches is notably larger than that of the parent branch, imposing a

heavy burden on it. The problem could be partly relieved by removing

one of the two parallel branches.

(xii) Photo T2-40: Stem B with curved and kinked branches

At the tipped end the long, upward curving but relatively slender branch,

an epicormic branch has emerged with an elbow joint (Photo T2-40). As

such a structure is potentially hazardous. As it carries a limited amount of

foliage, the parent together with its daughter branch is recommended for

removal.

(xiii) Photo T2-41: Stem B branch with elbow-joint at tipped end

On stem B, a tipped branch end has developed an epicormic branch

attached to its parent with an elbow joint (Photo T2-41). Evaluation at

close quarters should inform whether the decayed tip has sufficient sound

wood to hold the epicormic branch. If not, the daughter branch should be

reduced according to the amount of sound wood at the junction.

(xiv) Photo T2-42: Stem B branch with decay, cavity and bulgewood

The open wound on a branch of stem B has developed decay and cavity,

and the resulting loss of mechanical strength has induced response wood

formation in the form of bulgewood (Photo T2-42). The extent of decay

should be investigated at close quarters. The load on the defective branch

should be reduced in proportion to the amount of remaining sound wood.

(xv) Photo T2-43: Stem B branch with crowded and decayed sprouts

A branch of stem B has formed a cluster of crowded sprouts at its end

with some decayed branchlets (Photo T2-43). The dead branchlets should

55

be removed to prevent the spread of wood-decay fungi to adjacent healthy

wood, and to partly relieve the sprout crowding problem.

(xvi) Photo T2-44: Stem A branch with tipped end and no sprout

Stem A has a tipped medium-sized branch leaving a wound with no

sprout development (Photo T2-44). Whether the branch is dead or not

could be verified. If so, it should be removed to prevent the spread of

wood-decay fungi to adjacent healthy wood.

(xvii) Photo T2-45: Tipped branches with sprout clusters above Bonham

Road

Some branches hanging above Bonham Road and situated near the

buildings on the opposite side have been tipped with sprout clusters

emerging from the wounds (Photo T2-45). They should be trimmed by

removing the relatively weaker and thinner sprouts and leaving an even

spread of stronger branches.

(xviii) Photo T2-46: Crown extension towards buildings at St Stephen’s

Lane

The south part of the crown has extended towards the buildings at St

Stephen’s Lane (Photo T2-46). To resolve this conflict, the branches that

get too close to windows should be trimmed to provide a horizontal

clearance of 2 m. Only the standard reduction cut down to a notable fork

should be applied in shortening branches. The heading cut is forbidden.

A light-weight telescopic pruning pole could be used to prune the target

proximal branches from the nearest windows of the affected residential

flats. This way, the need to block the vehicular traffic to conduct pruning

from an elevated platform could be avoided.

(xix) Photo T2-47 and T2-48:Tipped branch above St Stephen’s lane with

epicormic branches

Some branches above St Stephen’s Lane near buildings have been tipped

with sprouts emerging from the wounds (Photo T2-47 and T2-48). If the

inspection at close quarters finds that the truncated end of the branch has

developed decay and structural defects, the attached epicormic branches

should be trimmed to reduce the burden on the parent stem.

56

(xx) Photo T2-49: Tipped branch above St Stephen’s lane with multiple

and crowded epicormic branches

A tipped branch above St Stephen’s Lane has developed multiple and

crowded sprouts from the wound (Photo T2-49). They should be trimmed

by removing the relatively weaker and thinner sprouts and leaving an

even spread of stronger branches.

(xxi) Photo T2-50: Crown extension above building podium at St

Stephen’s Lane

The south part of the crown has extended above the building podium at St

Stephen’s Lane (Photo T2-50). To resolve this building-tree conflict, the

branches that get too close to windows should be trimmed to provide a

horizontal clearance of 2 m. Only the standard reduction cut down to a

notable fork should be applied in shortening branches. The heading cut is

forbidden. A light-weight telescopic pruning pole could be used to prune

the target proximal branches from the nearest windows of the affected

residential flats. This way, the need to block the vehicular traffic to

conduct pruning from an elevated platform could be avoided.

(xxii) Photo T2-51: Stem B with large truncated and decayed limb

Stem B has a large truncated limb developing decay at the wound (Photo

T2-51). The stubs with decay should be removed using the standard

removal cut at the fork with the parent stem.

(xxiii) Photo T2-52: Truncated limb of Stem B with decay and bulgewood

The base of the truncated limb has a decayed and depressed wound with

bulgewood formation and a wasp nest (Photo T2-52). The recommended

inspection at close quarters should check the condition of the wound. If it

has developed advanced decay and structural defect that may compromise

its mechanical strength, to such an extent that it may not support its own

weight, the relatively long and stout truncated limb should be removed by

the proper removal cut at its fork with its parent stem.

(xxiv) Photo T2-53: Stem B with seam and bulgewood (microdrill point 4)

Stem B has a seam with bulgewood formation indicating internal decay

(Photo T2-53). The interior wood condition should be ascertained by

microdrilling, with the position and direction of drilling indicated on the

photograph. If the microdrilling finds that the wound has developed

advanced decay and structural defects that may compromise its

57

mechanical strength to such an extent that it may not support its load, the

weight of the branches supported by the stem should be trimmed. The

amount to be removed should be contingent upon the condition and

quantity of the remaining sound wood.

The microdrilling graph (Appendix Figure T2-P4) indicates that the drill

bit stop at 40 cm without exiting the stem, because it was too short to

penetrate the entire stem diameter. A microdrill with a longer drill bit or

an additional drilling entering the stem from the opposite direction could

have overcome this inherent instrumental constraint. Thus the results

provided by the contractor show only a partial picture of the internal

wood condition at this site of potential weakness. Based on the detected

portion of the stem, weakened wood due to decay is found from 16 cm to

21.5 cm. There is enough sound wood on the two sides to provide support.

The decay may continue to deteriorate to a more advanced stage and may

spread to affect more wood. This position of weakness needs to be

monitored continually in future inspections.

(xxv) Photo T2-54: Stem B with limb-loss wound and decay

At the lower part of stems B, a limb-loss wound is beset by decay and has

developed some sprouts (Photo T2-54). As callus tissues have formed

around the edge, the wound should be monitored for signs of continued

decay.

(xxvi) Photo T2-55: Stem B branch with open decayed wound (microdrill

point 5)

A decayed wound with incomplete callus formation is found on a wavy

branch of stem B. The internal condition of wood should be explored

using microdrilling with the arrow denoting the drilling position and

orientation (Photo T2-55). The load on the defective branch should be

reduced in proportion to the amount of remaining sound wood.

The drill bit went through the branch at point 5 (Appendix Figure T2-P5),

indicating a stem diameter of about 21.6 cm. No notable pocket of

advanced decay, cavity or crack has been detected along the drill path.

The decay at the open wound has not penetrated into the interior of the

stem. The results do not call for drastic pruning or end-load reduction

treatment.

58

(xxvii) Photo T2-56: Stem A1 with decayed stub (microdrill point 6)

Stem A1 has a decayed stub with bulgewood formation and wasp nest

(Photo T2-56). The interior wood condition should be ascertained by

microdrilling, with the position and direction of drilling indicated on the

photograph. If the microdrilling finds that the wound has developed

advanced decay and structural defect that may compromise its mechanical

strength to such an extent that it may not support its load, the weight of

the branches supported by the stem should be trimmed. The amount to be

removed should be contingent upon the condition and quantity of the

remaining sound wood. The stub-end wound with jagged edges or an

uneven surface should be very carefully trimmed using a sharp arborist

manual saw and with the help of a carpenter's chisel.

The drilling results are presented in Appendix Figure T2-P6. The wood

from about 9 cm to 17.5 cm has been degraded by decay which is likely

to have spread from the decayed stub shown in Photo T2-56. The decay

has not reached the advanced stage at this juncture, and together with the

development of response wood as expressed by the bulgewood on both

sides of the wound, there should be enough wound wood to hold the stem.

It is possible that the decay may progress in due course to the more

advanced stage and spread to a larger proportion of the stem. This

potential weakness should be continually monitored both visually and

with the help of instruments.

(xxviii) Photo T2-57: Stem A1 with depressed and decayed wounds and

bulgewood (microdrill points 7 and 8)

The south side of stem A1 has two decayed and depressed wounds with

wasp nests (Photo T2-57). The internal condition of the wood should be

ascertained by two microdrillings, with the positions and directions of

drillings annotated on the photograph. If the microdrilling finds that the

wound has developed advanced decay and structural defect that may

compromise its mechanical strength to such an extent that it may not

support its load, the weight of the branches supported by the stem should

be trimmed. The amount to be removed should be contingent upon the

condition and quantity of the remaining sound wood.

The microdrilling trace shown in Appendix Figure T2-P7 has not drilled

through the stem. It stopped inexplicably at 24 cm whereas the stem

diameter should be around 40 cm. It is possible that the drilling

erroneously followed an off-centre path, hence a shorter trace was

recorded. Thus the drilling graph has detected only a part of the stem

interior condition. Based on the part that has been drilled, the wood

59

condition does not demonstrate advanced decay, cavity or other structural

weakness, hence drastic pruning to reduce loading on the stem is not

warranted.

The microdrilling trace shown in Appendix Figure T2-P8 has gone

through the stem. It indicates two pockets of decay which have a limited

extent each of about 2 cm diameter. The decay at the open wound has

penetrated into the interior part of the stem, although the spread at the

present stage is limited. There is enough sound wood to support the stem.

As a precautionary measure, about 10% of the end load should be

trimmed. The pruning should selectively remove the thinner and weaker

branches with a view to leaving an even spread of remaining branches.

The weak point should be continually monitored to check whether it will

further aggravate to a more risky stage.

(xxix) Photo T2-58: Stem C large decayed wound of truncated limb

A large wound with decay is found at the lower part of stem C, left by the

truncation of a thick limb (Photo T2-58). The wound should be monitored

for signs of continued decay.

(xxx) Photo T2-59: Stem C with decayed and recessed wound

The lower part of stem C has a decayed wound in a recessed niche (Photo

T2-59). It could be very carefully trimmed with the help of a carpenter's

chisel.

(xxxi) Photo T2-60: Stem C with trapped rubbish

A piece of plastic rubbish is partly engulfed and trapped by wood growth

in Stem C (Photo T2-60). It should be pulled out manually, if it is

possible to do so. Refrain from using tools or excessive force in the

extraction process.

(xxxii) Photo T2-61: Stem C with cracks, decayed wound and crotch

(microdrill points 9 and 10)

The middle section of Stem C has developed cracks, seam, bulgewood,

decayed wound, and a decayed crotch (Photo T2-61). The adjacent branch

removal wound has developed decay. The internal condition of the wood

should be ascertained by two microdrillings, with the positions and

directions of drillings annotated on the photograph. The load on the

defective branch should be reduced in proportion to the amount of

remaining sound wood and the condition of the structural defect.

60

Microdrilling at point 9 (Appendix Figure T2-P9) shows a cavity of about

4.5 cm diameter. The t/r ratio at the thinner side of the wood shell is

11/17.5 = 0.63 which is above the critical threshold of 0.33. The almost

vertical edges on both sides of the trough imply that the “cavity” could be

a gap between the stem and a lignified aerial root. At point 10, drilling

was made through the crotch with surficial decay symptoms (Appendix

Figure T2-P10). The wood from 0 cm to 6 cm has been partly degraded

by decay. The remaining wood is generally sound and has the strength

and thickness to hold the stem. Overall, the stem does not require drastic

load reduction.

(xxxiii) Photo T2-62: Stem C branch with bulgewood

The bulgewood developed in a branch of Stem C indicates possible

internal decay (Photo T2-62). The load on the defective branch should be

reduced in proportion to the amount of remaining sound wood and the

condition of the structural defect.

(xxxiv) Photo T2-63: Stem C branch stub with decay

The decay of the stub on a branch of stem C may extend into the parent

branch (Photo T2-63). The stub should be removed using the standard


(xxxv) Photo T2-64: Stem C tipped branch with decay spread

The tipped branch of stem C is beset by the spread of decay from the

wound and dead sprouts (Photo T2-64). The dead branch should be

removed using the standard removal cut at the fork with the parent stem.

(xxxvi) Photo T2-65: Stem C branch with crack at crotch

A branch of stem C has developed a crack at the critical crotch position

(Photo T2-65). The crack should be assessed at close quarters to find out

its length and depth and whether internal decay has developed. The result

will determine the amount of wood to be removed from the affected

branch. In the extreme case, the entire branch should be removed along

the dotted line.

61



(i) Tree-wall relationship

T3 is one of the four stonewall trees dwelling on the eastern one-third of

the subject wall (Photo T3-1). It is situated near the staircase linking

Bonham Road to St Stephen’s Lane. Due to close spacing amongst the

row of trees, their crown spread tends to be laterally restricted by mutual

interference (Photo T3-2). The semi-mature tree perches on the crest of

the wall and hangs above the bus stop at Bonham Road (Photo T3-3). It

leans towards Bonham Road by about 40 degree at the base, but assumes a

more upright posture in the remainder of the trunk which tilts at 8 degree

from the vertical.

(ii) Tree dimensions and biomass structure

The tree measures 14.77 m tall. Some large branches have been tipped

(Photo T3-4). Three rather thick trunks with DBH respectively at A=30

cm, B=56 cm and C=74 cm support the confined and reduced crown

(Photos T3-5 and T3-6). The aggregate DBH reaches 100 cm. Trunk A

that tilts heavily towards T4 at its east side has been truncated, and it does

not carry any branches. Trunk C has been truncated at a low level and

carries no branches. It is connected to trunk B via a bundle of lignified

aerial roots. Trunk C’s main residual function is lending support to the

limbs and branches of trunk B.

