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INTRODUCTION

Within the last 100 years of the 20th century glaciers have been very quickly losing mass

and receding because of steadily increasing climatic temperatures. Before the 20th century

glaciers receded slowly, but in the 20th century these glaciers started to recede at an astounding

and problematic pace. The Athabasca Glacier of the Rocky Mountains only retreated 200 m

between 1844 and 1906, but retreated over 1000 m between 1906 and 1981 losing approximately

half of its mass (Clague et al. 2011). The significant amount of glacier recession in the 20th

century uncovers land surfaces, remains of buried forests, killed trees, sub-fossilized wood and

tilted trees that can be used in dendroglaciological studies to infer about the timing of past glacial

events. (Johnson and Smith 2012, Smith and Lewis 2007). Glacier recession is problematic

because it is indicative of the negative impacts of climatic change. Climates that are associated

with increased snowfall and lower temperatures favours positive glacial mass balance and thus

glacial accumulation, meaning the glacier increases in size and mass and advances forward

(Copland 2011). Climates that are associated with decreased snowfall amounts and higher

temperatures favor negative glacial mass balance and thus glacial ablation, meaning the glacier

melts, losing size and mass and thus retreats (Copland 2011). The pattern of changing glacial

mass balance itself is a product of annual atmospheric conditions such as temperature,

precipitation whereas the actual pattern of glacial retreat and advance is a response to the overall

change in climatic conditions (Shrestha 2011).

This paper will focus on the methodologies used by dendroglaciological researchers to

date glacial advance and retreat events. It will review the process of site selection, sample

collection, and dating methodologies used by dendroglaciologists. This paper will also focus on

the use of dating trees on moraines, abrupt changes in growth and symmetry, ice-scarring by

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direct glacial contact, glacial proximity affect on growth, and trees killed by glaciers.

Additionally, this paper will also focus on the connection between tree-rings, glaciers and climate

change and the usefulness of tree-rings to infer about the consequences of climate change.

DENDROGLACIOLOGICAL METHODOLOGIES

Site Selection

Site selection is of utmost importance in dendroglaciological studies. Glacial fore fields

are the sites of choice for tree selection when trying to reconstruct glacial fluctuations because

this is the area where the glacier advances and retreats. Luckman (1988) mapped out the glacial

forefield and historical ice-front positions using moraines. In the glacial forefield Luckman

(1988) collected tree core samples from trees in the vicinity of moraines, which are ridges of

sediment deposited by glaciers (Schomack 2011). Within the glacial forefield glaciers retreat and

advance. Researchers are able to find the where trees may have contacted the glacier in a glacial

forefield by using historical aerial photographs that gives them the rough time and positioning of

certain glaciers best tree sampling areas (Smith and Lewis 2007).

Sample Collection

According to Schweingruber (1989) there are two main types of wood that can be found

within a glacial forefield. One may find sub-fossilized trees that have been taken down by the

movement of the moraine, indirectly cause by glacial movements. One also may find sub-

fossilized trees or boles that were covered by the glacier at one point in time and then uncovered

later on that can be cross-dated to find out the trees age and when it was killed by the advancing

glacier (Schweingruber 1989). In the glacial forefield researchers also collect samples of detrital

tree remains, boles or stumps that are often buried in glacial till deposits due to advancing

glaciers. Trees that appeared to be sheared off are good indicators of glacial movement because

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their deaths occur as a direct consequence of glacier movement over-riding the tree (Luckman

1995). Thus, these sheared stumps show the position and date of the glacier at the time the stump

was killed (Luckman 1995).

Dating Methods

Glacially affected trees can be cross-dated using manual dendrochronological methods.

This means by counting individual growth rings and noting pointer years. By matching pointer

years to a master tree ring chronology trees can be dated to infer about glacial activity. Sub-

fossilized wood is ancient wood that is the remains of forests that were destroyed by glacial

advance (Schweingruber 1989). These old samples usually do not cross-date to living

chronologies thus they must be assigned ages using radio-carbon dating which is less accurate

and more expensive then cross-dating (Luckman 1988).

Tree Indicators of Glacial Activity

There are four main tree indicators that can be used by dendroglaciologists to date glacial

activities. The four indicators are trees growing in the glacial forefield, abrupt changes in tree-

ring symmetry as a result of glacially-caused tree tilting, ice-contact scars, as well as trees killed

by glacial activity (Smith and Lewis 2007).

Trees Growing in Glacial Forefield

Glaciers retreat because of warm temperatures that cause glacial ablation and loss of

glacial mass causing recession. As the glacier recedes it exposes the lands surface allowing

colonization by tree seedlings (Smith and Lewis 2007). Additionally, glacial retreat exposes sub-

fossilized wood that was buried by previous glacial advances (Wiles et al 1999). The new trees

that colonized the deglaciated forefield are youngest at close proximity to glacial front (Smith

and Lewis 2007).

