The Uniqueness of Habitats in Old Eucalypts:

8/14/2019 The Uniqueness of Habitats in Old Eucalypts:

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November 200917Tasforests Vol. 18

The uniqueness of habitats in oldeucalypts: contrasting wood-decay

fungi and saproxylic beetles of youngand old eucalyptsT. Wardlaw1,2*, S. Grove2, A. Hopkins3, M. Yee1, K. Harrison4 and C. Mohammed2,4,5

1Forestry Tasmania, GPO Box 207, Hobart, Tasmania 70012Co-operative Research Centre for Forestry, College Road, Sandy Bay, Tasmania 7005

3Forest Protection, Scion, Private Bag 3020, Rotorua 3010, New Zealand4Department of Agricultural Science, University of Tasmania, College Road, Sandy Bay,

Tasmania 70055CSIRO Sustainable Ecosystems, College Road, Sandy Bay, Tasmania 7005

*e-mail: [email protected] (corresponding author)

Abstract

Over the past decade, considerable research hasbeen done in Tasmania to beer understand theecological value of old eucalypts in temperate wet

forests with respect to communities of wood-decay fungi and wood-inhabiting (saproxylic)

beetles. Old eucalypts provide unique habitatsfor these organisms, and support a highspecies diversity and distinctive assemblagecomposition. However, the value of old eucalyptsin providing unique habitat extends well beyondthe time the trees are alive. Once fallen, theirdead remains may take several centuries todecompose, providing further habitat for fungiand invertebrates which are the most species-richcomponent of forest biodiversity, including rarespecies with very restricted ranges.

The development of communities of wood-decayfungi and saproxylic beetles in living eucalyptsat dierent stages of their life can be understoodin terms of the stages of crown development.Thus, young trees in the crown-liing phase arecharacterised by white-rot fungi colonising assmall branches are shed. In contrast, older treesin the crown-retraction phase harbour a diversesuite of fungi, many of them causing brownrot, associated with sizeable wounds resulting

from failure of large branches, dieback or re.

A similar transition is seen for beetles, witha diverse community of obligately saproxylicbeetles only appearing in mature trees thatcontain brown rot. The switch that providesconditions suitable for the colonisation of maturetrees by fungi that cause brown rot, whileecologically signicant, remains unresolved.

Importantly, research to date has shown thatthere is a continuum of habitats for decay fungiand saproxylic beetle assemblages in mature treesand the logs that these generate. Maintaining thehabitats provided by large logs therefore requiresa perpetuation of the mature growth stage ofliving trees. This in turn can only be achievedthrough maintaining a diversity of forest agesacross the landscape, and allowing a proportionof those forests to reach maturity at appropriate

scales of space and time.

Introduction

The management of old-growth eucalyptshas divided the Australian public for atleast the past two decades. In Tasmania,particularly, this debate has been hotlycontested, and rarely does an election passwithout the management of old-growth

eucalypts becoming a political issue. Themain focus of the old-growth debate in


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Tasforests Vol. 18 18 November 2009

Tasmania has been the tall, wet eucalyptforests dominated by Eucalyptus regnans, E.obliqua and E. delegatensis. Together, thesethree species sustain Tasmanias hardwoodsawmilling and veneer industries,with supply from Crown (State) forest

mandated through government legislation(Forestry Act 1994). While the massivesize aained by these three species makesthem very valuable for timber products,it also provides a powerful symbol forenvironmentalists: individual trees areanthropomorphised with mystical namessuch as Icarus Dream, Gandalfs Staand El Grande (see www.gianrees.com.au).

For over half a century, foresters haveargued, on ecological grounds, that thesewet eucalypt forests exist only as a result ofperiodic catastrophic disturbance resultingfrom wildres. Stand-replacing wildreproducing even-aged regeneration wasthe accepted paradigm for wet eucalyptforests. The silvicultural practice of clearfellharvesting followed by high-intensity

burning and aerial sowing of seed (CBS)was thus adopted widely, and was touted

as emulating this wildre disturbance.Increasingly, however, it is being recognisedthat in certain aspects CBS does not emulatewildre, and that when done on regular,80-100 year cycles CBS will result in foreststhat are much simpler in structure thanthose resulting from natural wildres(Lindenmayer and McCarthy 2003). Thisis because wildres in wet eucalypt forestsare oen not completely stand-replacing,particularly in areas where there has beena long re-free interval resulting in a mixedforest consisting of a mature eucalyptoverstorey with a rainforest understorey(Turner et al. 2009). This knowledgehas signicant implications for forestmanagement, where the goal is to sustainnot just wood supply but also the manyother values that forests provide, includingthe provision of sucient suitable habitatat appropriate spatial scales to allow the

perpetuation of forest-dependent speciesacross their range (Forestry Tasmania 2008).