(iii) Trunk B and low branch removal

Trunk B is the only one that has been left largely intact (Photo T3-5). The

tree’s crown is built mainly by branches attached to trunk B. However, a

notable number of limbs and branches formerly attached to trunk B have

been lost. The tree suffers from excessive removal or loss of its lower

branches. This defect could suppress the tree’s ability to develop a proper

trunk taper to compromise its mechanical strength at the critical trunk

base level (cf. Section 4.2.4).

(iv) Tree hazard rating


The main contribution to tree risk is the breakage of branches that are

structurally weak or decayed.

62


(i) Wall structure and surface root spread

The stone wall at T3 stands 3.27 m above the ground. Similar to the

condition at T2, the masonry blocks are larger in the lower two-third of the

wall, and smaller in the top one-third (Photo T3-7). Root spread on the

wall face is somewhat limited laterally and especially vertically. The

surface roots are confined mainly to the upper half of the wall. The more

or less straight and horizontal lower edge of the surface root mass indicates

that the lower half was cut away. Some roots on the east side have

intertwined with those of its neighbour T4, with some grafting of the two

root systems.

(ii) Root growth at wall toe and trunk base

A U-channel with concrete covers is present at the wall base (Photo T3-7).

The excavation associated with its installation would have removed some

of the surface roots at the lower half of the wall and roots that have grown

into the soil under the pavement. The trunk base is sitting near the wall top

(Photos T3-8, T3-9 and T3-10). Most of the tension roots that hold the tree

against overturning and take water and nutrients from the aft-soil are

believed to be situated underneath and in the vicinity of the trunk base.

(iii) Shift of masonry block

Just above the trunk base, a masonry block has been pushed away from the

wall face (Photo T3-10). At the wall top, the interlocking force that applies

a strong force against displacement is relatively weak. The shifting of one

piece of stone probably indicates the localized pressure exerted by root

growth.

(iv) Removing rubbish from surface roots

Measures can be implemented to improve the root growth conditions of

the tree. The rubbish that has deposited on the surface roots and the trunk

bases and crotches, together with the leaf litter, should be regularly

removed with the help of a brush to avoid moisture accumulation which

could induce decay (Photo T3-10). More importantly, keeping the gap

clean can avoid the potentially risky wedging effect which may destabilize

the tree (cf. Section 4.2.3).

63

(v) Removing cement sealing at masonry joints

The limitations to root growth due to the lack of open and penetrable joints

between masonry blocks, and the lack of accessible soil at the wall toe and

crest positions, could be ameliorated to enhance tree growth and

performance (Photo T3-7). The joints between masonry blocks have been

sealed recently by cement, thus stopping their penetration by tree roots.

The rigid cement seal also restricts expansion of existing roots and may

cause injurious girdling as they continue to thicken. Where the roots are

physically obstructed, consideration could be given to localized removal of

the cement seal to permit some new roots to grow into the joints and

existing roots to expand (cf. Section 4.2.2).

(vi) Installing soil strip at wall toe and crest


the wall toe at Bonham Road to allow growth of roots in the soil below the

pavement (Photo T3-7). Similarly, an open soil strip filled with a good-

quality soil mix could be installed at the wall crest to allow growth of roots

in the soil below the pavement at St Stephen's Lane (Photo T3-17) (cf.

Section 4.2.1).


Despite the somewhat small crown and a low live crown ratio of 40-70%,

the foliage density appears normal for the species (Photo T3-11). The leaf

size and colour also follow the norm. The peripheral branchlets and twigs

of the crown do not show symptoms of dieback (Photo T3-12). The crown

development is somewhat limited with reference to its height and

aggregate trunk diameter. The overall crown shape denotes reduction due

to a combination of confinement and branch losses (Photos T3-2, T3-3 and

T3-4). The tree has lost an appreciable amount of branches and foliage,

and suffers from excessive loss of lower branches that can incur instability

problems (cf. Section 4.2.4).


(i) Photos T3-13 and T3-14: Removing Stem A

The remnant part of Stem A, which was snapped in the past, has lost all its

natural branches and is harbouring only a small number of feeble sprouts

mainly at its broken end (Photos T3-13 and T3-14). The tip has an uneven

surface and has developed rather advanced decay. The trunk tilts heavily

towards the east to approach its neighbour T4. Not playing a functional

64

role in the overall scheme of the tree, trunk A is recommended for removal

by cutting at its base, taking care to minimize injury of adjacent trunk B

and associated lignified aerial roots.

(ii) Photos T3-15: Trimming Stem B limb-removal wound

The wounds left by limb removal from stem B have developed decay and

cavity (Photo T3-15). Their jagged edges or uneven surfaces or decayed

wood should be very carefully trimmed using a sharp arborist manual saw

with the help of a carpenter’s chisel. However, the well-formed callus

tissues should as far as practicable be left undisturbed.

(iii) Photos T3-16: Upward curving limb of Stem B

A low, upward-curving and truncated limb of stem B supports two long

ascending epicormic branches (Photo T3-16). For the one on the right-

hand side, evaluation of the cut wound at close quarters should inform

whether the decayed tip of the parent branch has sufficient sound wood to

hold the epicormic branch. If not, the daughter branch should be shortened

by a reduction cut. The amount to be removed shall correspond to the

condition of the wood around the cut face.

(iii) Photo T3-17: Long epicormic branch with elbow joint of Stem B

A long epicormic branch emerges with an elbow joint from a branch-cut

wound (Photo T3-17). Evaluation of the cut face at close quarters should

inform whether the decayed tip of the parent branch has sufficient sound

wood to hold the epicormic branch. If not, the daughter branch should be

shortened by a reduction cut. The amount to be removed shall correspond

to the condition of the wood around the cut face.

(iv) Photo T3-18:Tipped branch with long epicormic branch of Stem B

A top branch of stem B was tipped and has developed a series of long and

nearly parallel replacement epicormic branches (Photo T3-18). The

crowded branching habit could be partly relieved by removing one of the

three parallel branches. Evaluation a close quarters should inform whether

the decayed tip of the parent branch has sufficient sound wood to hold the

epicormic branches. If not, the daughter branches should be shortened by

reduction cut. The amount to be removed shall correspond to the condition

of the wood around the cut face.

65

(v) Photo T3-19: Decayed stubs on Stem B

Decayed wounds are found at the stubs of truncated branches at stem B

(Photo T3-19). The decay could spread to the parent stem. The stub with

advanced decay should be removed using the standard removal cut at the

fork with the parent stem.

(vi) Photo T3-20: Stub with advanced decay on Stem C

The truncated stem C has developed advanced decay at the large wound

(Photo T3-20). The terminal end of the stump with advanced decay should

be cut away, but the lignified aerial root linking stem C to stem B above

should not be cut.

66



(i) Tree location on stone wall

T4 is a member of the group of four stonewalls trees situated at the eastern

one-third of the old stone wall (Photo T4-1). They are of similar size and

configuration, and are growing close to each other. T4 is relatively

stronger than the other three trees (Photo T4-2). Its crown interlocks partly

with two neighbour trees T3 and T5 (Photo T4-3).

(ii) Tree dimensions and crown configuration

The twin-stemmed tree has an aggregate DBH of 77 cm. Tree height

attains 14.83 m, supporting a crown that spans 15.7 m parallel to the wall

alignment, and 18.7 m perpendicular to it. The crown is highly

asymmetrical, being restricted on the west and south sides.

(iii) Trunk lean

Trunk A leans at 52 degree towards Bonham Road, which is the most tilted

of the six stonewall trees at the site (Photos T4-2 and T4-4). Trunk B leans

backwards at 49 degree towards St Stephen’s Lane, which is also the most

tilted in the southward direction amongst the six trees. The curved lower

part of trunk A is propped by a bundle of lignified aerial roots to lend extra

strength to its basal part (Photo T4-5). The bulk of the crown weight is

carried by trunk A.


(i) Surface root growth pattern

The spread of surface roots on the wall face assumes an unusual pattern

(Photo T4-6). The east half of the root mass descends down to the wall toe

and penetrates into the soil below the pavement. The west half has been

truncated at the middle of the wall, as evidenced by the straight and

horizontal lower edge of the root mass. Some roots have intertwined with

those of T3 to its west and T5 to its east, involving some tissue grafting

and fusion.

67

(ii) Root growth at wall toe

The wall toe is equipped with a U-channel covered by concrete slabs

(Photos T4-6 and T4-7). The excavation necessary to install this drainage

channel would have severed or injured the roots that penetrated into soil

below the pavement. Some roots could grow below the channel to explore

the soil lying underneath and to capture more water and nutrients to

support tree growth.

(iii) Trunk base detachment from wall face

The trunk base is attached to the top part of the wall face (Photos T4-8 and

T4-9). A narrow gap is found between the wall face and the root mass,

indicating that the base of the tree has been detached somewhat from the

wall (Photos T4-8, T4-9, T4-10, T4-11 and T4-12). The tree anchorage

has failed to a certain extent by yielding a little. The remaining roots are

holding the tree against overturning and collapse.

(iv) Dense surface root mass

The dense network of surface roots to the east and west of the trunk base

embodies some roots that have turned backwards to move into the joints

and henceforth into the aft-soil. Together with the penetrated roots below

the trunk base, they afford critical tensional pull to hold the tree against

overturning (Photos T4-13 and T4-14). The two obsolete iron angle bars

situated near the trunk base could be removed. If their removal may harm

the surface roots, the exposed part could be sawn away without extracting

the embedded part.

(v) Root growth restriction




performance.

(vi) Removing cement seal at masonry joints

The joints between masonry blocks have been sealed recently by cement,

thus stopping their penetration by tree roots. The rigid cement seal also

restricts expansion of existing roots and may cause girdling injury as they

continue to thicken. Where the roots are physically obstructed,

consideration could be given to localized removal of the cement seal to

68

permit some new roots to grow into the joints and existing roots to expand


(vii) Installing soil strip at wall toe


the wall toe at Bonham Road, in lieu of the existing drainage u-channel, to

allow growth of roots in the soil below the pavement (cf. Section 4.2.1). If

the propping method C explained in Section 3.15e is adopted to support

the tree at the Bonham Road pavement, the design could be adjusted to

accommodate both the propping frame and the soil strip.

(viii) Installing soil strip at wall crest


the wall crest to allow growth of roots in the soil below the pavement at St

Stephen's Lane. A segment of the existing stone parapet wall will have to

be replaced by railing to permit the roots to reach this new soil strip (cf.

Section 4.2.1). If the cable bracing method B to pull the tree at the

northern edge of St Stephen's Lane explained in Section 3.15e is adopted,

the design could be adjusted to accommodate both the steel frame and the

soil strip.


(i) Two subcrowns supported by trunks A and B

The crown of T4 is composed of two portions supported respectively by

trunk A and trunk B. The bulk of the crown supported by trunk A hangs

above Bonham Road (subcrown A), and similarly the bulk of the crown

supported by trunk B hangs above St Stephen’s Lane (subcrown B)

(Photos T4-15 and T4-1). Subcrown A was subject to tipping in the past,

and it has since rebuilt its lost top by an expanded epicormic branch (Photo

T4-15). Subcrown A is notably larger and heavier than B.

(ii) Crown configuration and condition

The live crown ratio of the somewhat enfeebled tree stands at a medium

level of 40-70%. Foliage density is rated as sparse due to compromised

vigour, but the leaf size and colour are judged as normal. Dieback of twigs

is gauged as low at < 5%. The crown is not symmetrical, and some main

branches have been tipped. Excessive loss of lower branches is obvious,

which carries the undesirable consequence of failing to develop proper

69

trunk taper to compromise tree stability (cf. Section 4.2.4). Overall tree

vigour is judged to be average.


(i) Overall condition (microdrill points 1 and 2)

The truncated tip of Stem A is shouldering the heavy load of a large

replacement epicormic branch (Photo T4-15). The evaluation at close

quarters should inform whether the tip of the parent branch has sufficient

sound wood to hold the rather heavy epicormic replacement branch. If not,

the end weight of the daughter branch should be reduced by removing

some of the smaller and weaker branches. The amount to be removed shall

be commensurate with the condition of the wood around the cut face.

Two microdrillings were made by the contractor through Stems A and B

respectively (Photo T4-5). Both drillings do not penetrate the whole trunk;

they stop at 40 cm which is the length of the rather short drill bit. Thus the

results cannot detect the internal wood condition of the whole trunk

section. Based on the limited data, Stem A has sound wood (Appendix

Figure T4-P1). Stem B displays lowered strength in 0-17 cm, denoting the

presence of decay or weak wood in the lignified aerial roots attached to the

trunk (Appendix Figure T4-P2). The limited data show that the main trunk

has enough sound wood. No drastic pruning is necessary at this stage. As

the decay may progress in due course to a more advanced stage and spread

to a large volume of wood, the structurally compromised location should

be continually monitored.

(ii) Photo T4-16: Tipped branch of Stem A with epicormic branch and

elbow joint

A tipped branch of Stem A carries a replacement epicormic branch with an

elbow joint (Photo T4-16). The evaluation at close quarters should tell

whether the tip of the parent branch has sufficient sound wood to hold the

rather heavy epicormic replacement branch. If not, the end weight of the

daughter branch should be reduced by removing some of the small

branches. The amount to be removed shall be commensurate with the


(iii) Photo T4-17: Hanging rubbish trapped in crown

Hanging plastic rubbish is trapped in the crown (Photo T4-17). It should

be carefully extricated and removed in the course of pruning with the help

of a hydraulic platform.

70

(iv) Photo T4-18: Stem A branch with basal curvature and decayed crotch

(microdrill points 3, 4 and 5)

A branch of stem A has a curved basal section and a decayed wound at

crotch (Photo T4-18). The internal wood condition at the critical juncture

should be probed by microdrilling, the position and direction of which are

shown in the photograph. The results should inform whether the critical

crotch position has sufficient sound wood to hold the two thick branches

and the thinner and elongated epicormic branch. If not, the epicormic

branch should be removed. If the decay has affected over one-third of the

wood in the notional cross-section, the end weight of the two thick

branches will have to be reduced by trimming some small branches. The

amount to be removed shall correspond to the condition of the wood

around the cut face.