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In a study performed by Heusser (1956) he selected the oldest tree on the surface outside

the trim-line to find the minimum amount of years that have passed since ice advance or retreat.

The easiest way to ensure one is sampling the oldest tree is to sample as many trees as possible,

as well as to look for indicators of age that vary depending on species (Coulthard and Smith

2013). Sampling the oldest tree in the area of terminal and lateral moraines also allows dating of

their stabilization (Koch, Clague and Osbourne 2007). Cross dating of the oldest tree in a moraine

can provide information on the date of maximum glacier extent if sampling occurred at the most

distal moraine. Additionally cross dating the oldest trees can provide dates for the fluctuation of

glaciers because as glaciers retreat they leave multiple moraines behind. As glaciers retreat, they

leave behind surfaces that can be colonized by new tree saplings, as the saplings the glacier

retreats further, allowing more saplings to germinate and colonize the surface. This creates a

gradient of younger trees being closer in proximity to the glacier and older trees further from the

glacier (Smith and Lewis 2007, Hubbard and Glasser 2005). A limitation when using this method

is that researchers must account for the ecesis period, which is defined as the period of time

between surface clearance by glacier recession and tree-germination (Smith and Lewis 2007).

The ecesis period is determined by many factors such as exposure, soil type, precipitation,

temperature, and tree species and thus can be relatively fast, 3-10 years, or can take a long time,

50-70 years (Hubbard and Glasser 2005). In research done by Koch et al. (2007) the ecesis period

was studied in detail to ensure as accurate dating as possible of the oldest living trees on a

stabilized moraine. Without a correction for ecesis the older tree age from a given stabilized

moraine will always be underestimated. Thus, Koch et al. (2007) made corrections that account

for the ecesis period as well as the time it takes trees to grow to the height at which coring takes

place. He based his correction off the local age-height, width, and pith ring patterns. When Koch

and group corrected the dates for ecesis they took into account the aspect, depth of snow, and

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other environmental factors that may affect the tree.

Abrupt Changes in Growth Rates and Tree-Ring Symmetry

Glacial expansion or recession often causes trees to become tilted. This tilting event may

cause abrupt changes in growth rates, tree-ring with, and eccentric tree-ring symmetry (Coulthard

and Smith 2013). Additionally, trees that survive the initial tilting event form narrower upslope

tree-rings and wider downslope tree rings which are characteristic of reaction wood that functions

to restore the tree to upright position (Coulthard and Smith 2013). Trees that have their roots

disturbed by glacial movement may show abrupt decrease in growth (Schweingruber 1989). In an

examination of a cross section of Heusser’s log researchers cross dated the abrupt onset of

compression wood to 1714 indicating the Athabasca glacier advanced during the 18th century

tilting the tree in 1714 (Luckman 1988). Dating these abrupt changes allow precise dating of the

exact year of glacial events (Luckman 2000). A limitation of using tilted trees is sometimes the

tilting event is so severe that it results in the death of the tree. In study performed by Reyes et al.

(2006) they sampled tilted White Spruce trees along the outermost moraine of the Kaskawulsh

Glacier. Reyes et al. (2006) concluded that these trees were killed at the time of tilting because

their outermost rings did not have an abrupt change in growth rate and also did not produce

reaction wood in response to the tilting event. In cross dating these trees to a regional living

master white spruce chronologies they found that the last ring in the only sample they found that

had bark indicated that the east lobe of the Kaskawulsh Glacier reached its Little Ice Age

maximum in 1757 (Reyes et al. 2006).

Tree Ring Growth when Trees in Close Proximity to Glacier

Trees that grow in close proximity to glaciers are prone to cold temperature stress. The

cold conditions may cause the tree to form narrow rings or frost rings (Coulthard and Smith

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2013) Wiles et al (2006) sampled trees that grew 10 meters past the outermost terminal moraine,

thus the trees never came in direct contact with the Ultramarine Glacier. These trees had varied

responses, some had an abrupt decrease in ring width, and some stopped growing all together.

These trees often showed abrupt onset of growth indicating that the glacier was in a period of

recession. Wiles et al (2006) compared the ring width series of two samples in close proximity

with the glacier with a control chronology of mountain hemlock not in close proximity with the

glacier. They found that as the glacier advanced the two sample trees had suppressed growth

beginning in 1872 and 1850. They both then had and increase in growth in 1900 and 1880

respectively due to glacier recession. These samples also both had missing rings due to close

proximity temperature stress. (Wiles et al 2006).