The public and scientic focus on thevalue of old-growth eucalypts for speciesdependent on the habitats they provide hastended towards charismatic mammal and

bird species. However, in the wet eucalyptforests of Tasmania and elsewhere, these

groups make up a very small proportion ofthe forest biodiversity. Studies around theWarra Long-Term Ecological Research (LTER)site in southern Tasmania, and based mostlyin wet eucalypt forests, have found thattotal species richness is dominated by muchsmaller life-forms, particularly the beetles (andother invertebrates) and the fungi (Figure 1).Not only do these two groups dominate therichness of species lists in these wet eucalyptforests, they also provide vital roles in

ecosystem function as ecosystem engineers(creating habitat for other species), nutrientrecyclers (Edmonds and Marra 1999) and soil

builders (Lavalle et al. 1997).

Over the past decade, a substantial researcheort has therefore been devoted to a beerunderstanding of invertebrate and fungaldiversity in wet eucalypt forests in southernTasmania, particularly those elementsthat are dependent on old trees and on

structures originating from such trees. Inthis paper, we review that body of researchand draw conclusions that may guide futuremanagement of these forests.

Paerns of tree development associated withageing of eucalypts

Under natural conditions, eucalypt seedlingsin wet forest establish as an age cohort

following a disturbance event sucientlyintense to create large openings in the densecanopy of the standing forest, expose mineralsoil, and reduce populations of seed-feedingants and browsing herbivores. Typically, suchdisturbance events are extensive wildres.While small-scale disturbance, such as tree-falls, can trigger eucalypt regeneration, theseedlings typically become suppressed dueto shading from the surrounding intact forest(Alcorn 2002).


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Regardless of whether temperate weteucalypt forests regenerate from stand-replacing wildres producing single-aged

forests, or from non-stand-replacing resproducing multi-aged forests, each cohortof eucalypt regeneration typically follows

0

30

60

90

0-4 5-6 7-10 15-30 40-80 100-200 200-400+

Age: time since disturbance (years)

Heightrang

e(metres)

Seedling hicket Sapling Pole Spar Mature Over-matureAge class:

Crown-lifting

Crown-deepening

Crown-retraction

Stage of crowndevelopment

Figure 2. Stages in the development ofEucalyptus regnans across its natural life-span (illustrated as heightranges at particular age-classes). Adapted from Ashton (1975).

3 7 11 2157

125

228262

1135

1160

0

200

400

600

800

1000

1200

Amphibians Reptiles Fish Mammals Birds Lichens B ryophytes Vascular Fung i Beetles

Nu

mberofspecies

Figure 1. Number of species in dierent life-form groups recorded in the Warra Long-Term Ecological Research(LTER) site, southern Tasmania (www.warra.com). The number of species shown for the dierent life-forms is notbased on equal sampling eort for each life-form across the range of ecological zones present in Warra. The numbers

of species shown for vascular plants and lichens, in particular, are likely to be underestimates.

Crown-deepening

Crown-lifting

Crown-retraction

Stage of crown

development

Heightrang

e(metres)

90

60

30

00-4 5-6 7-10 15-30 40-80 100-200 200-400

Age: time since disturbance (years)

Age class: Seedling Thicket Sapling Pole Spar Mature Over-mature

Amphibians Reptiles Fish Mammals Birds Lichens Bryophytes Vascular Fungi Beetles

plants

Numberofspecies

1200

1000

800

600

400

200

03

1160

1135

262228

572111

125

7


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a characteristic paern of development(Figure 2). Aer seedlings have becomeestablished three distinct phases of crowndevelopment can be recognised: crown-liing, crown-deepening and crown-retraction(Figure 3).

The crown-liing phase, which occupiesmost of the rst century of development, isa race to the sky, as the light-demandingeucalypts get their heads above theirneighbours and above the dense layer ofregenerating understorey species. Aritionrates are high for individuals, and barelyone tree in ten will make it past the rst 30years (Jackson 1968). During this period ofrapid height growth, branches at the base of

the crown have a relatively short life as theyquickly become shaded. As these branchesdie, they are quickly lost through an ecient

branch-shedding mechanism (Jacobs 1955),

resulting in a straight, branch-free trunk - anaribute much admired by sawmillers.