All three drillings which went through the branch show sound wood

throughout (Appendix Figures T4-P3, T4-P24and T4-P5). Thus no

pruning or load-reduction action is recommended.

(v) Photo T4-19: Truncated branch of Stem A with epicormic branches

and elbow joint

A truncated branch of stem A has a decayed wound and two epicormic

branches with elbow joint (Photo T4-19). Evaluation at close quarters

should inform whether the broken end of the branch has sufficient sound

wood to hold the two closely spaced epicormic branches. If not, the load

on the parent stem should be reduced by an amount that is commensurate

with the condition and quantity of sound wood around the wound. From

slight to severe loss of wood mechanical strength, the following sequence

of actions could be adopted: shorten the thin branch, remove the thin

branch, shorten the thick branch, and remove the thick branch.

(vi) Photo T4-20: Stem A branch with fractured wound and decayed stub

A branch of stem A has a fractured wound and a nearby decayed stub

(Photo T4-20). The rather fresh fractured wound with jagged edges should

be carefully trimmed using a sharp arborist manual saw with the help of a

carpenter’s chisel. The decayed stub should be removed using the standard


71

(vii) Photo T4-21: Branch removal wound with decay on stem A

Stem A has a branch-removal wound with decay (Photo T4-21). The

wound with jagged edges and uneven surface should be very carefully

trimmed using a sharp arborist manual saw with the help of a carpenter’s

chisel. However, the well-formed callus tissues should as far as practicable

be left undisturbed.

(viii) Photo T4-22: Branch removal wound with decay on stem A

Stem A has a branch-removal wound with decay (Photo T4-22). It should

be left alone but be monitored to see if the decay would move to the

advanced stage.

(ix) Photo T4-23: Branch removal wound with decay on stem A

Stem A has a branch removal wound with decay (Photo T4-23). The

wound with jagged edges and uneven surface should be very carefully

trimmed using a sharp arborist manual saw with the help of a carpenter’s

chisel. However, the well-formed callus tissues should as far as practicable

be left undisturbed.

(x) Photo T4-24: Branch stub with decay on stem A

Stem A has a branch stub with advanced decay (Photo T4-24). It should be

removed using a standard removal cut at the fork with the parent stem.

(xi) Photo T4-25: Meandering shape of Stem A

The lower section of the heavily leaned trunk A assumes a meandering

shape (Photo T4-25). The curved parts should be monitored for possible

development of cracks.

(xii) Photo T4-26: Stem B curving backwards towards St Stephen’s Lane

The heavily leaned trunk B assumes a meandering shape (Photo T4-26).

The curved parts should be monitored for possible development of cracks.

(xiii) Photo T4-27: Stem B stub with advanced decay

A stub of stem B with an irregular tip has developed advanced decay

(Photo T4-27). It should be removed using a standard removal cut at the

fork with the parent stem.

72

(xiv) Photo T4-28: Stem B stub with decay

A branch stub of stem B has developed decay (Photo T4-28). It should be

removed using a standard removal cut at the fork with the parent stem.

(xv) Photo T4-28: Existing Cobra bracing using Stem B to hold Stem A

An existing Cobra cable bracing has been installed using Stem B which

points backwards towards St Stephen’s Lane to hold Stem A that leans and

hangs above Bonham Road. Stem B may not have the strength to provide

support to Stem A. Please see the alternative support systems explained

below (Section 3.1.5e).

(xvi) Prevention of wedging effect and decay at trunk base

The rubbish that has deposited on the surface roots and the adjacent trunk

base, together with the leaf litter, should be regularly removed with the

help of a brush to avoid moisture accumulation which could induce decay

at the critical trunk base position (Photos T4-11 and T4-12). More

importantly, objects trapped in the wedge gap caused by the detachment of

the tree from the wall could aggravate the detachment process due to the

wedging effect. They must be diligently and thoroughly removed from the

gap to prevent aggravation of the detachment risk (cf. Section 4.2.3).

(e) Proposal for tree support systems for T4

(i) Tree hazard status

Tree hazard assessment has a score of 9 plus an extra score due to

detachment of the trunk base from the wall face. The total hazard score of

10 has pushed the tree from top of the medium risk category into the base

of the high risk category (Table 4G). Of the six stonewall trees at the site,

T4 has been accorded the highest hazard score. The tensional roots that

should pull the tree against overturning have yielded a little to create a gap

between the tree base and the wall face (Photos T4-8, T4-9, T4-10, T4-11

and T4-12). The main contribution to tree risk is the potential for the

whole tree to be uprooted from the wall. If the tree were to fail, it is likely

that the whole tree will collapse on Bonham Road.

(ii) Three tailor-made tree support systems

It is pertinent to provide artificial, effective and long-lasting support to the

tree at the earliest opportunity to prevent tree failure. Due to the severe

73

site constraints at both Bonham Road and St Stephen’s Lane, innovative

methods have to be tailor-made to stabilize the tree without obstructing

pedestrian or vehicular traffic. It could be realized in two modes, namely

pulling it towards the south by cable bracing, or propping it from the north.

Three options in order of preference are proposed below.

(iii) Method A: Cable bracing

The tree's roots have partly detached from the stonewall face, leaving a

gap between the wall and the trunk base. Trunk A is heavily tilted at 52

degree from the vertical towards Bonham Road. It carries the bulk of the

tree's mass. As the tree hangs above the pavement and the carriageways, a

long-term and effective solution to secure it must be implemented at the

earliest opportunity to forestall further detachment, and to abate the risk of

tree failure. The present cable bracing shown in Photo T4-29 using trunk

B to hold trunk A has doubtful capability to prevent tree failure. A more

direct, effective and relatively less costly approach is strongly

recommended, by way of a cable bracing system to hold both stems. The

cable should have sufficient elasticity to permit the stems to move within a

certain limit.

The cables can be anchored on the columns of the nearest building to the

south at St Stephen's Lane at an elevated level to permit vehicular and

pedestrian headroom clearance (Photo T4-30). The Highways Department

with the help of the District Council and Home Affairs Bureau could

negotiate with the property owners and building management to obtain the

permission to do so. The relevant property owners could be persuaded in

the interest of preserving the tree and the associated amenities which could

benefit them in the long term. The maintenance of the cable bracing

system and the affected parts of the columns or beams should be

shouldered by the government. Please see Photo T4-30 to illustrate the

concept design of this cable bracing system. The detailed design should be

elaborated in conjunction with a structural engineer.

(iv) Method B: Cable bracing

If permission could not be obtained in implement the Method A cable

bracing system, the less desirable alternative is to install a strong steel

frame near and parallel to the parapet wall at St Stephen's Lane to hold the

tree (Photo T4-31). The frame should be firmly anchored in the ground.

Cables can then be installed to link the frame to the two trunks. Due to the

low bracing position relative to tree height, the amount of swing in the

wind would be rather limited. Thus the cable should have a

correspondingly lower degree of elasticity.

74

As the Lane is used for vehicular access, the steel frame has to be

positioned as near as possible to the parapet wall to maintain sufficient

vehicular lateral clearance. It implies that excavation will have to be

conducted near the tree which may harm the roots that have penetrated the

soil lying below the Lane. To minimize this impact, the positions of the

two anchors for the frame could be placed respectively between T4 and T3,

and T4 and T5. Moreover, the impact could also be reduced by developing

a technique with the help of an engineer to install the anchors with a

minimum excavation limit. Please see the Photo T4-31 which illustrates

the concept design of this alternative cable bracing system. The detailed

design should be elaborated in conjunction with a structural engineer.

(v) Method C: Propping

The two cable bracing methods explained above are preferred to stabilize

the tree and to abate the tree failure hazard. Propping the tree in front of

the stone wall is inordinately difficult due to the narrow pavement and its

use also as a bus stop. If both cable bracing methods cannot be

implemented, the least desirable propping option will have to be enlisted.

In view of the severe site constraints, this option has to be considered as a

compromise that is less effective in holding the tree. A strong steel frame

will have to be constructed at the pavement near the wall face to minimize

disturbance to pedestrian flow and to reduce risk to buses and other

vehicles getting too close to the frame (Photo T4-32). To leave sufficient

wide berth for pedestrian flow, the frame below the normal pedestrian

vertical clearance will have to abut onto the wall face.

The vertical support column has to be firmly anchored in the ground and

the wall to hold the load of the tree and to prevent overturning. A strong

cantilever arm will extend from the top of the frame to bear the weight of

the tree. The interface between the arm and the trunk should be properly

cushioned to prevent bruising injuries. The exact positions of the anchors

should be carefully chosen to minimize impacts on surface and penetrated

roots. To avoid injuries to the tree, great care in conjunction with physical

protection and other precautionary measures should be adopted in

installing the steel frame. Please see Photo T4-32 which illustrates the

concept design of the proposed propping system. The detailed design

should be elaborated in conjunction with a structural engineer.

(vi) Method D: Short-term contingency measure to abate tree risk

Three long-term options in order of preference have been proposed in the

above sections to abate the risk of T4 on targets. It is envisaged that one

75

of them will be adopted in due course. As the implementation may incur a

lead time to overcome the obstacles in terms of space, traffic and property

rights, in the meantime it is deemed necessary to take a short-term measure

to reduce the risk of tree failure. Drastic reduction of branches has to be

ruled out, because it could impose irreparable damages on the tree and

may dampen its vigour and health to render it more hazardous to targets.

The critical weakness is the incipient detachment of the trunk base from

the wall face (Photos T4-8 to T4-12; Section 3.1.5biii). A temporary cable

system could be installed to be anchored in the ground near the parapet

wall at St Stephen’s Lane to hold the trunk base and prevent further

progression of detachment. Holes can be drilled in the parapet wall to

allow cables to extend from the anchor to the trunk base to provide

tensional pull to hold the tree against failure. A structural engineer could

provide professional input on the design of this ground anchor method. A

schematic drawing of this temporary cable bracing method is shown in

Photo T4-33.

(vii) Alternative to long-term cable bracing or propping

In the unlikely and undesirable event that none of the three long-term

methods (Methods A, B or C) to stabilize the tree can be implemented, the

alternative will have to be drastic pruning to reduce the load and hence the

risk level. Such an approach will disfigure the tree and reduce its vigour,

and may drive the tree towards an irreversible decline spiral to render it

more unstable and hazardous. Furthermore, the load-reduction effect of

drastic pruning may not last long, as the tree in the post-treatment years

will struggle and send out a notable amount of sprouts and epicormic

branches in its attempt to regain some of the lost photosynthetic capability.

Thus the tree will have to be heavily pruned repeatedly for some years, a

process that is tantamount to a protracted death sentence. As there are

feasible ways to stabilize the tree, drastic and repeated pruning cannot be

recommended.

76




T5 is one of the four trees that are clustered at the eastern one-third of the

stone wall (Photo T5-1). It stands at 14.97 m tall, which is similar to its

three companions. The tree sits near the top of the wall. The height of the

first branch is elevated well above the ground at 5.18 m from the trunk

base. The tree leans towards Bonham Road at 21 degree.


Unlike its three stronger and multiple-stemmed partners, it has a single

trunk with DBH of 41 cm, which is rather slender in comparison with its

height (Photos T5-2 and T5-3). The height to DBH ratio of the trunk is as

high as 36.4, which signifies an inherently unstable shape. The crown is

also unusually narrow especially in the direction parallel to the wall

alignment at merely 9.3 m. Perpendicular to the wall, the crown is a little

wider at 11.9 m. Some branches interlock with its two neighbours, namely

T4 at its west and T8 at its east.


(i) Surface root distribution on wall face

The wall gets shorter towards the east end, and at T4 it is 2.54 m above the

pavement level (Photo T5-4). The surface roots extend from the trunk base

to the wall toe, with some penetration into the soil below the pavement.

One surface root in the middle of the root mass is particularly thick and it

lends important support to the trunk.

(ii) Interface between trunk base and wall face

The contact interface between the trunk base and the wall face is

somewhat limited in comparison with its partners on the same wall

segment (Photos T5-5 and T5-6). Organic litter and rubbish have

accumulated in the gap between the trunk base and the wall face (Photo

T5-7). They should be regularly removed to avoid the wedging effect and

wood decay (cf. Section 4.2.3).

77

(iii) Root growth restriction




performance.

(iv) Removing cement sealing from masonry joints






permit some new roots to grow into the joints and existing roots to expand


(v) Installing soil strip at wall crest





Section 4.2.1). If the cable bracing Method B explained in Section 3.1.6f

is adopted, the design could be adapted to accommodate both the steel

frame and the soil strip.

(vi) Installing soil strip at wall toe


the wall toe at Bonham Road, in lieu of the existing drainage U-channel

and pavement covered with unit pavers, to allow growth of roots in the soil

below the pavement (cf. Section 4.2.1). If the propping method C

explained in Section 3.16f is adopted to support the tree at the Bonham

Road pavement, the design could be adjusted to accommodate both the

propping frame and the soil strip.

(c) Assessment of large basal cavity

(i) Photos T5-8 and T5-9: Large cavity at trunk base with decay

As the large basal cavity is closely associated with root development and

hence compensatory reinforcement, the topic is discussed in this dedicated

section. It assumes a typical inverted V-shape of a basal cavity, and is

78

nearly as wide as the trunk base itself (Photos T5-8 and T5-9). The

internal surface of the cavity is covered with brownish decayed wood,

some of which has a loose consistence that can be removed with the hand.

The thickness of the residual wall around the cavity and especially the

amount of remaining sound wood should be ascertained by microdrilling.

Three drilling positions have been annotated on the photographs (Photos

T5-8 and T5-9). The results can inform the amount of sound wood in the

residual wall and whether it is adequate to support the tree’s biomass.

Field inspection indicates that the residual wall is rather thin.

The edges on both sides of the cavity has developed prominent strips of

response wood to compensate for the loss of wood strength due to cavity

formation at the critical position of the trunk (Photos T5-8 and T5-9). The

response wood tends to have a higher density, higher mechanical strength

and higher resistance to fungal decay. They contribute significantly to

reinforcing the trunk base against snapping failure. At the opposite side of

the trunk lean and at the basal position, the response wood affords critical

tensional pull. Moreover, the response wood development has extended

above the cavity to the lower-middle part of the trunk to offer notable ribs

of tension wood to pull and stabilize the leaning tree (Photo T5-10).

