1
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
2
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
3
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).
4
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
5
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
6
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
7
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
8
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.
9
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
10
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
11
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.
12
References
Clague J. J., Menounos, B., and Wheate, R. 2011. Canadian Rockies and Coast Mountains of Canada. In: Encyclopedia of Snow, Ice and Glaciers. Springer. pp. 109.
Copland, L. 2011. Retreat/advance of glaciers. In: Encyclopedia of Snow, Ice and Glaciers. Springer. pp. 934.
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.
Johnson, K., and Smith, D. 2012. Dendroglaciological reconstruction of late-Holocene glacier activity at White and South Flat glaciers, Boundary Range, northern British Columbia coast mountains, Canada. The Holocene. 22(9): 987-995.
Luckman, B. H. 1988. Dating the moraines and recession of Athabasca and Dome glaciers, Alberta, Canada. Arctic and Alpine Research. 20(1): 40-54.
Luckman, B. H. 1995. Calender-dated, early “Little Ice Age” glacier advance at Robson Glacier, British Columbia, Canada. The Holocene. 5(2): 149-159
Luckman, B. H. 2000. The little ice age in the Canadian Rockies. Geomorphology 32(1): 361.
Reyes, A. V., Luckman, B. H., Smith, D. J., Clague, J. J., & Van Dorp, R. D. 2006. Tree-ring dates for the maximum Little Ice Age advance of Kaskawulsh Glacier, St. Elias Mountains, Canada. Arctic. 58(1): 14-20.
Schomack, A. 2011. Moraine. In: Encyclopedia of Snow, Ice and Glaciers. Springer. pp. 747.
Schweingruber, F.H. 1989. The history of glaciers. In: Tree rings: basics and applications of dendrochronology. Kluwer Academic Publishers, Dordrecht, Holland. pp. 194-198.
Shrestha, A. B. 2011. Climate change and glaciers. In: Encyclopedia of Snow, Ice and Glaciers. Springer. pp. 149.
Smith, D., and Lewis, D. 2007. Dendroglaciology. In: S.A. Elias, ed., Encyclopedia of Quaternary Science, Vol. 2. Elsevier, Amsterdam. pp. 986-994.
13
Wiles, G. C., Calkin, P. E., and Jacoby, G. C. 1996. Tree-ring analysis and Quaternary geology: principles and recent applications. Geomorphology. 16(3): 259-272.
Wood, C., and Smith, D. 2004. Dendroglaciological evidence for a neoglacial advance of the Saskatchewan Glacier, Banff National Park, Canadian Rocky Mountains. Tree-Ring Research. 60(1): 59-65
Wood, L. J., Smith, D. J., and Demuth, M. N. 2011. Extending the Place Glacier mass-balance record to AD 1585, using tree rings and wood density. Quaternary Research, 76: 305-313.
Zhu, H., Xu, P., Shao, X., and Luo, H. 2013. Little Ice Age glacier fluctuations reconstructed for the southeastern Tibetan Plateau using tree rings. Quaternary International, 283: 134-138.