Aer about 70-100 years since disturbance,the crowns of the surviving eucalypts aregrowing in a more open light-environment;

branches at the base of the crown persist,and the rate of crown-liing slows.This marks the beginning of the crown-deepening phase. Further height growthover the next 100-200 years results in thedevelopment of deep crowns (Bar-Ness2005). Because natural branch senescenceinduced by shading slows down or stopsaltogether, the branches can grow to largesizes. Mortality of these large branches ismuch less predictable, and follows stochastic

events such as wildres (Bar-Ness 2005),dieback episodes triggered by climaticstresses such as drought (Wardlaw 1989),or mechanical failure while the branches

Figure 3. The three distinct phases of crown development in eucalypts: crown-liing (le), crown-deepening(centre) and crown-retraction (right).


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are still alive. In this mature stage (100-300years aer disturbance), the breakage oflarge branches, either dead or alive, or thekilling of patches of the stem followingwildre, produces large wounds that mayeventually develop into hollows.

By about 200 years aer disturbance, theeucalypts have reached their maximumheight (Ashton 1976), and further agingresults in a progressive decline in heightthrough dieback or top breakage. This isthe crown-retraction phase, which continuesuntil the oldest survivors of the cohortreach their maximum life-span of about450 years. While old trees decline in heightduring the laer stages of their life, they do

continue to increase in diameter (Ashton1975), and some grow to become massivelyfat trees of immense volume (see www.gianrees.com.au).

The terms regrowth and mature arewidely used in forestry to group growthstages. The crown-liing and transition tothe crown-deepening phase correspondwith regrowth, while mature correspondswith the majority of the crown-deepening

and crown-retraction phase. The termover-mature corresponds with the crown-retraction phase.

Throughout the natural life-span of a cohortof eucalypts, a proportion of the standingtrees (either dead or alive) fall over or suerstem breakage, and in doing so provide asupply of downed logs to the oor of theforest. Once on the forest oor, the downedlogs, commonly called coarse woody

debris (CWD), slowly decay until they areeventually fully consumed and returned tothe soil organic maer pool. This processof decay follows a recognisable sequence ofdecay stages (Grove et al. 2009):

Decay class 1: Structurally intact oralmost so; bark or small branches stillaached; few signs of wood-decay;wood mostly retains original colour.

Decay class 2: Structurally less intact butstill hard when kicked; small branchesabsent; lile or no bark present; earlysigns of wood-decay or discoloration.

Decay class 3: Clearly decaying but still

supports its own weight; may be slightlyso when kicked; may be hollow inplaces; no bark; moss and fungi may beprominent.

Decay class 4: Cannot support its ownweight; so to kick (but may still behard in places, in which case may beextensively hollow); moss, fungi andinvading roots likely.

Decay class 5: No longer retains originalshape; wood very so or largelydisintegrated; sometimes only outlinevisible beneath moss and invading roots.

Large eucalypt logs can persist on the forestoor for a long time: studies in southernTasmania have found that the progress ofeucalypt logs through these ve stages ofdecay and on to the soil organic maer poolmay take more than two centuries (Grove

et al. 2009). Because of their longevity andthe ongoing injection of fresh CWD, thevolumes of CWD on the oor of temperatewet eucalypt forests can be among thelargest globally, commonly exceeding800 m3 ha-1 (Grove and Meggs 2003).

Successional paerns in wood-decay fungi

Wood-decay fungi establish quite early in

the life of a eucalypt. Studies in regrowthforests by Wardlaw (1996, 2003) foundthat, by age 20-40 years, more than 90% ofvigorous, unsuppressed E. obliqua, E. regnansand E. delegatensis trees had at least onecolumn of wood decay within the lowerstem. Extensive columns of decay haddeveloped in about 20% of these trees. Theoverwhelming majority of decay columnsin these young trees become establishedduring the branch-shedding process via


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dead branches (Wardlaw 1996, 2003) (Figure4), with an increasing probability of decayestablishment with increasing branch size(Wardlaw 2003).

Species within the family Hymenochaetaceae

and a species of Dichostereum dominatethe communities of wood-decay fungiestablishing through dead branches inyoung eucalypts, and accounted for 50% ofthe isolates of wood-decay fungi obtainedin cultures taken from decay columnsthat originated from dead branches inTasmanian wet eucalypt regrowth forests(Wardlaw 2003). Davidson and Tay (2008)also found a species of Hymenochaete wascommon in discoloured wood in young

karri (E. diversicolor) in Western Australia.Armillaria (putativelyA. novae-zelandiae)is the dominant wood-decay fungusestablishing through the roots, where it

causes a white rot of the heartwood. Enzymetests suggest that the majority (>80%) ofwood-decay fungi establishing in youngeucalypts in Tasmania cause white rot(T. Wardlaw, unpublished data); taxa withinthe Hymenochaetaceae all cause white rot

(Hawksworth et al. 1995).