(ii) Photos T5-11 to T5-13: Trunk reinforcement by lignified aerial roots,

thickened root pro, and response wood

In addition to the reinforcement adjoining to the cavity, the tree has

developed two rather strong and upright lignified aerial root stands

adjacent to the trunk base (Photos T5-11 and T5-12). They provide

supplementary support to the tree at the critical load-bearing position, and

are instrumental in securing the tree against breakage at the hollowed trunk

base. The root stands have assumed a distinctive flattish shape

perpendicular to the wall face to maximize the tensional pull strength

against the overturning moment (Photo T5-13).

(d) Assessment of tree crown

(i) Narrow and confined crown development

The narrow crown has developed few branches (Photo T5-14). Two major

limbs follow a rather upright growth habit. One of the limbs displays

evidence of past tipping which has triggered the development of epicormic

branches from the wound. The two neighbour trees have literally

sandwiched and trapped the relatively weaker T5 in the intervening space.

79

The foliage density is rated as sparse, although leaf size and colour are

normal. Twig dieback is not evident, with less than 5%.

(ii) Competition with neighbour trees and loss of low branches

The tree has lost a good deal of its branches, and branch growth has been

confined by proximity and competition with neighbour trees which are

situated too close to it. The live crown ratio is put in the lowest category

of < 40%. Excessive loss of lower branches is noted, which can suppress

the tree’s ability to develop a proper trunk taper and compromise its

stability (cf. Section 4.2.4). Overall tree vigour can be reckoned as average.

(e) Assessment of trunks and branches

(i) Photos T5-8 and T5-9: Large cavity at trunk base with decay

(microdrill points 1, 2, 3 and 4)

The loose decayed wood in the large basal cavity could be cleaned to

reduce the chance of moisture accumulation and hence to dampen fungal

invasion of the remaining wood in the residual wall (Photos T5-8 and T5-

9). Only the badly decayed and partly detached wood in the large wound

should be very carefully removed. Do not attempt to excavate or remove

woody tissues that have not partially detached. Do not overdo this cleaning

exercise. The detached wood debris accumulated at the bottom of the

cavity should also be removed, if necessary with the help of a powerful

vacuum cleaner. The cavity cleaning could help to dampen fungal growth.

As a precautionary measure, a professional extermination company should

be enlisted to check thoroughly whether the tree has been infected by

termites. If so, the modern hormone bait method should forthwith be

applied to get rid of the wood consuming insects.

Two drillings penetrate the remaining wood shell of the large basal cavity.

At point 1, the shell thickness is merely 8 cm with somewhat degraded

wood (Appendix Figure T5-P1). Wood with advanced decay has been lost,

leaving the partly decayed wood in the shell. Lower down at point 2, some

wood with advanced decay still lingers on the cavity wall in 0-8 cm

beyond which the wood has indication of incipient decay (Appendix

Figure T5-P2). The wood shell measures 13 cm. As the cavity is open on

the south side, the t/r ratio is not applicable. The lignified aerial roots and

root props at and around the trunk base have reinforced it and compensate

for the loss of wood strength due to decay and cavity formation (Photos

T5-11 to T5-12). Moreover, the liberal development of response wood on

80

the upper side of the trunk base serves as pertinent tension wood

reinforcement to pull and help to stabilize the tilted tree (Photo T5-13).

Two more drillings were made above the basal cavity to explore whether

the decay has moved upwards into basal large cavity. A large internal

cavity of 15 cm diameter is found which signifies the longitudinal

extension of the decay column (Appendix Figure T5-P3). The cavity is

found on the west side, with a 33 cm residual wood shell of sound wood

concentrated on the east side. Further up at point 4 (Appendix Figure T5-

P4), the size of the internal cavity is smaller at 4 cm diameter and again

situated on the west side. Combining the results of points 3 and 4, the

decay column is notably constrained as it moves up the trunk. The

additional lignified roots and thickened root prop (Photo T5-11 to T5-13)

provide substitute support above point 3 to compensate for the loss of

strength due to cavity formation. The tree has been able to respond to the

cavity formation by developing supplementary support.

Overall, the saving grace is that the crown load of the tree is limited due to

restricted crown development (Section 3.1.6d). The defective trunk does

not need to hold a large and heavy crown, hence the pressure on the trunk

is somewhat reduced. Despite the large basal cavity, the tree has been able

to support itself by a combination of supplementary supports. The risk

level could be contained by installing a support system as explained in

Section 3.1.6f.

(ii) Photo T5-10: Removal of limbs from the trunk

The multiple wounds on the trunk were due to removal of limbs in the past

(Photo T5-10). Their jagged edges or uneven surface or decayed wood

should be very carefully trimmed using a sharp arborist manual saw with

the help of a carpenter’s chisel. However, the well-formed callus tissues

should as far as practicable be left undisturbed.

(iii) Photos T5-11 to T5-13: Trunk reinforcement by lignified aerial roots,

thickened root prop and response wood (microdrill point 4)

The loss of wood strength due to the large cavity at the trunk base has been

compensated by the development of lignified aerial roots linking the trunk

to the root mass on the stonewall face, thickening of a root prop just below

the trunk base, and response woos at the lower section of the trunk.

The microdrilling results at point 4 (Appendix Figure T5-P4), which is

related to the large basal cavity, are discussed above together with points 1

to 3 in Section 3.1.6ei.

81

(iv) Photo T5-14: Truncated branch with long epicormic branches and

elbow joint

The truncated limb has formed epicormic branches from the wound, and

lower down at its middle a long and heavy epicormic branch with an

elbow joint (Photo T5-14). Evaluation at close quarters should inform

whether the tip of the truncated parent branch has sufficient sound wood to

hold the epicormic replacement branches. If not, the end weight of the

daughter branch should be reduced by removing some of the small

branches. The amount to be removed shall be commensurate with the


In addition, the end weight of the long and heavy epicormic branch in the

middle of the parent limb (Photo T5-14) should be reduced by one-third by

selectively removing by reduction cut the weaker and thinner branches,

with a view to leaving an even spread of remaining branches. The heading

cut must not be used to trim the branches.

(v) Photo T5-15: Upright limbs with v-crotch

The narrow V-crotch between the two rather upright limbs should be

examined at close quarters to see if it has formed included barks (Photo

T5-15). It should be checked for the presence of crack and internal decay

at the interface. If so, the two rather upright limbs should be linked by an

elastic cable to prevent splitting at the weakened fork.

(vi) Photo T5-16: Branch stub on trunk with decay

A thick branch stub on the trunk has developed decay (Photo T5-16).

As callus tissues have formed rather well around the edge of the cut face,

and the decay has not reached the advanced state, it could be left alone and

monitored.

(vii) Photo T5-17: Seam and bulge on heavy limb (microdrill point 5)

A notable seam with slight bulgewood has formed on the heavy limb, with

possible internal decay (Photo T5-17). The wood condition inside and

around the seam should be explored by microdrilling, the position of

which has been annotated on the photograph. If more than one-third of the

notional cross section has been degraded by decay or cavity, the end

weight of the long and heavy branch should be reduced. The amount to be

removed shall be commensurate with the condition of the wood around the

82

seam. The removal could aim at the weaker or thinner branches, with a

view to leaving an even spread of remaining branches.

A large internal cavity is found by microdrilling in the heavy limb

(Appendix Figure T5-P5). It measures about 11.5 cm in diameter, but the

wood shell on both sides has sound wood. The t/r ratio for the thin (west)

side is 10/15.5=0.64 which is above the critical threshold of 0.33. There is

enough sound wood to support the load, hence no drastic pruning is

needed. The end weight of the long and heavy branch should be reduced

by about one-third. The pruning should aim selectively at the weaker or

thinner branches, with a view to leaving an even spread of remaining

branches. The decay, however, could continue to move into the sound

wood shell to reduce wood strength in due course. The weakness should

be continually monitored.

(viii) Photo T5-18: Existing Cobra bracing using T5 to hold T4 Stem A

The existing cobra cable bracing using T5 to pull T4 may not be able to

help the latter (Photo T5-18). T5 suffers from the significantly weakened

trunk base due to the large basal cavity. Despite the development of

response wood and lignified root stands, it is unlikely to have much spare

strength to hold T4. If T4 were to fail, it is highly likely that T5 will also

be brought down collaterally. It is recommended that this cable should be

removed to relieve T5 of a burden that it can hardly afford and the

associated risk of collateral failure. Instead, T4 and T5 each should have

its own supporting system (cf. Sections 3.1.5e and 3.1.6f).

(ix) Preventing decay at trunk base

The rubbish that has deposited on the surface roots and the adjacent trunk

base, together with the leaf litter, should be regularly removed with the

help of a brush to avoid moisture accumulation which could induce decay

at the critical trunk base position (Photo T5-7). More importantly, objects

trapped in the gap could induce the wedging effect to detach the trunk

base from the wall face. Such materials must be diligently and thoroughly

removed from the gap to prevent the detachment risk (cf. Section 4.2.3).

(f) Proposal for tree support systems for T5


Tree hazard assessment has a score of 8 which can be slotted at the middle

of the medium risk category (Table 5G). Moreover, it has been assigned an

extra score due to the alarmingly large basal cavity. Of the six stonewall

83

trees at the site, T5 has been accorded the second highest hazard score

trailing after T4. The main contribution to tree risk is the potential for the

trunk to snap near the base due to advanced decay and formation of a large

cavity. If the tree were to fail, the whole tree may fall down on Bonham

Road.

(ii) Three tailor-made tree support systems

In view of the high risk status, something concrete, effective and long-

lasting have to be applied to abate the hazard. Due to the severe site

constraints, which are similar to those faced by T4, special non-routine

measures will have to be developed to stabilize the tree without

obstructing pedestrian or vehicular traffic.

As the tree has a limited biomass due to the limited crown size and amount

of branches and foliage, a cable bracing method is proposed to provide the

pulling force from the south side. The large decayed wound at the trunk

base has degraded into a large cavity. It occurs at the back of the inclined

trunk, which is the most critical position where strong tension wood should

have developed to hold the tree's weight. The tree has developed

supplementary and substitute supporting structures in the form of notable

ribs of tension response wood, two lignified aerial root props, and a

thickened root prop (Photos T5-11 and T5-12). They have been

instrumental in maintaining tree stability despite the significant loss of

support at the trunk base.

Three supporting systems are proposed, the design of which has taken into

account the rather severe site constraints. They are listed below in order of

preference.


The pavement and carriageway under the tree, however, are quite

frequently used. The tree manager has to do something to abate the risk of

tree failure and possible damage or injury of the targets. The most direct

and cost-effective way is to install a cable bracing system to hold the trunk.

The cable system should have sufficient elasticity to permit the stems to

move within a certain limit. The cable can be anchored on the columns of

the nearest building to the south at St Stephen's Lane at an elevated level

to permit vehicular and pedestrian headroom clearance (Photo T5-19).

The Highways Department with the help of the District Council and Home

Affairs Bureau could negotiate with the property owners and building

management to obtain the permission to do so. The relevant property

84

owners could be persuaded in the interest of preserving the tree and the

associated amenities which could benefit them in the long term. The

maintenance of the cable bracing system and the affected parts of the

columns or beams should be shouldered by the government. Please see

Photo T5-19 which illustrates the concept design of this cable bracing

system. The detailed design should be elaborated in conjunction with a

structural engineer.


The second option is to install a strong steel frame near the parapet wall at

St Stephen's Lane to hold the tree using a cable-bracing device (Photo T5-

20). The frame should be firmly anchored in the ground. Cables can then

be installed to link the frame to the trunk at a position above the cavity.

Due to the low bracing position relative to tree height, the amount of swing

in the wind would be rather limited. Thus the cable should have a

correspondingly lower degree of elasticity.

As the Lane is used for vehicular access, the frame has to be positioned as

near as possible to the parapet wall to maintain sufficient vehicular lateral

clearance. It implies that excavation will have to be conducted very

carefully near the tree to avoid harming the roots that have penetrated the

soil lying below the Lane. Moreover, the impact could also be reduced by

developing a technique with the help of an engineer to install the anchors

with a minimum excavation limit. Please see Photo T5-20 which illustrates

the concept design of this cable bracing system. The detailed design should

be elaborated in conjunction with a structural engineer.

(v) Method C: Propping

If both cable bracing methods are found to be not feasible, the last resort of

mounting a strong steel propping frame at the Bonham Road pavement

will have to be considered (Method C). The design concept will be similar

to the case of T4, hence the same drawing and description will be

applicable (cf. Section 3.1.5e and Photo T4-32).

(vi) Method D: Short-term contingency measure to abate tree risk

Three long-term options in order of preference have been proposed in the

above sections to abate the risk of T5 on targets. It is envisaged that one

of them will be adopted in due course. As the implementation may incur a

lead time to overcome the obstacles in terms of space, traffic and property

rights, in the meantime it is deemed necessary to take a short-term measure

to reduce the risk of tree failure. Drastic reduction of branches has to be

85

ruled out, because it could impose irreparable damages on the tree and

may dampen its vigour and health to render it more hazardous to targets.

As the trunk base and lower trunk with a large basal cavity is the weakest

part of the tree, it cannot be used for cable attachment. Instead, the cable

has to be mounted above the large basal cavity and its extension as internal

cavity (cf. Section 3.1.6ei). The make-shift cable bracing can be attached

to anchors in the ground just behind the parapet wall at St Stephen’s Lane.

Holes can be drilled in the parapet wall to allow cables to extend from the

anchor points to the trunk. A structural engineer could provide

professional input on the design of this cable bracing method. A

schematic drawing of this temporary cable bracing method is shown in

Photo T5-21.

