Download Plant Community Analysis and Dating of the Asulkan

Plant Community Analysis and Dating of the Asulkan Glacier: A Study of Plant
Community Succession Over Time in a Glacial Forefield
By: Ciara Sharpe, Laura Grant and Karl Domes
Instructor: Dr. Dan Smith
Department of Geography
University of Victoria
December 2008
Table of Contents
Abstract................................................................................................................................3
Introduction..........................................................................................................................4
Study Site.............................................................................................................................6
Methods................................................................................................................................7
Dendrochronology...................................................................................................7
Lichenometry............................................................................................................8
Plant Community Analysis.....................................................................................10
Results................................................................................................................................11
Dendrochronology.................................................................................................11
Lichenometry..........................................................................................................12
Plant Community Analysis.....................................................................................14
Discussion..........................................................................................................................20
Dendrochronology.................................................................................................20
Lichenometry..........................................................................................................21
Developing a timeline of retreat............................................................................23
Plant Community Analysis.....................................................................................24
Conclusion.........................................................................................................................30
Acknowledgements............................................................................................................31
References..........................................................................................................................31
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Abstract
Succession of plant communities after a disturbance is characterized by the
progressive replacement of one community by another. The slow rate of retreat of a
glacier allows for a unique opportunity to create a representative view of succession as a
function of time. In this study, performed at the Asulkan Glacier in British Columbia,
plant community succession was assessed using species richness and indicator species. A
transect was constructed from the terminal moraine to the glacial tongue along which 5m
x 5m plots were studied every 100 m. Dendrochronology and lichenometry were used to
generate a timeline of the glacier’s retreat along the transect. Species richness increased
from the glacial tongue to the terminal moraine. Presence or absence of nitrogen soil
indicators further confirmed that the sites closer to the terminal moraine, exposed for over
100 years, were in early to secondary successional stage. The sites closer to the tongue of
the glacier, exposed for less than 100 years, were in pioneer successional stage. The
ability to predict plant community succession as a function of time will allow an
understanding of ecosystem response to increasingly frequent natural disturbances.
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Introduction
Webster’s dictionary defines succession as ‘the act or process of following in
order’ (Merriam and Webster, 2010). In an ecological sense, it is the gradual process of
change in an ecosystem following a disturbance. Characterized by the progressive
replacement of one community by another, succession continues until a stable end stage
or climax is reached. Though succession can be viewed in many ways it is most
commonly examined by assessing community composition and species diversity.
Mechanisms in determining the sequence of species found after a perturbance fall under
three alternative models. All models state that certain species will appear first because
they have evolved colonizing characteristics (Connell and Slatyer, 1977). Where the
models differ are in the mechanisms that determine how new species appear later in the
sequence of succession. The most common model is the facilitation model, which
predicts that early successional species modify the environment, in turn determining the
later successional species (Whittaker, 1975). This model is applicable to situations where
substrate has not been influenced by organisms beforehand, such as the barren ground
exposed by receding glaciers.
Glacial retreat provides a unique setting for the study of primary succession. The
newly exposed substrate lacks plant life; therefore, a glacier’s gradual recession allows
for progression of primary succession in the glacial forefield. The advance and retreat of
a glacier is dependent on annual mass balance fluctuations; if more mass accumulates in
the winter months than is lost through melt during the summer months, the glacier will
advance over time. Conversely, if ablation exceeds accumulation, the glacier will recede.
Since the Little Ice Age, which ended in the late 19th century, glaciers across the globe
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have been gradually receding, yielding a marked increase in retreat rate in the past two
decades (IPCC, 2007). As the retreat rates of British Columbia’s glaciers increase, the
importance of understanding subsequent environmental responses becomes increasingly
significant.
Dendrochronology and lichenometry are methods in which the historic retreat of
glaciers can be outlined. Dendrochronology is the use of tree growth rings to date
environmental events and conditions. The rings, which grow radially outwards from the
center of the tree trunk, are formed annually. These rings can be visually distinguished by
the variation in shade from the lighter spring growth to the darker fall growth. The age of
a tree can be determined by simply counting its number of growth rings.
Lichenometry was developed in the 1950’s as a means to date the time of
exposure of a substrate. Lichen growth rate and age can be extrapolated by measuring the
width of the lichen thallus (Luckman, 1977). The most commonly used species to date
substrate is Rizocarpon geographicum because it grows slowly, relatively radially, and
has a long life span (Luckman, 1977). Using these methods, a timeline for succession can
be developed and compared to plant communities found within the glacial forefield.
Ecologists often describe habitats by species assemblages or indicator species
(Dufrene and Legendre, 1997). An indicator species is any biological species that defines
a specific trait or characteristic about the environment. Indicator species and species
diversity indices assist us in analyzing succession; habitat change as a function of time.