A chronosequence study in a mixed-agestand of E. obliqua within the Warra LTERsite found low levels of wood decay in 69-and 105-year-old cohorts of trees (Hopkinset al. 2006). However, there was a signicantincrease in both the amount of wood decayand the diversity of wood-decay fungiin the oldest cohort (>150 years), whichhad declining crowns characteristic of the

crown-retraction phase. This study foundlile overlap in species composition ofwood-decay fungi between the two youngerage-cohorts and the oldest cohort. Several

Figure 4. The origins of decay columns that had established in the stems of young (20-40 year old) Eucalyptus

spp. in Tasmania. Based on Wardlaw (2003).

Decay entering through

stem wounds

(3% of all origins)

Decay associated

with insect galleries

in the stem

(7% of all origins)

Origins not identifed

(4% of all origins)

Decay

entering through

roots or lower stem

(butt rot)

(8% of all origins)

Decay entering

through the stubs of

dead branches

(52% of all origins)

Decay entering through the

crotch of dead branches


Decay associated with insect

galleries in dead branches



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species from the genus Postia predominatedin the communities of wood-decay fungiwithin the trees over 150 years old, but werelargely absent from younger trees. Althoughtwo species of the Hymenochaetaceae werefound in trees over 150 years old, their

general paucity in the chronosequence studycontrasted with the ndings of Wardlaw(2003) in younger trees in Tasmanian forests.

The contrasts in the assemblages of wood-decay fungi inhabiting younger and matureE. obliqua were also reected in contrasts ofthe type of wood decay. In regrowth trees,the great majority of decay columns werewhite rot (Figure 5), either simultaneouswhite rot or white pocket rot (Hopkins

2007; T. Wardlaw, unpublished data).Brown rot was rare in regrowth E. obliqua,

but became more common in mature trees(Hopkins 2007). The increase in abundanceof brown rot in mature trees coincided withthe prevalence of Postia, a genus widelyconsidered to cause brown rot.

The paern of types of rot found inregrowth and mature E. obliqua trees wasmirrored in downed logs. Yee (2005) foundsignicantly more brown rot in large logsoriginating from mature trees than insmaller logs originating from regrowth

trees (Figure 6). In that study, Yee foundthat columns of brown rot were primarilyconned to the central heartwood regionof the logs, suggesting that the decaycolumns had established while thetree was still standing. This idea wassupported by Hopkins (2007), who foundthat one species of Postia was common todecay columns in both living trees anddead and/or downed logs. Smaller logsoriginating from younger regrowth trees

were dominated by white (pale) rot andwhite pocket rot decay types, typicallyspreading from the outside of the loginwards, suggesting that decay columnsin these logs had established aer the treehad died (Yee 2005).

0 1 2 3 4 5 6

No rot

Discoloured wood

Pale stringy rot

Pale spongy rot

Small pocket rot

Combination pocket and dark rot

Dark blocky crumbly rot (dry)

Dark blocky crumbly rot (wet)

Dark blocky fibrous rot

Number of trees

69 y.o. 105 y.o. >150 y.o.

Brownrot

White

rot

Figure 5. Frequency of occurrence of roen wood types in six trees each of 69-year-old regrowth, 105-year-oldregrowth, and >150-year-old mature E. obliqua. Adapted from Figure 5.3.4 in Hopkins (2007).

69 y.o. 105 y.o. 150 y.o.

Brownrot

White

rot

Dark blocky fbrous rot

Dark blocky crumbly rot (wet)

Dark blocky crumbly rot (dry)

Combination pocket

and dark rot

Small pocket rot

Pale spongy rot

Pale stringy rot

Discoloured wood

No rot

Number of trees

0 1 2 3 4 5 6


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Succession paerns of saproxylic beetles

Lile is known about the diversity ofsaproxylic (dead-wood-dependent) insectsin young regrowth eucalypts. However,Wardlaw (2003) found ample evidence of

stem-borers associated with decay columnsin 20-40 year-old eucalypt regrowth. Nearly20% of the encountered decay columnscontained galleries of wood-boring insects(Figure 4). Nearly two-thirds of the borergalleries were associated with dead

branches, typically in the crotch of thebranch. Cerambycid beetles of the generaPhoracantha and Epithora are thought to

be the main wood-boring insects in thesebranch-crotch galleries (Dick Bashford,

pers. comm.). The genus Phoracanthacontains many species known to aackyoung eucalypts in both native forestand plantation situations, occasionallyachieving pest status and causingconsiderable economic damage (Ellio etal. 1998). These species are seldom found inmature eucalypts, suggesting that they are

specialists on young trees, although theirinitial araction may be to stressed trees.