(vii) Alternative to long-term cable bracing or propping

In the unlikely and undesirable event that none of the three long-term

solutions (Methods A, B or C) to stabilize the tree can be implemented, the

alternative will have to be drastic pruning to reduce the load and hence the

risk level. Such an approach will disfigure the tree and reduce its vigour,

and may drive the tree towards an irreversible decline spiral to render it

more unstable and hazardous. Furthermore, the load-reduction effect of

drastic pruning may not last long, as the tree in the post-treatment years

will struggle and send out a notable amount of sprouts and epicormic

branches in its attempt to regain some of the lost photosynthetic capability.

Thus the tree will have to be heavily pruned repeatedly for some years, a

process that is tantamount to a protracted death sentence. As there are

feasible ways to stabilize the tree, drastic and repeated pruning cannot be

recommended.

86




T6 is located at the far eastern end of the wall (Photo T6-1) close to the

junction with Park Road. It is the shortest of the group of four stonewall

tress clustered closely at the eastern stretch of the old wall. Standing at

12.37 m, it is tilted at 22 degree towards Bonham Road. It also leans

significantly towards the east, which is away from its neighbour T5 (Photo

T6-2).


The twin-stemmed tree has a thicker trunk A with 33 cm DBH and trunk B

at 21 cm (Photos T6-3 and T6-4). The aggregate DBH is 39 cm. The

crown shape is notably asymmetrical. Parallel to the wall face, the crown

is mainly developed on the east side. On the west side it is decidedly

stifled. Perpendicular to the wall face, the crown development is biased

towards the front facing Bonham Road (Photo T6-4). Overall tree vigour

is rated as average.


(i) Surface root distribution on wall face

The wall is at its shortest at the eastern end, measuring only 2.13 m above

the adjoining pavement level at T6 (Photo T6-5). Surface root density is

higher at the upper one-third, and notably decrease in the lower two-third

where the roots tend to descend towards the wall toe in four main clusters.

Some roots have entered the gap at the wall toe to explore the soil under

the pavement. Unlike other trees along the same wall, the wall toe at T6 is

not equipped with a U-channel (Photo T6-6).

(ii) Trunk base interface with wall face

The roots penetrate the joints mainly at the wall crest (Photos T6-5 and

T6-7). Organic litter and rubbish have accumulated in the gap between the

trunk base and wall face (Photos T6-7 and T6-8). They should be removed

diligently on a regular basis to avoid the wedging effect which could

contribute to detachment of the trunk base. The accumulation of moisture

87

in the gap, which could facilitate fungal growth, can also be avoided (cf.

Section 4.2.3).

(iii) Mortar displacement and trapped litter

The entry of roots into joints is critical for the tensional pull of the tree

against overturning (Photo T6-9). Some mortar strips have been detached

but not broken by root growth at the west side of the trunk base. The

detachment is likely to be triggered by the diameter growth of the root that

has penetrated the joint. The adjacent masonry blocks have remained in

their original positions. The litter trapped in the surface root mass can be

removed on a regular basis.

(iv) Root growth restriction




performance.

(v) Removing cement sealing at masonry joints






permit some new roots to grow into the joints and existing roots to expand.

(vi) Installing soil strip at wall crest





Section 4.2.1). If a steel frame has to be installed at the edge of the Lane

(cf. Section 3.1.7e) to hold the cable to pull the tree, the soil strip design

can be adjusted accordingly to accommodate both.

(vii) Installing soil strip at wall toe


the wall toe at Bonham Road, in lieu of the unit pavers, to allow growth of

88

roots in the soil below the pavement. Similarly, a soil strip can be installed

at the wall crest to permit roots to grow backwards to pull the tree.


(i) Crown configuration and loss of low branches

The crown is somewhat small vis-à-vis its height, with a live crown ratio

in the lowest category of < 40% (Photo T6-10). The crown outline is

highly asymmetrical with heavy development mainly on the eastern side

coupled with heavy leaning towards the east. Excessive loss of lower

branches from the trunk is quite obvious, and this may dampen the ability

of the tree to develop a normal trunk taper. Without sufficient thickening

of the lower trunk, the tree may not have enough strength there to support

its biomass (cf. Section 4.2.4).

(ii) Foliage and vigour

The foliage density is rated as sparse, whereas leaf size and leaf colour are

considered as normal. The branch dieback is in the low category of < 5%.

The overall tree vigour is judged to be average.


(i) Upright epicormic branch

A rather upright epicormic branch has emerged with an elbow joint from

the parent stem A (Photo T6-11). It bears resemblance to the tree-on-tree

structural defect. In due course, its further expansion will impose an

increasingly heavy burden on the parent stem. It should be removed whilst

it is still small.

(ii) Loss of low branches

Notable loss of lower branches is recorded in both trunks (Photo T6-12).

The impact on tree structure has been discussed in Section 3.17c above (cf.

Section 4.2.4). The undesirable pruning practice of preferential removal of

lower branches should forthwith be stopped. New epicormic branches or

sprouts emerging from the lower part of the trunks should be assessed to

keep those with the potential to develop into replacement branches.

89

(iii) Branch removal wound with decay on Stem B

Stem B has a branch removal wound with decay (Photo T6-13). Its jagged

edges, uneven surface and decayed wood should be very carefully trimmed

using a sharp arborist manual saw with the help of a carpenter’s chisel.

However, the well-formed callus tissues should as far as practicable be left

undisturbed.

(e) Proposal for tree support systems for T6


The hazard assessment yields a total score of 6 which falls on the top end

of the low risk bracket. The main concern is about falling of weakly

structured branches, and the coincidence of tree lean with the

asymmetrical crown.

(ii) Two tailor-made tree support systems

The tree is heavily tilted toward the road and the east side. A large

proportion of its branches and foliage is also concentrated on the east side

of the crown. The pavement and carriageway under the tree are quite

frequently used. The tree manager has to do something to abate the risk of

tree failure and possible damage or injury to the targets. Two cable bracing

systems are proposed.


In the preferred Method A, the cable can be anchored on the nearest

building to the south at St Stephen's Lane at an elevated level to permit

vehicular and pedestrian headroom clearance (Photo T5-19). The

Highways Department with the help of the District Council and Home

Affairs Bureau could negotiate with the property owners and building

management to obtain the permission to do so. The relevant property

owners could be persuaded in the interest of preserving the tree and the

associated amenities which could benefit them in the long term. The

maintenance of the cable bracing system and the affected parts of the

columns or beams should be shouldered by the government. Please see

Photo T4-30 which illustrates the concept design of this cable bracing

system. The detailed design should be elaborated in conjunction with a

structural engineer.

90


If Method A is not feasible, the alternative Method B has to be employed.

A strong steel frame is proposed to be installed near the parapet wall at St

Stephen's Lane to hold the tree using a cable-bracing device. The frame

should be firmly anchored in the ground. Cables can then be installed to

link the frame to the trunk at a position above the cavity. Due to the low

bracing position relative to tree height, the amount of swing in the wind

would be rather limited. Thus the cable should have a correspondingly

lower degree of elasticity.

As the Lane is used for vehicular access, the frame has to be positioned as

near as possible to the parapet wall to maintain sufficient vehicular lateral

clearance. It implies that excavation will have to be conducted very

carefully near the tree to avoid harming the roots that have penetrated the

soil lying below the Lane. Moreover, the impact could also be reduced by

developing a technique with the help of an engineer to install the anchors

with a minimum excavation limit. Please see Photo T4-31 which

illustrates the concept design of this cable bracing system. The detailed

design should be elaborated in conjunction with a structural engineer.

91

3.2 Tree values, potential risks and preservation

3.2.1 Value of the stonewall trees

(a) World-class and unique urban ecological gem

Stonewall trees denote a special if not unique urban ecological

endowment of our city. Very few places in the world have so many stone

retaining walls concentrated in a relatively small area, and with so many

trees of sizeable biomass and high landscape quality to dwell on them.

They represent the product of our difficult terrain working in tandem with

our labour and ingenuity in creating a city literally from scratch. The

traditional Chinese masonry technology has found pragmatic expressions

in the territory. It has lent tremendous help to maximize the amount of

developable land on hillslopes. The stonewall trees denote a pleasant by-

product of this laborious endeavour.

(b) Spontaneous nature in built-up areas

Beginning with a natural environment, we have created the city artefact.

On the artificial cliffs embedded in the city, we have inadvertently

facilitated nature to return to ameliorate the excesses of urban growth. We

did not plan for nature in the cramped built-up areas, yet nature would

come spontaneously to embellish them. The infilling of the vertical

stonewall habitats by trees reflects faithfully and emphatically nature’s

tenacity and resourcefulness in claiming the seemingly inhospitable sites.

In building the stone retaining walls, we aimed squarely and narrowly at

winning land for city development. Nature would make good use of the

opportunities offered by the vacant niches to flourish in the sea of

artefacts. It was serendipity and providence that have steadfastly invited

them to settle as permanent residents amidst our homes.

(c) Multiple amenity functions to people

Once they arrive and establish themselves on the stone structures, people

would begin to appreciate them. They are welcomed as our beloved

friends and benefactors amidst our neighbourhood and close to our homes

and offices. As children grew up and adults got older, the trees became

bigger and stronger. Despite limitations and stresses, they manage to

soldier on against heavy odds to become outstanding ambassadors of

nature. In close proximity to people, they generously share their valuable

ecosystems services with people. They enhance urban landscape quality

and enrich urban biodiversity.

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(d) Historical and heritage significance

The collection of a row of six stonewall trees on one of the oldest stone

walls in Hong Kong, and along one of the oldest roads that was built

about 150 years ago, would connote historical and heritage values. They

could be construed as objects to fill the collective memory of the

community. Traditional stone retaining walls are no longer built, and

many existing ones have been demolished or degraded by unsympathetic

modifications. The stonewall trees often suffer collateral damages due to

unfriendly treatment of the walls. From time to time, nature will take its

toll on the weaker members of the hanging tree cohort.

(e) Diminishing and threatened resource

The stonewall trees denote a diminishing natural-cum-cultural heritage of

our city. The fortuitous confluence of events in the past has permitted

their existence, but such events are becoming rare if not non-existent. The

present generation has fortunately received a wonderful bequest from our

forefathers and from Mother Nature. We owe the future generations the

duty to preserve them in a healthy state so that they could become a

sustainable endowment of our community. We should not allow them to

degrade and become more threatened.

3.2.2 Risk of the stonewall trees

(a) Wall instability versus tree instability

The risk of the stonewall trees can be linked to two related aspects,

namely the stability of the stone wall that holds them, and the stability of

the trees themselves. The wall itself is managed by the Highways

Department with technical advice given by the Government’s

Geotechnical Engineering Office of the Civil Engineering and

Development Department. Hong Kong has accumulated plenty of

experience and expertise especially in the last few decades in making

slopes including retaining walls safe employing mainly engineering

methods. The walls that are at risk are closely monitored by the relevant

authorities.

(b) Stone wall failure versus stonewall tree failure

A review of the history of wall failures in Hong Kong (Chan, 1996; GEO,

2011; Sections 2.2.4, 2.2.5 and 2.2.6) indicates that few of the cases

involved stonewall trees. The causes of wall failures were often attributed

93

to poor design, poor workmanship, poor maintenance, and sometimes

associated with leaking pipes situated behind the wall. Only a small

number of the failed walls had trees growing on them. The stonewall trees

could sometimes be brought down due to en masse wall collapse. In one

case, a stonewall tree was hinted as one of the possible causes of wall

failure. For the few cases of stonewall tree failures rather than stone wall

failures per se, the wall structure was hardly affected by tree uprooting or

collapse.

(c) Causes of stonewall tree failure

In two documented cases of recent stonewall tree failures, in the form of

uprooting, the causes could be more convincingly ascertained. They

involved tree failure without damage to the wall structure. The two

stonewall trees that toppled in strong wind associated with typhoon or

rainstorm were found to be inadequately anchored in the aft-soil. The lack

of tension roots that grow backwards into the joints and hence into the aft-

soil is interpreted as the main cause of failure. Without sufficient tensional

pull, the tree may overturn under extreme gusts. The lack of joints or

avenues for roots to penetrate the wall is considered as the fundamental

cause of insecure anchorage.

(d) Tree risk at subject site

The six trees at the subject site are located at a busy road with heavy

vehicular and pedestrian traffic during the daytime. The probability of a

failed tree hitting a target at the frequently used road is reckoned as rather

high. Such a level of road use would add scores to the tree hazard

assessment. T4 and T5 and to a lesser extent T6 express symptoms of

instability that call for remedial measures to abate their risk. As they are

situated right above a busy bus stop where people tend to gather and stay,

the risk level has been correspondingly raised. More assured methods

have to be implemented to suppress the risk to an acceptable level.

The remaining trees do not display visual clues to indicate that they are

unduly unsafe. The structural defects and decay problems of individual

trees have been identified and specific arboricultural treatments have been

recommended to minimize risk due to breaking or falling branches. Please

refer to Section 3.1 for relevant details on individual trees. A condensed

summary of tree problems and remedial treatments is given in Section 4.1

below.

94

3.2.3 Potentials and limitations in tree preservation

(a) Liberal availability of crown growth space

The six stonewall trees are all growing at or near the top of the wall,

where they are raised above the level of pedestrians and hence do not

obstruct their movement. This is more so for trees on the west side and

less so on the east. Although the branches hang above the carriageway of

Bonham Road, they are sufficiently elevated to provide clearance to

vehicular traffic. The elevated space above Bonham Road at the front and

St Stephen’s Lane at the back provide adequate room that is rather free

from building and other obstructions for their crown to extend. They do

not conflict with traffic and hence do not demand regular pruning to abate

this conflict. In comparison, stonewall trees at other sites in Hong Kong

are often beset by the lack of growing space especially at the back where

buildings often abut onto the wall to restrict crown development on one

side.