A study done by Nicolussi and Patzelt (1996) showed similar findings as Wiles et al

(2006). They also showed that Pinus cembra tree species growing in close proximity to the

Gepatschferner glacier had decreased growth. The Pinus cembra trees sampled within a few

meters of the 1850 Little Ice Age moraine. These trees all showed a decrease in tree-ring width

during the mid 19th century indicative of the glaciers maximum advance in 1850. Most of the

trees had their narrowest ring in the mid 1850s. These trees when index and compared to a

control Pinus cembra index chronology that shows that only the trees in close proximity to the

glaciers experienced decreased growth and thus had narrow rings (Nicolussi and Patzelt 1996).

Glacial-contact or Ice-contact Scarring in Trees

When glaciers come into direct contact with trees they can cause ice contact scar

formation and the formation of reaction wood by tilting the tree (Wiles et al. 2006). These glacial

abrasion events may cause the development of callous tissue. These ice scars can be cross-dated

to the exact year of the scarring event by counting the rings (Coulthard and Smith 2013). These

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scars may provide information about the direction of glacial movement or the direction in which

the tree was tilted. In a study by Zhu et al. (2013) reaction wood was used to date the year of

tilting by Gawalong glacier movement. The onset of the reaction wood was in 1987, and then

there was an abrupt decrease in growth after 1991. This data indicates that the Gawalong glacier

advanced up to the tree. The advance caused the tree to tilt and caused the tree to have stunted

growth (Zhu et al. 2013).

Ice scars proved to be useful information when Luckman (1988) examined a Picea

engelmannii tree that was left scarred by Dome Glacier in Alberta. The inner face of the scar

closest to the pith cross-dated to 1843. However this date is incorrect as 3 rings were lost due to

wear and tear. The outer edge of the scar that has been overgrown with new callous growth dates

to 1846 the actual date of the Dome Glacier scarring event (Luckman 1988).

Trees Killed by Glacial Activity

Sometimes glaciers are responsible for killing of trees. This can be by advancing onto the

tree, shearing off the crown or by cold temperatures close to the glacier. Luckman (1995)

sampled sheared stumps that were over-ridden by the Robson Glacier. Some of these stumps

were in growth position and some were moved and deposited by the glacier. One limitation of

using logs that were deposited by Robson glacier is that these logs only provide when the glacier

killed the tree since the original growth position is unknown. Despite not being able to infer

about glacial positioning, these trees can be used to estimate roughly how long the period of

glacial advance was (Luckman 1995). The stumps that were found in growth position reveal the

exact date and position of glacier at time the tree was killed. Thus by mapping out the position

and dates of these trees one can map out the position of any glacier and the movement it had over

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time.

APPLICATION OF DENDROCHRONOLOGICAL METHODS IN CASE STUDIES

Wood and Smith (2004) were interested in the Neoglacial event, a world-wide advance of

glaciers that occurred after a period in the Holocene where glaciers were at their smallest. The

advances during the Neoglacial period are estimated to have occurred between 8000 14C before

present (B.P.) to approximately 3000 14C B.P. They studied the sub-fossilized stumps and

detrital boles that were recently exposed by the retreat of the Saskatchewan Glacier in the

Canadian Rocky Mountains. In this area of sub alpine forests and alpine tundra’s the dominant

tree species consist Engelmann spruce (Picea engelmanni), lodgepole pine (Pinus contorta),

subalpine fir (Albies lasiocarpa), whitebark pine (Pinus albicaulis) and krum-holtz spruce. They

took cross-sectional discs of wood from 19 sub-fossilized stumps and 69 ice-proximal detrital

boles. They constructed floating chronologies for each disc using standard dendrochronological

methods, verified their data with COFECHA programming and standardized it with ARSTAN

programming. They tried cross dating the sub-fossilized samples to living master chronologies,

when cross-dating failed it ensured that the sample was not of recent origin. These samples of

sub-fossilized wood were submitted for radio-carbon dating and dated to be killed in 287060

14C years before present and 283060 14C years before present. The age of these samples agrees

with the past estimated dates of Neoglacial advances (between 8000 14C years B.P and 3000 14C

years B.P) but suggested that the advances were closer to the date of 300014C years B.P. In this

instance, the radiocarbon-dated ring-width chronologies derived from sheared stumps and detrital

wood at the Saskatchewan Glacier support previous findings of increased glacier ex- tent ca.

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3,000 14C years B.P. in the Canadian Rocky Mountains. One of the caveats of this study done by

Wood and Smith (2004) is the abundant use of 14C dating which is less accurate than ring

matching and cross-dating, but in many cases necessary when working with sub-fossilized

samples.