Older regrowth and mature E. obliquatrees support a rich diversity of beetles.Harrison (2007) recorded a total of 143

species of saproxylic beetles within thesame eighteen E. obliqua trees sampled forfungi by Hopkins (2007). The process of treeaging strongly inuences the assemblagesof beetles inhabiting living E. obliqua trees.In particular, the transition from regrowthhabit (150 years)is associated with marked shis in beetleassemblages, with mature trees having amuch richer diversity of species compared toregrowth trees (Harrison 2007). Nearly 40%

of the total beetle diversity was unique tomature trees (Figure 7). The distinctivenessof mature trees is due mostly to obligatelysaproxylic beetles that live within thestem wood (Harrison 2007). A substantialproportion of that unique saproxylic beetlefauna in mature trees was associated withhollows and dead tops.

Figure 6. Frequency of roen wood types found in 21 large (le) and 21 small (right) E. obliqua logs, all in an

intermediate stage of decomposition. Roen wood types have been grouped according to the region in the log theyoccur. Adapted from Figure 4.4 in Yee (2005). (BR) = brown rot types; (WR) = white rot types.

Number of logs

25 20 15 10 5 0 5 10 15 20

Inner heartwood

Mudguts (BR)

Dark blocky crumbly rot - dry (BR)

Dark blocky fbrous rot (BR)

Discoloured woodHeartwood

Outer heartwood

Localised

Surface

Large logs Small logs

Fibrous surface rot

Jelly surface rot

Wet cracks

Pale pocket rot (WR)

Pale stringy rot (WR)

Yellow dry slaty rot (BR)

Dark cubic friable rot (BR)


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Harrison (2007) found that speciesdiversity among the saproxylic beetles wasstrongly associated with dierent rot-typeswithin E. obliqua trees. A greater numberof species occurred more commonly inwood containing brown rot than in wood

containing white rot (Figure 8). The greatdiversity of saproxylic beetle speciesrestricted to mature trees is correlated withthe strong association between saproxylic

beetles and the brown rots that Hopkins(2007) found to be most prevalent inmature E. obliqua.

Yee (2005) found similar saproxylic beetlespecies richness in small and largeE. obliqua logs, with nearly 50% of the

species in each of the small and large logsize-classes being unique to that size-class.As with mature living trees, regions of

brown rot, primarily located in the innerheartwood, contained particularly richsaproxylic beetle communities (Figure9). There was also a saproxylic beetle

community associated with brous rot onthe outer surface of the logs. This zone,which typically occurs at the soil-loginterface, is known to be an important larvalhabitat for soil-dwelling species (Grove2006).

There are some common elements in thesaproxylic beetle communities associatedwith brown rot in the inner heartwood ofmature E. obliqua trees and large logs arisingfrom such trees. Six of the seven beetlespecies that Harrison (2007) found to besignicantly associated with mature treeswere also commonly encountered in largelogs (Yee 2005). Interestingly, some of thesespecies, which are common in Tasmanian

forest (e.g. Prostomis atkinsoni, PycnomerusTFIC sp. 2, Cossonus simsoni), have relativesin European forests that have becomeendangered or regionally extinct followinga long history of intensive forestry andland clearance. The sensitivity to intensiveforestry of saproxylic beetle species reliant

Figure 7. Venn diagram showing the total number of saproxylic beetle species extracted from six living E. obliqua

trees in each of three age classes (young = 69 years old, medium-aged = 105 years old and mature = >150 years old).Adapted from Figure 5.1 in Harrison (2007).

>150-year-old

trees

(115 species of

beetle)

69-year-old trees(56 species of beetle)

105-year-old trees

(72 species of beetle)


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0 2 4 6 8 10 12 14 1

Brown mudgut rot

Wet brown cubic rot

Red-borwn block fibrous rot

Dry brown cubic rot

Combination pocket and brown rot

Termite damage

Cerambycid larvae damage

Discoloured wood

Small pocket rot

Stringy rot

White spongy rot

Number of beetle species

Whiterot

Brownro

0 2 4 6 8 10 12 14 16

Mudguts (BR)

Dark blocky, dry crumbly rot (BR)

Dark blocky fibrous rot (BR)

Discoloured wood


Yellow dry slaty rot



Wet cracks

Jelly surface rot

Fibrous surface rot (WR)


Surface

Localised

Outer heartwood

Heartwood

Inner hartwood

Figure 9. Number of saproxylic beetle species that were recovered on more than two occasions each for particular

types and locations of rot in small and large E. obliqua logs. Adapted from Table 5.1 in Yee (2005). (BR) = brownrot types; (WR) = white rot types.