(b) Proximity of branches to nearby buildings

The proximity of some long branches to buildings on the opposite site of

Bonham Road and at the back along St Stephen’s Lane could cause

nuisance to the residential units. However, this conflict is not serious

enough to consider removing or drastically prune the trees. It can be

rectified by periodic and skilful shortening of selected branches. A

horizontal clearance of 2 m is recommended to abate the nuisance. The

limited amount of pruning conducted periodically will have little impacts

on tree health or structure. It is very important to forbid the harmful

heading cuts in any attempt to shorten branches.

(c) Tree support by propping system

Stonewall trees that have potential risk of failure demand remedial

measures in the form of a supporting system such as cable bracing and

propping. Three of the trees, namely T4, T5 and T6 have displayed

symptoms of instability which call for human assistance. Propping can

provide a strong and assured way to buttress the distressed trees.

However, the narrow pavement in front of the wall cannot provide

adequate space to accommodate a normal propping frame design.

A specially designed propping system with firm anchorage in the ground

and the wall could lend support to a sufficiently sturdy cantilever arm to

hold the tree’s weight. A tailor-made design of the propping system has

to be developed by a tree specialist in conjunction with a structural

95

engineer. This method may be more difficult to apply in comparison with

conventional propping, but it is not impossible. The need to puncture the

old stone wall face to install the support frame could be considered as

damage to the heritage feature. The installation of the anchors in the stone

wall will have to be conduct with great care to minimize the impact.

(d) Tree support by cable bracing anchored on adjacent building

In view of the rather tight site constraints, the alternative cable bracing

system is more suitable. The best approach is to anchor the cables on the

structural columns or beams of the buildings situated at the back of the

wall at St Stephen’s Lane. As the Lane allows vehicular access, the cables

have to be mounted at a height to permit sufficient vertical clearance. The

property owners concerned may not give consent to have the cables

attached to their buildings. The Highways Department could solicit the

help of the District Council and the Home Affairs Bureau to convince the

owners to offer a public service to help the trees. To allay the worries of

increase in maintenance cost due to the installation of cable anchors, the

government can take up the responsibility of maintenance for the affected

columns and beams.

(e) Tree support by cable bracing attached to steel frame

If the property owners cannot be persuaded to mount the cable anchors on

their buildings, an alternative cable bracing method will have to be

developed. It has to be fitted to the tight site conditions to leave sufficient

berth for both vehicles and pedestrians. Behind each tree in distress, a

strong steel frame is proposed to be anchored on the ground near the

parapet wall. The frame should be placed as close to the wall as possible,

but it must avoid injuring the roots that are likely to spread below the

paving near the tree base. The cable can be attached to the top of the

frame to hold the tree.

(f) Ecosystem services of stonewall trees to neighbourhood

The installation of the tree support systems can sustain the trees. The

associated pleasant and welcomed ecosystem services, including

landscape, environmental and amenity benefits provided by the lovely

stonewall trees, could thus be preserved to benefit the property owners

and residents. The sale and rental values of the adjoining properties can be

correspondingly lifted by the high-quality proximal greenery. Without the

support systems, the stability state of the trees, particularly T4 and T5,

could continue to deteriorate, and in the worst-case scenario they may

have to be felled in the interest of public safety. It will be a pity to lose

96

these outstanding members of nature embedded in the densely developed

neighbourhood.

(g) Human and social dimensions of tree preservation

The human and social dimensions, often delicate if not sensitive, have to

be considered in tree preservation. The stonewall trees have existed in the

neighbourhood for a long time. T2 is likely to be the oldest resident who

has dwelt on the wall for more than a century. It has spanned notionally

four human generations, and witnessed the coming and going of buildings

and people in its environs. The residents have literally grown up with the

trees, especially the younger members such as T1, and to a lesser extent,

T3 to T6. It is natural for them to have developed a fond and probably

sentimental attachment and a collective memory to their arboreal friends.

They harbour an expectation that the government, represented by the

Highways Department, will keep the trees in a healthy and safe state. It

will not be easily to convince the people to remove the trees without

taking efforts and exhausting the means to help them. On the contrary, it

will be easy to gain their support of tree preservation endeavours.

97

4. RECOMMENDATIONS ON MAINTENANCE OF THE

STONEWALL TREES

4.1 Short-term practical maintenance measures for individual trees

For the six stonewall trees at the subject site, the results of the visual tree

assessment has provided the scientific basis to develop a short-term and

long-term tree care package. The detailed recommendations on

arboricultural works for individual trees have been elaborated in Section

3.1. They could be condensed as follows.

4.1.1 Maintenance of T1

A small tree with little structural or decay problem and a low risk level. It

is vigorous and robust and relatively free from major problems. Two small

branches have been recommended for corrective pruning to pre-empt

aggravation into serious structural problems in the future. Thus far, the

tree has received little pruning in the past, hence it has escaped from the

common damages of improper pruning that have been imposed widely on

local trees.

The performance of this tree is contingent upon the quality of future tree

care. Unnecessary, indiscriminate and unprofessional pruning should be

assiduously avoided. It should be given the chance to grow up of its own

accord as a robust wall tree without the undue disturbance of unnecessary

tree works and other impacts. The modern design of the wall with joints

tightly sealed by mortar would restrict the ability of roots to penetrate into

the aft-soil. As a result, the tree’s longer term prospect could be curtailed

by the lack of new sources of water, nutrient and anchorage.


This is the largest of the six trees and one of the largest and finest

exemplar of stonewall trees in Hong Kong. It signifies the landmark if not

the signature specimen tree of the site. It is one of the largest trees in the

neighbourhood, even though it finds it footing on an unusual vertical

habitat rather than on the ground. The tree is overall in good health and

has a well-formed scaffold. The risk level is rated as low. Some branches

are suffering from structural weaknesses or decay, most of which were

associated with a long period of improper pruning and lack of proper care

98

in the past. These accumulated defects have been identified and a

comprehensive range of arboricultural treatments have been

recommended to rectify them (cf. Section 3.1.3).

A particularly alarming improper treatment is the systematic and repeated

removal of almost all the lower branches over a considerable period. This

type of erroneous and unprofessional pruning is widely practised by local

landscape contractors. The loss of lower branches over an extended period

would suppress the ability of the tree to develop a normal taper. With

inadequate thickness at the trunk base but continual expansion of the

crown, the tree could suffer from the risk of snapping. It is regrettable that

the damage has been quite thoroughly done.

This defect is not easy to correct and it may take a long time to nurture

some new lower branches. It is a somewhat probabilistic venture because

one can only choose from the new epicormic growths to identify the

potential future new branches. If the tree sends out few epicormics or few

qualified epicormics, it may not be possible to accomplish the task in

good time. The exercise demands a high level of skill, supervision and lots

of patience, and it has to be consistently applied over some years to take

effect.


This is a semi-mature Chinese Banyan with a much confined scaffold

development. Two of its three trunks have been truncated. The remaining

one constitutes the bulk of the tree’s biomass, which has been reinforced

by clusters of lignified aerial roots linking between branches and from

branches to trunks. The crown development is limited in terms of its

spread and branch density. It has lost a notable number of limbs and main

branches. The live crown ratio is low, and its vigour is rated as average.

No visible symptoms could be discerned to indicate high risk. The hazard

rating is classified as low. The tree can be helped by removing trunk A

which has been badly decayed with no branches except some sparse

sprouts. The remaining stump of trunk C cannot be removed as it lends

crucial support to trunk B via a bundle of lignified aerial roots. The

truncation tip of trunk C with advanced decay could be removed. Some

wounds left by past limb or main branch removal could be carefully

trimmed. The burden on two branch removal wounds could be alleviated

by selective pruning of their epicormic daughter branches.

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This is the relatively stronger tree amongst the group of four stonewall

trees growing at the eastern part of the wall. The twin-stemmed tree has

strong trunks well reinforced by lignified aerial roots. No trunk is missing

or has been truncated. The crown development, stifled by competition for

space with neighbour trees, has been curtailed. A notable number of limbs

and main branches have been removed or have broken off. Most branch

loss wounds are beset by decay. The foliage density is sparse and the live

crown ratio is rather low at less than 50%.

Trunk A is tilted at 52 degree towards Bonham Road and trunk B at 49

degree towards St Stephen’s Lane. Both leaning angles are the highest

amongst the six trees. The bulk of the tree’s crown weight is carried by

trunk A. It is alarming that the trunk base has been slightly detached from

the wall face. It indicates the incipient failure of the root anchorage. This

critical defect could aggravate and lead to tree collapse in the worst-case

scenario. Something concrete and effective must be implemented as soon

as possible to support the tree and stop the drop. A cable bracing system is

proposed, preferably to be anchored on the nearest building at St

Stephen’s Lane. If this is not feasible, a strong steel frame can be mounted

on the ground of St Stephen’s Lane adjacent to the parapet wall. If cable

bracing is found not feasible, the last resort is to construct a specially-

designed spacing-saving steel frame at the narrow Bonham Road

pavement to prop the tree.

The decayed and fractured wounds should be trimmed and cleaned up.

The end weight of tipped branches carrying heavy loads of epicormic

replacement branches should be appropriately reduced. Stubs should be

removed.


This is the feeblest of the four trees found at the eastern part of the wall.

The single trunk of the slender tree supports a disproportionally small

crown that is squeezed tightly between T4 and T6. Most of the lower

limbs and branches have been removed or lost. The cutting wounds have

been infected by wood-decay fungi. They need to be cleaned up by

careful trimming or remedial pruning. Two specific structural defects

demand attention, including the V-crotch between the only two ascending

limbs, and a seam with bulgewood on the heavy limb.

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The most critical issue is the development of large basal cavity with

continued wood decay on its greatly reduced residual wall thickness. The

tree has responded to the reduction in wood strength by developing strong

response wood on and around the cavity. It has also formed two strong

lignified aerial roots around the trunk base and a thickened root prop

below the trunk base to provide compensatory reinforcement. Despite

these self-help by the tree itself, the high pedestrian vehicular volume at

the site demands additional assurances. A cable bracing system is

recommended to pull the tree towards the south. The cable preferably

should be anchored on the adjacent building at St Stephen’s Lane. If it is

not possible, a strong steel frame can be mounted on the edge of the

parapet wall at the Lane’s edge to hold the cable.


This is situated at the far eastern end of the wall. The main trunk of the

twin-stemmed tree is tilted heavily towards the east and Bonham Road.

Akin to its companions on the same wall section, its crown development

has been stifled due to the lack of growth space. With freedom from

competition, its eastward crown extension is notably better. The resulting

crown is highly asymmetrical, with most of the biomass concentrated on

the east side. Due to the heavy pedestrian and vehicular traffic at the site,

the tree can be rendered safe by installing a simple cable bracing system

that is similar to the one recommended for T5. The branch removal

wounds with decay and the heavy epicormic growths can be trimmed.

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4.2 Long-term practical tree maintenance measures

After conducting the long overdue remedial tree works, future tree care

must follow the best international practice to prevent introducing or

inducing new problems. In the long term, measures could be taken to

enhance their growth and safety. Some generic measures can be

implemented to enhance long-term welfare of the six stonewall trees.

4.2.1 Install soil strips at wall crest and toe

A soil strip could be installed at the crest to promote growth of new roots

so as to reinforce tensional pull at the critical position. A strip of the

concrete paving at the wall crest adjacent to the trunk base of the six

stonewall trees can be removed to expose the underlying soil. If necessary,

soil amendments and surface mulches can be added to improve the soil

condition. Roots have the tendency to grow backwards into the soil strip

to provide a new means of tension pull to strengthen tree stability. The

additional supply of water and nutrients will also facilitate tree growth

and health.

Similarly, a soil strip can be installed at the wall toe to enlarge the soil

catchment volume for additional supply of water and nutrients. It means

removal of the present U-channel at the wall toe for T3, T4 and T5. The

surface water at present drained away via the U-channel can infiltrate into

the soil via the new soil strip. For T1, T2 and T6, the unit pavers laid

against the wall toe could be removed to expose the soil and facilitate root

entry into the soil. This new soil strip can bring immediate benefits to T2

to T6, because their surface roots have already reached the pavement

level and penetrated into the soil under it. More new roots are expected

to make use of the new opportunity to grow into the soil strip.

However, for T1 which is still small and does not yet have long surface

roots extending to the wall toe, the soil strip installation could be

postponed. In the long term, the surface roots of T1 could eventually

grow into the wall toe soil strip to furnish a new and important source of

sustenance to the tree. As the wall supporting T1 is of modern design with

little accessible joints for root penetration, the provision of soil strips at

both the wall crest and wall toe can notably enhance its future growth.

They can literally provide a new lease of life to the otherwise sequestered

tree roots.

To preserve the walkable width at the narrow pavement and to avoid

human-foot trampling of soil, a metal grille could be placed on the soil

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strip. The advice of a geotechnical engineer should be sought in the

design of the soil strips. If a continuous soil strip is not feasible, an

intermitted strip could be adopted instead.

4.2.2 Remove joint seals to permit new root penetration

The continued entry of roots into joints is crucial to provide a new crop of

roots for the tensional pull of the tree against overturning. As the trees

grow bigger, they need to have a corresponding increase in roots to hold

their biomass and capture more water and nutrients. However, the joints

between masonry blocks on the old retaining wall have been sealed

thoroughly in recent years by cement, thus stopping their penetration by

new tree roots. The rigid cement seal plastered around existing roots also

restricts their diameter expansion and may cause girdling injury as they

continue to thicken. The wall supporting T1 is an outlier as it is of

modern design with joints originally sealed by mortar.

Losing the ability to send new roots into the aft-soil and at the same time

losing existing roots could jeopardize tree stability. Where the roots are

physically obstructed, localized removal of the cement seal could be

considered to permit some new roots to grow into the joints and existing

roots to expand. This way, the continued growth and stability of the trees

will not be unduly suppressed. Such a relief can release the trees from a

stifling if not daunting bondage, and ameliorate a potential risk factor

faced by the stonewall trees.

4.2.3 Prevent wedging effect on tree stability

The organic litter and rubbish that tend to accumulate in the gap between

the trunk base and wall face could pose a danger to the trees. As a

stonewall tree tends to swing slightly in strong wind, the gap could be

temporarily widened by a small bit to allow the accumulated debris to

drop further down into the crevice. The trapped debris would create the

wedging effect to prevent the tree to bounce back to its original position.