In a different dendroglaciological study that was described by Luckman (1988) Heusser

sampled a tree in 1984 that was growing out of the side of an outer terminal moraine made by the

Athabasca Glacier. He made an upper cut that had a cross-dated pith date of 1862. Further down

a core sample was taken that had a pith date of 1858. Even further down the trees length where

the tree grows horizontally out of the terminal moraine he made another cut that had a cross-

dated pith date of 1750 (Luckman 1988). His findings suggest that the main trunk he sampled

was actually a leader stem that dominated in growth after an original earlier stem was knocked

over and almost killed due to moraine movement by glacial dynamics. Heusser also noted an

abrupt decrease in the growth of annual rings starting in 1843 that lasted for approximately 12

years until 1855 when there was an increase in the growth of annual rings (Luckman 1988).

These changes in ring width imply that the original tree stem was damaged in 1843, and the

suppression in growth from then on is a result of completion from nearby saplings. In 1855 the

tree recovered from the glacial disturbance and resumed normal growth with wide ring widths of

1-2 mm (Luckman 1988). The 1750 date for the assumed original stem is confirmed by the

presence of callous tissue surrounding the entire stem in the trunk that formed from a leader stem.

TREE RINGS, GLACIERS AND CORRELATION WITH CLIMATIC CHANGES

An increase in climatic temperature promotes a negative glacial mass balance. Together in

combination these factors may encourages rapid tree growth and the production of large annual

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growth rings (Dolezal et al. 2014). Thus, dendroglaciologists are able to use tree ring widths to

reconstruct glacial dynamics and climate changes. Dolezal et al. (2014) are some of the

researchers that have studied these connections. They did this by converting a Betula ermanii

ring-width chronology from near the Koryto Glacier to indices. The compared the 60-year

indexed chronology with both the averaged July temperatures from 1940 to 2000 to the summer

Koryto Glaciar mass balance records. This comparison revealed very narrow and wide pointer

years that corresponds with extreme climate in that year. Overall, this study revealed that warmer

climatic temperatures in July was correlated with negative glacial mass balance and increased

growth rates that produce wider annual growth rings in trees (Dolezal et al. 2014).

In a separate study performed by Wood et al. (2011) they aimed to use annual growth

rings that respond to climate in order to represent the response of Place Glacier to climate change.

The researchers collected samples from high-elevation old-tree stands to make sure the climate

was limiting to growth (Wood et al. 2011). They also got climate data from climate station

records. They analyzed their samples and formed ring-width chronologies, from Engelmann

spruce and Douglas-fir trees within the region surrounding the Place Glacier. The Engelmann

spruce ring-width chronology was used to reconstruct what Place Glaciers glacial mass balance

would have been in the last 400 years. They are able to do this because in warmer climates,

glaciers lose mass and retreat allowing trees to have an increase in growth. Thus large rings in

tree samples indicate that the glacier has a negative mass balance.

CONCLUSION

Because the world is warming dendroglaciologists, now more than ever before, are able to

use glacially affected trees to infer about glacial dynamics caused by increasing climate changes.

This review has shown the usefulness of tree rings, as a proxy for dating glacial events is very

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accurate to within one year when dating glacially caused scarring events, shown in the example

of an ice scar dated accurately to the exact year 1846 (Luckman 1988). The present is the key to

the past. In dendroglaciology, this is seen in ancient tree stumps that have been uncovered by

retreating glaciers that hold evidence historical glacial events, shown in examples of stumps that

radiocarbon date to be nearly 3000 14C years old telling dendroglaciologists stories about glacial

advances in the Neoglacial period (Wood and Smith 2004).

Tree rings that have been affected by glaciers are also useful tool to infer about climatic

changes. This was seen in the example of warm average July temperatures being correlated with

negative glacial mass balance (glacial retreat and melting) that favours increased annual growth

and wide growth rings (Dolezal et al. 2014).

Overall, dendroglaciology is an extremely valid method to date glacial activity and infer

about climate change processes. Future research that could be considered in the field of

dendroglaciology is to explore other factors besides climate that may have an effect on glacial

dynamics. Another field that may be interesting is looking at the effects that glacial melt water

and the isotopes and minerals it contains has on trees in glacial forefield.

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Dolezal, J., Altman, J., Vetrova, V. P., and Hara, T. 2014. Linking two centuries of tree growth and glacier dynamics with climate changes in Kamchatka. Climatic change, 124(1-2), 207-220.

Hubbard, B., and Glasser, N. F. 2005. Relative-age dating techniques. In: Field techniques in glaciology and glacial geomorphology. John Wiley & Sons. pp 362-363.

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