Figure 8. Number of saproxylic beetle species collected from dierent roen wood types occurring in 18 E. obliquatrees across a range of ages. Figure 6.4 in Harrison (2007).

Cerambycid larvae damage

Discoloured wood

Termite damage

Combination pocket and brown rot

0 2 4 6 8 10 12 14 16

Mudguts (BR)

Dark blocky, dry crumbly rot (BR)

Dark blocky fbrous rot (BR)

Discoloured wood


Yellow dry slaty rot



Wet cracks

Jelly surface rot

Fibrous surface rot (WR)


0 2 4 6 8 10 12 14 16

Brown mudgut rot

Wet brown cubic rot

Red-brown block fbrous rot

Dry brown pocket rot

Brownrot

White spongy rot

Stringy rot

Small pocket rot


Inner heartwood


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on habitat that develops in mature trees(and large logs arising from them) isreected in the preponderance of this groupamong European red-listed species nearly

45% of Swedens 1000+ known saproxylicbeetle species are red-listed (Jonsell andNordlander 2002). Tasmania, with a muchshorter history of forestry, is fortunate tohave very few CWD-dependent specieslisted as endangered. In the southern forests,where we know most about the biodiversitydependent upon mature trees and CWD,only one saproxylic beetle species, Lissotesmenalcas, is listed as vulnerable, chiey

because of its restricted distribution (Meggs

and Taylor 1999). Recent research has

conrmed that this log-dwelling speciesalso has a strong association with brown rotin the centre of logs (Belinda Yaxley, pers.comm.).

Interdependencies among eucalypts, wood-decay fungi and saproxylic beetles

In providing habitat for wood-decay fungiand saproxylic beetles, characteristics ofeucalypts during their development are offundamental importance. We would expectpaerns in branching characteristics to bea key driver of assemblage composition for

wood-decay fungi and saproxylic beetles,because of the importance of branches as

Age-class Standing trees Fallen logs

Youngregrowth (20-

40 years old)

Crown-liing: regular senescence of lower

branches, many small dead branches.

Many decay columns (mostly small), Hymenochaetaceaeand Dichostereum spp.predominateWhite rot prevalentCerambycid ( Epithora, Phoracantha)

galleries in crotches of dead branches

Low heartwood extractivescontent.

Decay progresses from the surface of the log inwardsWhite rot prevalentLile/nooverlapbetweenwood-decay fungi in standing tree and

fallen log

Regrowth (70-100 years old)

Crown-deepening: senescence of lowerbranches stops.

Few decay columns establish, few species ofwood-decayfungipresentRot, when present, is white rotRelatively low diversity of saproxylic beetles

As above (initially)

Mature (>150years old)

Crown-retraction: branches persist and growto large size; branch breakage, re damageand branch death (from dieback) creating largewounds.

Large stem diameter.Larger volume of decaySuitable conditions for hollow formation

Heartwood becomes less suitable for white-rotfungi.

Brown rot becomes more prevalent, higherdiversityofwood-decayfungithaninregrowth treesDiversity of saproxylic beetles (associated withbrown rot) increases

High heartwood extractives

content.Surface decay restricted to

sapwood

Brown rot in inner heartwood(established when tree was

standing).High saproxylic beetle diversityassociated with standing trees is

maintained

Large log diameter.Persistent and stable environment

for saproxylic beetles with poordispersal abilities

Table1.Keyaributesofdierentage-classesofE. obliqua trees and logs, and (italic dot-points) proposedconsequencewithrespecttowood-decayfungiandsaproxylicbeetles.


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entry points for these organisms. Acrossthe age-span examined in the studies byWardlaw (1996, 2003), Hopkins (2007) andHarrison (2007), three distinct phases incrown development occur: crown-liing,crown-deepening, and crown-retraction.

Table 1 summarises the key ndings fromthese studies and their linkage to the threestages of crown development. Studies byYee (2005) were of logs generated from mid-aged regrowth trees and mature trees, likelyreecting the crown retraction and crown-deepening phases respectively.