The process could be repeated, and the cumulative effect of repeated

wedging could be alarming.

Gradually, the gap could be widened to jeopardize tree stability. The

debris in the gap should be removed diligently and thoroughly on a

regular basis to forestall a precipitating cause of stonewall tree

detachment which may lead to tree failure. A non-destructive method

using a trash picker in conjunction with a brush with long and somewhat

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firm bristles could be employed to clean the gap on a regular basis. In

addition to preventing the wedging effect, the cleaning could prevent the

accumulation of moisture in the gap so as to avoid fungal growth at the

critical trunk base position.

4.2.4 Stop repeated removal of lower branches

All the trees except the very young T1 have lost an inordinate amount of

lower branches from their trunks. This wholesale loss of lower branches

is a rather common phenomenon afflicting other trees in Hong Kong. It is

unlikely that natural causes such as strong wind could preferentially

remove the lower branches so thoroughly on so many trees. Field

observation of pruning works in action would reveal the widely-adopted

but erroneous pruning practice.

This highly undesirable pruning method is probably based on the

mistaken idea that the lower branches are relatively shaded and hence

they play little role in photosynthesis and plant food production. It is also

possible that lower branches are relatively easy to reach and remove in

comparison with the higher-up branches. The common use of the

hydraulic platform in pruning work in Hong Kong, instead of employing

a tree climber, means that the upper part of the crown of large trees could

not be easily reached. This may induce the concentration of pruning in the

lower reachable parts of the crown.

The repeated removal of lower branches can reduce the natural ability of

trees to develop a proper trunk taper. The base of the trunk, deprived of

sufficient and timely increase in diameter, in time may not be able to

support the expanding crown with increasing weight. Thus an originally

healthy and well-structured tree could be unintentionally and

unknowingly transformed over the years into a potential hazard tree by

this unprofessional pruning practice. The affected tree is prone to snap at

the trunk base. Contractors could be coached the proper pruning methods

and be weaned from the harmful approach. Briefing and close supervision

of contractors are necessary to forthwith stop this rather pervasive

negative impact.

4.2.5 Stop unprofessional and unnecessary pruning practice

Unnecessary removal of non-defective and non-hazardous branches

appears to be rather common in tree care works in Hong Kong, including

the management of the six stonewall trees. Every pruning task should

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have a clearly considered and explained objective, which should be based

firmly on detailed scientific assessment of the tree in the field. The

principal reasons for pruning should be removal of defective branches and

abatement of hazard or nuisance. Pruning without a defined objective is

akin to shooting without aiming.

A pruning proposal that can result in significant reduction in crown size,

such as crown lifting, crown lowering, crown shaping, crown reduction,

and substantial truncation and shortening of large branches, must not be

approved if they are not supported by scientifically justifiable reasons. A

pruning proposal that can incur deformation of the natural crown form,

such as wholesale removal of branches in a given part of the crown, must

not be approved. Contractors should not be given a free hand to prune

without a well justified written pruning plan, which must be approved by

a relevant professional officer who is equipped with specialist tree

knowledge and practice experience.

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4.3 Preventing grave impacts of excavation

Excavation work at the wall crest platform and wall toe can impose grave

harms on tree roots and hence on tree health and stability. This source of

potential massive injury which could irreversibly dampen and destabilize

stonewall trees must be avoided. If it cannot be avoided, the excavation

must follow strictly the following guidelines to minimize its short- and

long-term impacts:

Demarcation of tree protection zone (TPZ) around the affected

stonewall tree. A recommended TPZ should cover the wall toe

platform, wall face and wall crest platform, with a radius equal to the

average crown diameter. It should include the soil below the wall toe

platform down to 1 m, and the soil behind the wall face and below the

wall crest platform. It should also include the air space above the TPZ

to exclude impacts on the crown.

As far as possible, do not permit excavation in the TPZ. Alternative

location, routing and detour for the proposed excavation must be

seriously considered.

Excavation should be located at a maximum possible distance from

the TPZ.

Digging within the TPZ should only be adopted as absolutely the last

resort.

Excavation depth and width should be minimized.

The duration of opening must be minimized.

If the work has no choice but to intrude into the TPZ, the no-dig, or

trenchless, or micro-tunnelling, or precision directional drilling

techniques should be employed in lieu of open trenching within the

area defined by the TPZ.

Within the TPZ, only the required pipe or cable should be installed.

Other ancillary but obtrusive installations such as junction boxes

which will cut roots and permanently eliminate soil volume for root

growth must not be placed inside the TPZ.

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Assess impact of excavation on tree stability, and if necessary, provide

cable bracing or propping to prevent tree failure due to the degradation

of root anchorage.

Install hard, rigid and effective protective shields to prevent

mechanical damage to any tree parts, including roots, trunk, branches

and foliage. They should be removed as soon as they have served their

purpose.

Prevent rainwater or construction wastewater from flowing into the

opened pit or trench. Install water-tight bunds around the TPZ

perimeter to prevent flowing of water from the work site into the TPZ.

Use manual tools for excavation in and near TPZ. Light-weight and

low-impact pneumatic digging machines could be used only for

breaking the paving, and should be approved before any operation.

After rainfall, water that stands in the pit or trench (not draining away

by gravity) for more than 30 minutes should be pumped out within 30

minutes.

Take away tall excavated paving materials from the site; do not dump

them into the pit or the trench.

Minimize the cutting of roots encountered in the course of digging.

Ensure proper protection of roots encountered in the course of digging

from mechanical damage and desiccation. All exposed roots should

be wrapped by three layers of hessian and should be kept moist all the

time by spraying with water.

Refill the pit or trench with a good quality soil mix.

Prevent soil degradation and root damage in the course of installing

the new paving.

Depending on the specific conditions of the tree, wall and environ, a

dedicated method statement must be developed and approved before

any excavation work can start.

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4.4 Guidelines to inspect and monitor tree structural stability and health

The visual tree assessment method, specially developed to cater to

stonewall trees, has been adopted in this study (cf. Tables 1 to 6). By

applying this dedicated method, stonewall trees could receive a

comprehensive, scientific and objective assessment that is commensurate

with the unique tree and habitat characteristics. In the course of field

assessment, individual trees can be subject to a systematic and thorough

evaluation of all the major tree parts. The table is divided into nine

sections to organize the field assessment in a logical sequence, and to

ensure that all the critical features are covered.

Part A covers the basic attributes of the wall, habitat and environs, as well

as recording the pertinent measurements of the wall and tree dimensions

and related geometrical properties. Part B prompts the assessor to inspect

the main components of the tree, beginning with the surface roots, aerial

roots, and ground roots at wall toe. Part C draws the assessor attention to

the crown and the main scaffold. Part D tackles the trunk. Part E focuses

on the crotch between the trunk and limbs, and Part F the limbs and main

branches.

Integrating the assessment results of Parts A to F, Part G conducts a

hazard tree assessment to rate the tree and establish its hazard rating score.

Part H begins to apply the knowledge acquired thus far to practical tree

management. It offers a wide range of specific arboricultural

recommendations to ameliorate the identified tree problems and to

improve tree growth and safety. Part I is linked to Part H to explain the

rationales of the critical recommendations, and elaborate on the treatment

methods.

The stonewall tree assessment should be implemented by a tree specialist

with conceptual and practical experience in this special type of habitat and

their companion trees. All recommended arboricultural works should be

implemented by a skilful and experienced arborist. The minimum

qualifications of the arborist should be a recognized degree in a relevant

field, including arboriculture, forestry, horticulture, ecology or botany. It

should be accompanied by not less than three years of relevant experience

acquired in Hong Kong or in the humid-tropical Asian region with similar

tree species composition and climatic conditions. Besides the above

formal training and qualification, a relevant certification by a recognized

international or national tree care professional body is required.

The critical issues identified in the baseline survey could be given special

attention in the fellow-up monitoring surveys. Thereafter, the stonewall

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trees can normally be inspected once per year using the dedicated method.

For trees beset with high risk factors, the inspection frequency should be

raised to half-yearly or quarterly. In other words, the inspection frequency

could be increased in tandem with the degree of hazard. New critical

issues should be recorded in the course of the monitoring surveys. For

trees affected by construction activities, the monitoring frequency should

be monthly. The monitoring survey should be conducted as soon as

possible after a typhoon strike with signal 8 and above.

As tree risk assessment is dependent very much on clear visual images,

the baseline survey should build up a detailed photographic record.

Subsequent monitoring surveys should take photographs of existing and

new critical issues. All photographs should be of high quality and high

resolution of preferably not less than 2M per file and in jpeg format. The

photographs should be amply annotated. The relevant items in the

assessment report should be liberally linked to the photographs by citing

the reference number.

The approach begins with collection of comprehensive data on each tree,

from which practical tree care recommendations are inspired or distilled.

Officers with a sound tree knowledge base could learn the technique and

apply it to stonewall trees. It will be helpful if the officer charged with this

duty could learn the concepts and practice in both the classroom and the

real-world environment.

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5. EXECUTIVE SUMMARY

5.1 Preamble

Six stonewall trees are dwelling on an old masonry retaining wall at a

prominent location at Bonham Road near the Park Road junction. The

Highways Department has been assigned the management responsibility of

the wall and its companion trees. The road, the wall and at least one of the

trees have existed in the neighbourhood for more than a century. With

historical and heritage significance, they furnish outstanding landmark

features and conjure up collective memory for the community. As prized

assets, they deserve special attention and care to ensure their sustainability.

The important stonewall trees have been managed on a regular basis to

maintain their health and vigour. As stonewall trees have rather unique

growth habits and habitat conditions, general knowledge and practice in

tree science are not directly applicable. A detailed and dedicated scientific

study could yield useful information and insights to further enhance the

quality and effectiveness of management.

As the road under the trees is heavily used by both pedestrians and

vehicles, it is necessary to ensure that they are stable and safe. They call

for a thorough risk assessment using the state-of-art methodology. From

the results and findings of the comprehensive study, specific arboricultural

recommendations could be distilled to tackle tree growth and structural

issues. They can also provide a firm basis to develop a long-term

management strategy.

5.2 Study objectives

This study has been commissioned by the Highways Department to fulfil

eight key objectives:

Review the general development of stonewall trees on retaining

structures in Hong Kong.

Analyse the effect of stonewall trees on retaining structures based on

tree failure records.

Identify the factors affecting the stability and health of stonewall trees

based on literature review.

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Conduct detailed tree risk assessment of the six subject trees.

Appraise the value and potential risks of the six trees to the local

community.

Review the potentials and limitations in preserving them.

Formulate short and long term maintenance measures to enhance their

sustainability.

Prepare guidelines to inspect and monitor the structural stability and

health of the stonewall trees.

The deliverables include a report and a presentation at a seminar. The main

results, findings and recommendations are condensed below.

5.3 Background and general concerns

The extensive construction of masonry retaining structures echoes a

pragmatic response to our urban development history in a difficult

terrain.

Stone retaining walls embedded in built-up areas offer artificial cliff

habitats that are colonized by spontaneous vegetation.

Due to the highly stressful habitat conditions, the probability of

successful tree establishment on stone walls is rather low.

Banyan trees, especially those with strangler fig characters, are pre-

adapted to grow on the harsh vertical habitats.

Stonewall trees constitute an outstanding landscape and ecological

endowment of the city.

No other place in the world has such a rich endowment of stonewall

trees, making them particularly valuable as both cultural and natural

heritage.

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5.4 Effect of stonewall trees on retaining structures

Past retaining wall failures as indicated by records were largely

attributed to inadequacies in wall design, workmanship and

maintenance.

Most stone wall failures did not involve stonewall trees, and stonewall

trees were attributed as one of the possible causes in the collapse of a

small wall.

There is no record of stonewall tree collapse causing notable damage

to masonry retaining walls.

Stonewall tree failure could be induced by human disturbance of

cutting of critical tension roots.

Stonewall trees could collapse in strong wind due to inadequate growth

of tension roots through the joints into the soil behind the wall.

A dense network of surface roots covering the stone wall face may

protect and stabilize the masonry structure.

5.5 Factors on tree stability and health

Intrinsic wall factors include gaps between masonry blocks; stone size

and length of joints; stone shape and horizontal joints; wall inclination;

wall height; original joint filling; cement sealing of dry wall joints;

weathering state of masonry blocks; seepage on wall face; and weep

holes.

The quality of the wall environs exerts its influence through the factors

of moisture supply of aft-soil; volume of aft-soil; quality of aft-soil;

wall-crest slope with unsealed soil; wind exposure; solar access; air

quality; and proximity to buildings and roads.

The extrinsic impacts on walls and environs includes soil nail

installation and grouting damage; stabilization treatment of contiguous

wall crest and toe slopes; soil disturbance at contiguous paved areas at

wall toe and wall crest; skin wall impact; and trenching damage of tree

roots at wall crest and wall toe.

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The stonewall tree factors include tree species; tree dimensions; tree

biomass structure; tree lean; root growth habit; and quality of pruning

work and tree maintenance.

5.6 Tree values, potential risks and preservation

The value of stonewall trees is due to their status as world-class and

unique urban ecological gem, spontaneous nature in built-up areas,

multiple amenity functions to people, historical and heritage

significance, and diminishing and threatened resource.

The risk to local community is related to wall instability and tree

instability; historical records indicate minimal contribution of tree

failure to wall failure; in-depth understanding, assessment of tree risk

coupled with preventive and ameliorative measures could minimize

tree failures and their impacts.

Tree preservation is favoured by site conditions with little conflicts

with surrounding buildings and roads; welcomed ecosystem services

bestowed on the neighbourhood; sentimental attachment of local

community to the trees and hence support of tree preservation.

Tree preservation is limited by the lack of space and anchor positions

for cable bracing or installation of propping frame, and the need to

puncture the heritage wall feature if the propping option is chosen.