During the crown-liing stage, when youngtrees achieve rapid height growth, thereis ongoing senescence of relatively young

and small branches in the lower crown.The great abundance of dead branches

being shed provides numerous potentialentry points for wood-decay fungi andstem-boring insects. In particular, severalspecies of decay fungi from the familyHymenochaetaceae appear well-adaptedto infecting the bole as small branches are

being shed. In addition, Hopkins (2007)found that 69-year-old trees tended to becolonised by wood-decay fungi from genera

that are regarded by Northern Hemisphereworkers as primary colonisers. Some speciesof cerambycid beetles from the generaPhoracantha and Epithora are similarlyopportunistic in establishing galleries in thecrotches of dead branches during this phaseof crown development.

By mid-age, when crown-liing stops (Bar-Ness 2005) and the eucalypts commence aperiod of crown-deepening, branches persistto much older ages and grow to larger sizes.The lower levels of wood decay and lowerdiversity of wood-decay fungi in mid-agedeucalypts reported by Hopkins (2007) isconsistent with a greater branch persistenceand hence with fewer opportunities forwood-decay fungi to gain entry into thestem of the eucalypt. These low levels ofdecay are also reected in a lower diversityof saproxylic beetles relative to matureeucalypts, particularly those taxa that areobligately saproxylic (Harrison 2007).

There is an interesting contrast betweenthe prevalence of wood-decay fungi in theyoung trees reported by Wardlaw (1996,2003), and the paucity of such fungi andtheir associated decay in mid-aged treesas found by Hopkins (2007). In particular,

trees with extensive decay columns, whichwere common in young eucalypts, werenot encountered in the mid-aged treessampled by Hopkins (2007). We wouldexpect a proportion of the young treeswith extensive decay columns to break oor fall over before they reached mid-age.This has been observed by the authors inyoung wet eucalypt forests, in which treeswith extensive bu-rot due toArmillariainfection may be particularly vulnerable.

Some young eucalypts with extensive decaydo, however, reach mid-age (T. Wardlaw,unpublished observation). Their absencefrom the mid-aged trees sampled by thestudies reported by Hopkins (2007) probablyreects the small sample size relative to thenatural degree of variation in decay amount(Wardlaw 1996, 2003).

In mature eucalypts, the crown-deepeningphase continues until the trees aain their

maximum height, a process which maybe truncated prematurely by stochasticevents such as re, storm or crown diebackepisodes, the laer triggered by stress eventssuch as severe drought (Wardlaw 1989).Branches persist for much longer in thisphase and can become so large that they failmechanically or succumb to the stochasticevents just mentioned. The wounds createdwhen such branches break, die or are burntprovide large entry-points for wood-decayfungi and saproxylic beetles (Lindenmayeret al. 1993). Over the long time-span thatmature trees persist, the accumulation ofsuch events produces large volumes ofroen wood in the stem and branches witha high diversity of wood-decay fungi andsaproxylic beetles. This rich diversity isnot just a function of the large volumes ofavailable habitat (dead wood), but also ofthe distinctiveness of the habitat. While

physically distinct structures such ashollows and dead tops contribute to this, the


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shi to a greater prevalence of brown rotsin the heartwood of mature trees is also asignicant factor. All of the seven saproxylic

beetle species that Harrison (2007) foundto be indicative of mature eucalypts wereassociated strongly with brown rot.

The ultimate cause of the age-related shifrom decay fungi that cause white rots tothose that cause brown rots remains unclear.One possible factor is an age-inducedchange in the chemical properties of theheartwood. For example, more extractivesare deposited in the heartwood of oldereucalypts (Wardlaw 1990). These renderthe heartwood more resistant to microbialaack, and would provide an explanation

for the tendency for wood decay to spreadfrom the outer heartwood inwards in smalleucalypt logs, while in large logs or maturetrees the spread is from the inner heartwoodoutwards (Yee 2005; Hopkins 2007). It isuncertain, though, whether it is the increasein heartwood extractives in older trees thatfavours brown-rot fungi over white-rotfungi, or some other chemical change.

The composition of lignin also has a

powerful eect on selecting for white- orbrown-rot fungi. Most brown rot fungioccur in conifers, whereas most white rotfungi occur in angiosperms (Nakasone1996). Conifer lignin is dominated byguaiacyl lignin, which is quite resistant todegradation by wood-decay fungi (Highley1982). This renders conifer wood anunfavourable substrate for white rot fungi,which aack both lignin and cellulose.Angiosperm lignin, on the other hand,contains a mixture of guaiacyl and syringyllignin in varying proportions. The syringyllignin is more susceptible to decay by whiterot fungi than is the guaiacyl lignin (Highley1982; Obst et al. 1994), so angiosperm woodwith a high syringyl:guaiacyl ratio may thustend to be degraded more readily by whiterot fungi. Although poorly understood,there is evidence that the lignin in matureheartwood of E. regnans diers qualitativelyfrom the lignin in younger trees (Bland1960). It would be of great ecological

interest if it were demonstrated that suchqualitative dierences between young andmature eucalypt wood reected changes inthe syringyl:guaiacyl lignin ratio, and if itcould be determined whether such changesrender mature wood less favourable as a

substrate for white-rot fungi.

The distinctive wood-decay fungi andsaproxylic beetle communities that developin mature eucalypts also appear in largeeucalypt logs aer the trees fall to theforest oor. This is particularly so for thesaproxylic beetle fauna, where ve of theseven species found by Harrison (2007)to be signicantly associated with matureeucalypts were among the common species

inhabiting brown-roed regions of theinner heartwood of large downed logs(Yee 2005). While Hopkins (2007) foundsome overlap in the wood-decay fungiof mature eucalypts and large downedlogs, the association was much strongerfor the saproxylic beetle fauna. Takentogether, these elements of commonalityin communities of saproxylic beetles andwood decay fungi in mature trees and largelogs strongly suggest that the communities

that develop in the standing tree persistaer the tree falls to the forest oor. Thelarge size of the downed logs generatedfrom mature eucalypts and their longresidence time on the forest oor (Grove etal. 2009) provides for a very stable habitatthat is well-buered from uctuations inmoisture and re (Yee and Grove 2007).This persistent and stable habitat isparticularly important for those saproxylicspecies that are thought to have limiteddispersal abilities, allowing them to gothrough many generations without havingto move out of the log. Several invertebrates(e.g. stag beetles and velvet worms) listedas rare or endangered under TasmaniasThreatened Species Act (1999) fall into thiscategory.

In contrast with the continuum betweenmature trees and large downed logs, thereappears to be lile linking the communitiesof wood-decay fungi and saproxylic


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beetles of young trees and small logs.Because of their low heartwood extractivecontent (Wardlaw 1990), small logs decayreadily from the surface inwards, allowingpredominance by fungi that colonise aerthe tree falls to the forest oor. While we

could nd no evidence of a continuumin the communities of wood-decay fungiin regrowth trees (particularly those inthe active crown-liing phase) with thecommunities in mature trees, we cannotconclude that such a continuum does notexist. Our knowledge of the communities ofwood-decay fungi in older regrowth treescommencing the crown-deepening phase,and in mature trees, has developed from asmall sample of trees in a restricted area.

In addition, evidence of latent colonisationof apparently sound wood by wood-decayfungi (Hopkins 2007) suggests that samplingfocussed just on decay columns (as wasdone in the young regrowth trees) mayoverlook decay fungi that bridge the growthstages.

The lack of evidence linking the wood-decay fungi and saproxylic beetle biotaof young trees with those of mature trees

does not greatly impact on managementconsiderations. Prescriptions for mid-rotation thinning of native forests aim toselectively cull trees with high levels ofinternal decay. However, the area of nativeforest targeted for such management is lessthan 2% of the area of State forest (ForestryTasmania 2007). In addition, thinningwill never remove all trees containinginternal decay (Wardlaw and Bashford2007). Therefore, even if future researchdoes uncover a successional link in thecommunities of wood-decay fungi and/orsaproxylic beetles between young regrowthtrees and mature trees, the consequence ofany disruption to that link by thinning islikely to be minimal.

The link between the biota colonising maturetrees and that of large logs indicates theneed to ensure the continuity of maturetrees in space and time throughout forestedlandscapes, at appropriate scales to allownatural dispersal of the dependent biota.

A forest landscape with a diversity ofage-classes, including mature trees, willachieve this. The system of Comprehensive,Adequate and Representative (CAR)reserves provided through the TasmanianRegional Forest Agreement, coupled withcomplementary management outsidereserves, is designed to cater for theconservation needs of forest-dependentspecies in production forests. The challengefor management is to understand the

scales at which forest age-classes need tobe dispersed in space and time in order toachieve the this continuity.

Two research projects have commencedwithin the Warra LTER to provide thisunderstanding. One is examining the spatialarrangement of reserved mature forestswithin the production forest landscape,and testing whether that arrangement issuccessfully maintaining species dependent

on mature forests. The other is usingmolecular methods to resolve how saproxylic

beetles, including species strongly associatedwith mature trees or large logs, movethrough the production forest landscape tocolonise suitable habitat.

Acknowledgements

The work presented here evolved from an

oral presentation given by the senior authorat the Inaugural Asia-Pacic Conference ofthe International Society of Arboriculture,held in Brisbane 9-13 May 2008. Commentson earlier dras of this manuscript by DrSteve Read and two anonymous referees aregratefully acknowledged.


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