5.7 Tree risk assessment and arboricultural recommendations

T1: Small tree with few structural and decay problems; high live crown

ratio; free from legacy of past unprofessional pruning; only requires

minor corrective pruning.

T2: Largest, oldest and most outstanding stonewall tree; significant

height, crown spread and trunk diameter; well balanced and quite full

crown development; one of the top-ranking stonewall trees in Hong

Kong; designated as OVT; well-formed scaffold with three codominant

trunks; multiple branch defects and wound decay induced largely by

past poor pruning practice; some heavy epicormic load on decayed

tipping wounds; excessive removal or loss of lower branches with

implications on tree stability; no visible imminent high risk symptoms.

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T3: Two of three trunks truncated with decayed tip; limited and

confined crown development; extensive loss of limbs and branches;

excessively loss of lower branches; low live crown ratio; liberal

reinforcement of tree structure by bundles of lignified aerial roots;

most branch loss wounds beset by decay; two heavy epicormic

branches; no visible imminent high risk symptoms.

T4: Twin stemmed tree with relatively stout trunks; heaviest lean of

the six trees; low live crown ratio; confined crown development;

wound decay at branch removal wounds; heavy epicormic branches

attached to decayed tipping wounds; excessive removal or loss of

lower branches; alarming risk due to incipient anchorage failure; needs

support by cable bracing or propping.

T5: Single trunk; high height to DBH ratio; low live crown ratio;

narrow and confined crown; only two limbs and few branches;

compression fork between two limbs; decayed wounds; alarming risk

due to large basal cavity partly compensated by response wood at

cavity edge, two lignified root stands and a natural root prop; needs

support by cable bracing or propping.

T6: Twin-stemmed tree; one trunk carrying the bulk of the crown load

is heavily tilted; low live crown ratio; excessive removal or loss of

lower branches; decayed branch loss wounds; risk due to unbalanced

biomass distribution plus heavy lean; needs support by cable bracing.

5.8 Long-term practical tree maintenance measures

Install soil strips at wall crest and toe to facilitate root growth to

stabilize anchorage and increase supply of water and nutrients.

Remove joint seals at suitable locations to permit new root penetration

and existing root expansion, so that the continued increase in tree size

and weight could be matched by corresponding increase in new roots.

Prevent wedging effect on tree stability by regular removal of organic

debris and rubbish deposited at the gap between trunk base and wall

face.

Stop repeated removal of lower branches and unprofessional and

unnecessary pruning practice by demanding well-justified tree pruning

proposals and reinforcing coaching of contractors.

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Prevent extremely harmful impacts of excavation in the tree protection

zone of stonewall trees, and establish strict guidelines on work inside

the zone.

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REFERENCES

Agriculture, Fisheries and Conservation Department, 2004. Check List of Hong Kong

Plants 2004. Hong Kong Herbarium, Bulletin 1, Government of HKSAR.

Agriculture, Fisheries and Conservation Department, 2007, 2008, 2009, 2011. Flora

of Hong Kong, Volumes 1 to 4, edited by Hong Kong Herbarium and

South China Botanical Garden, Government of HKSAR.

Aslan, M., Atamov, V., 2006. Flora and vegetation of stony walls in south-east

Turkey (Şanliurfa). Asian Journal of Plant Sciences 5, 153-162.

Bolton, D.E., 1985. Living walls of Exeter. The Journal of the Devon Trust for Nature

Conservation 6, 51-66.

Breuste, J., Feldmann, H., Uhlmann, O., 1998. (Eds.) Urban Ecology. Springer, Berlin.

Chan, Y.C., 1996. Study of Old Masonry Retaining Walls in Hong Kong. GEO

Report No. 31, Geotechnical Engineering Office, Civil Engineering

Department, Hong Kong.

CEDD, 2012. Slope Information System. Civil Engineering and Development

Department, HKSAR Government.

http://hkss.cedd.gov.hk/hkss/eng/slopehttp://hkss.cedd.gov.hk/hkss/eng/slopei

nfo/disclaimer1.asp?type=sisinfo/siswelcome.asp, accessed on 01 August

2012.

Clark, J.R., Matheny, N.P., 1993. A Handbook of Hazard Tree Evaluation for Utility

Arborists. International Society of Arboriculture, Savoy, IL, 34 pp.

Darlington, A. (Ed.), 1981. Ecology of Walls. Heinemann Educational, London.

Deakin, R., 1855. Flora of the Colosseum of Rome. Groombridge and Sons, London.

de Neef, D., Stewart, G.H., Meurk, C.D., 2008. Urban biotopes of Aotearoa New

Zealand (URBANZ) (III): spontaneous urban wall vegetation in Christchurch

and Dunedin. Phyton Horn Austria 48, 133–154.

dos Reis, V.A., Lombardi, J.A., de Figueiredo, R.A., 2006. Diversity of vascular

plants growing on walls of a Brazilian city. Urban Ecosystems 9, 39-43.

Duchoslav, M., 2002. Flora and vegetation of stony walls in East Bohemia (Czech

Republic). Preslia, Praha 74, 1-25.

http://hkss.cedd.gov.hk/hkss/eng/slopehttp:/hkss.cedd.gov.hk/hkss/eng/slopeinfo/disclaimer1.asp?type=sisinfo/siswelcome.asp

http://hkss.cedd.gov.hk/hkss/eng/slopehttp:/hkss.cedd.gov.hk/hkss/eng/slopeinfo/disclaimer1.asp?type=sisinfo/siswelcome.asp

116

Figueiredo, R.A., Motta Junior, J.C., Vasconcellos, L.A.D.S., 1995. Pollination, seed

dispersal, seed germination and establishment of seedlings of Ficus

microcarpa, Moraceae, in southeastern Brazil. Revista Brasileira de Biologia

55, 233-239.

Francis, R.A., Hoggart, S.P.G., 2009. Urban river wall habitat and vegetation:

observations from the River Thames through central London. Urban

Ecosystems 12, 465-485.

GCW, 2011. Ficus microcarpa (Moraceae). Global Compendium of Weeds,

http://www.hear.org/gcw/species/ficus_microcarpa, accessed on 20 September

2011.

Geotechnical Engineering Office (GEO), 1993. Guide to Retaining Wall Design,

second edition. Geoguide 1, Civil Engineering Department, Hong Kong

Government, Hong Kong.

Geotechnical Engineering Office (GEO), 2010. Review of the 3 June 2007 failure of a

masonry retaining wall on hillside adjacent to no. 84 Peak Road. GEO Report

No. 250, Geotechnical Engineering Office, Civil Engineering and

Development Department, Geotechnical Engineering Office,

Geotechnical Engineering Office (GEO), 2011. Study on Masonry Walls with Trees.

GEO Report No. 257, CM Wong & Associates, Geotechnical Engineering

Office, Civil Engineering and Development Department, The Government of

the HKSAR.

Geotechnical Engineering Office (GEO), 2011. Technical Guidelines on Landscape

Treatment of Slopes. GEO Publications No. 1/2011. GEO, Civil Engineering

and Development Department, the Government of the Hong Kong Special

Administrative Region.

Geotechnical Engineering Office (GEO), 2011. Study on Masonry Walls with Trees.

GEO Report No. 257. GEO, Civil Engineering and Development Department,

the Government of the Hong Kong Special Administrative Region.

Ghosh, R.B., 1960. Preliminary observations on the flora of dilapidated walls and

buildings of Calcutta and its suburbs. Journal of Indian Botanical Society 39,

584-557.

Ghosh, R.B., Das, D., 2002. Observation in seasonal variation of wall vegetation of

the district of Bankura, West Bengal. Journal of Economic and Taxonomic

Botany 26, 557-561.

117

Gilbert, O., 1992. Rooted in Stone: The Natural Flora of Urban Walls. English Nature,

Peterborough, UK.

Gilman, E.F., 2002. An Illustrated Guide to Pruning, 2rd ed. Delmar, Thomas

Learning, Albany, NY.

Hendry, A.W., Khalaf, F.M. (Eds.), 2001. Masonry Wall Conservation. Spon, London.

Hruška, K., 1987. Syntaxonomical study of Italian wall vegetation. Vegetatio 73, 13-

20.

IFLA, 2011. Ficus microcarpa, laurel fig. Institute of Food and Agricultural

Sciences, University of Florida, http://plants.ifas.ufl.edu, accessed on 20

September 2011.

International Society of Arboriculture, 2011. Glossary of Arboricultural Terms. ISA,

Champaign, IL.

Jim, C.Y., 1990. Trees in Hong Kong: Species for Landscape Planting. Hong Kong

University Press, Hong Kong.

Jim, C.Y., 1994. Champion Trees in Urban Hong Kong. Hong Kong Flora and Fauna

Series, Urban Council, Hong Kong.

Jim, C.Y., 1998. Old stone walls as an ecological habitat for urban trees in Hong

Kong. Landscape and Urban Planning 42, 29-43.

Jim, C.Y., 2006. Chinese Banyan: From Wishing Trees to Wishes for Trees.

Agriculture, Fisheries and Conservation Department, Friends of the Country

Parks and Cosmos Books, Hong Kong.

Jim, C.Y., 2008. Urban Greening in Hong Kong: Exemplary Designs and Delights.

C&C Joint Printing and Green Fun Committee, Hong Kong

Jim, C.Y., 2008. Urban biogeographical analysis of spontaneous tree growth on stone

retaining walls. Physical Geography 29, 351-373.

Jim, C.Y., 2010. Old masonry walls as ruderal habitats for biodiversity conservation

and enhancement in urban Hong Kong. In: Müller, N., Werner, P., Kelcey, J.G.

(Eds.) Urban Biodiversity and Design. Blackwell Publishing, Oxford, pp. 323-

347.

118

Jim, C.Y., Chen, W.Y., 2010. Habitat effect on vegetation ecology and occurrence on

urban masonry walls. Urban Forestry and Urban Greening 9: 169-178.

Jim, C.Y., Chen, W.Y., 2011. Bioreceptivity of buildings for spontaneous arboreal

flora in compact city environment. Urban Forestry and Urban Greening 10:

19-28.

Karschon, R., Weinstein, A., 1985. Wall flora and vegetation at Qal‘at Nimrud, the

Castle of Banyas. Israel Journal of Botany 34, 59-64.

Kent, D.H., 1961. The flora of Middlesex walls. The London Naturalist 40, 29-43.

Kolbek, J., Valachovic, M., 1996. Plant communities on walls in North Korea: a

preliminary report. Thaiszia Journal of Botany 6, 67-75.

Laman, T.G., 1995. The ecology of strangler fig seedling establishment. Selbyana 16,

223-229.

Larson, D.W., Matthes, U., Kelly, P.E., Lundholm, J.T., Gerrath, J.A., 2004. The

Urban Cliff Revolution: New Findings on the Origins and Evolution of

Human Habitats. Fitzhenry and Whiteside, Markham, Toronto.

Lisci, M., Pacini, E., 1993. Plants growing on the walls of Italian towns 1. sites and

distribution. Phyton 33, 15-26.

Lisci, M., Pacini, E., 1993. Plants growing on the walls of Italian towns 2.

reproductive ecology. Giornale Botanico Italiano 127, 1053-1078.

Lo, H.L., 1971. The early history of masonry works in Hong Kong and their

relationship with local construction industry. Food and Commodity Monthly 9,

459-462.

Lonsdale, D., 1999. Principles of Tree Hazard Assessment and Management. HMSO,

London.

Matheny, N.P., Clark, J.R., 1994. A Photographic Guide to the Evaluation of Hazard

Trees in Urban Areas, 2nd ed. International Society of Arboriculture, Savoy,

IL.

Mattheck, C., 2002. Tree Mechanics. Karlsruhe Research Centre, Karlsruhe, Germany.

Mattheck, C., 1998. Design in Nature: Learning from Trees. Springer, Heidelberg,

Germany.

119

Mattheck, C., Breloer, H., 1994. The Body Language of Trees. HMSO, London.

McPherson, J.R., 1999. Studies in urban ecology: strangler figs in the urban parklands

of Brisbane, Queensland, Australia. Australian Geographical Studies 37, 214-

229.

Morgenstern and Geotechnical Engineering Office, 2000. Report on the Kwun Lung

Lau landslide of 23 June 1994. GEO Report No. 103, Geotechnical

Engineering Office, Civil Engineering and Development Department, The

Geotechnical Engineering Office,

Payne, R.M., 1978. The flora of walls in south-eastern Essex. Watsonia 12, 41-46.

Pokorny, J.D. (coordinating author), 2003. Urban Tree Risk Management: A

Community Guide to Program Design and Implementation. USDA Forest

Service Northeastern Station, St. Paul, MN.

Rishbeth, J., 1948. The flora of Cambridge walls. Journal of Ecology 36, 136-148.

Shigo, A.L., 1983. Tree Defects: A Photo Guide. General Technical Report NE-82,

U.S. Department of Agriculture Forest Service, Northeastern Forest Experiment

Station, Durham, NH.

Shigo, A.L., 1988. A New Tree Biology: Facts, Photos, and Philosophies on Trees and

their Problems and Proper Care, 2nd

ed. Shigo and Trees, Durham, NH.

Shigo, A.L., 1989. Tree Pruning: A Worldwide Photo Guide for the Proper Pruning of

Trees. Shigo and Trees, Durham, NH.

South China Morning Post, 1925. 18 July 1925, 29-31 July 1925, 8 August 1925, 3

and 5 September 1925

Starr, F., Starr, K., Loope, L., 2003. Ficus microcarpa, Chinese Banyan, Moraceae.

United States Geological Survey, Biological Resources Division, Haleakala

Field Station, Maui, HI.

Sukopp, H., Hejný, S., Kowarik, I., 1990. (Eds.) Urban Ecology: Plants and Plant

Communities in Urban Environments. SPB Academic, The Hague.

Segal, S. (Ed.) 1969. Notes on Wall Vegetation. Dr. W. Junk, The Hague.

Woodell, S., 1979. The flora of walls and pavings. In: Laurie, I.C. (Ed.) Nature in

Cities. Wiley, Chichester, pp. 135-157.

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