For example, the presence of red alder (Alnus rubra) indicates nitrogen-rich
environments, whereas the presence of Alpine Fireweed (Epilobium latifolium) indicates
nitrogen-poor soils (Klinka et al.,1989). This idea can be applied to various ecosystems
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including those found in the glacial forefield. As a glacier recedes over time, ground
previously barren to plant life is gradually exposed. Therefore, indicator species in theory
should illustrate a time line for glacial recession. Species found closer to the tongue of
glacier should be early successional, whereas those found closer to the terminal moraine
should be later successional. In conclusion, one would predict that from the tongue of a
glacier to the terminal moraine, succession is evident. Though succession of bacterial
communities inhabiting the glacial forefields has been thoroughly investigated in the past,
plant communities in the forefield are relatively unexplored (Crivii et al., 2002). In this
study, plant community composition was assessed along the Asulkan glacial forefield in
Glacier National Park, British Columbia, Canada. One would expect that from the tongue
of the glacier to the terminal moraine species diversity would increase, as the time the
community has had to establish is greater. Furthermore, soil and nutrient establishment
should increase as a function of time and thus species found closer to the terminal
moraine should indicate richer soils. These measures in conjunction with a timeline
created through dendrochronology and lichenometry, should illustrate a clear example of
succession. This approach in determining plant community composition as a function of
time, along the Asulkan glacial forefield, may provide a tool for future analysis of
succession after natural disturbance.
Study site
The Asulkan glacier is located in Glacier National Park (117°29’00’’W,
51°41’00’’N). In the Selkirk range of southeastern BC’s Columbia Mountains, this
glacier is found between Revelstoke and Rogers Pass. Regionally, the climate is
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relatively mild compared to the rest of the province, characterized by high annual
precipitation levels and heavy snow packs. Lower elevation ecosystems are dominated by
cedar-hemlock temperate rainforests, and higher elevations are characterized by mountain
hemlock and subalpine fir.
The study site was located at the tree line in the Asulkan glacial forefield, and can
be accessed from the Asulkan valley trail via the terminal moraine. A 1400 m transect
was constructed from the terminal moraine to the tongue of the glacier with 5m x 5m
study plots every 100 m. The transect followed a braided river system which ran from the
terminal moraine to the 600 m mark through quartzite dominated glacial till. At the 600
m mark the substrate changed abruptly to an exposed quartzite bedrock canyon, which
continued for 400 m until reaching an exposed bedrock ridge. This ridge continued for
300 m until the tongue of the glacier and the end of the transect was reached. At from
1000m from the terminal moraine, the proposed study plot was inaccessible and is
omitted from our data collection.
Methods
Dendrochronology
Cores were taken from the largest subalpine fir (Abies lasiocarpa) at every site
along the 1400 m transect, from the terminal moraine to the glacial tongue. The largest
trees were assumed to be the oldest thus effort was made to core these trees at each site.
However, after the first 200 m, tree growth became sparse, and selection was limited for
tree cores. In addition, there were no trees present in the vicinity of plots 8 to 13 therefore
lichenometry methods were employed in lieu.
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Trees were cored using a 5 mm increment borer. The cores were taken
perpendicular to the stem, and as close to the ground as possible. This was done to ensure
maximum presence of growth rings, and to minimize risk of underestimation of tree age.
After allowing the cores to dry, they were mounted into grooved boards and sanded until
the growth rings were clearly visible. Under a microscope, the rings on each core were
counted from the outer bark to the pith and the age of each tree was determined with an
error margin of +/- 2 years. Once the ages from all the samples were determined, only
data from the oldest tree at each site was kept.
In order to date the time of exposure, a dendrochronology ecesis interval was
developed for the study area. The ages of three sites were determined through the use of
historical photographs: the terminal moraine, and two recessional moraines. Photographs
showed the historical margin of the glacier at these sites, dating the terminal moraine to
1889 and the recessional moraines to 1905, and 1918. The differences between the date
of exposure, as determined photographically, and the date of sampling (2010) were
calculated for each site. The known tree ages were then subtracted from these differences
to generate an ecesis interval for the Asulkan Glacier with an error margin of +/- 5 year.
Lichenometry
Lichen measurements were taken from Rizocarpon geographicum growing on
exposed quartzite boulders and bedrock at 10 of the 14 study plots along the transect. No
lichen growth was present at sites 8, 12, and 14, and due to inaccessibility, site 10 was
not sampled. Measurements were also taken from the foundation of the Glacier House
resort, down-valley from the Asulkan Glacier. The foundation was exposed in 1929 after
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the resort’s demolition (Rogers Pass National Historic Site).
Digital calipers were used to measure lichen size to the nearest tenth of a
millimeter. However, due to rainy weather, the calipers short-circuited before completion
of the study. A ruler was used in its place, and subsequent measurements were made to
the nearest millimeter. The lichen’s largest diameter was measured; a thin black
perimeter defining the outermost edges of growth. Measurements were only taken for
those lichen that had a circular shape and were not immediately bordered by other lichen.
Lichen growth is maximized in favorable conditions, therefore, measurement of the
largest lichen ensures similar sampling methods and represents the maximum time of
exposure for each site. (Luckman, 1977; Bingham and Pitman, 2008).
In order to determine the year of exposure at each site, the lichen diameters were
plotted on a growth curve published by McCarthy 2003, as seen in Figure 1. This curve
was developed for lichen growing on moraines of the neighboring Illecillewaet glacier.
The curve took into account the timed rate of growth of R. geographicum after a
determined ecesis interval at various locations on the Illecillewaet glacier. Ideally, a
growth curve specifically for the Asulkan glacier would be used. However, the
Illecillewaet curve was the only available option and it was assumed that due to its
proximity to the Asulkan, it represented a similar environment. Ages of substrate
exposure for the various sites were extrapolated from McCarthy’s curve with a suggested
error margin of +/- 10 years (McCarthy, 2003) (Figure 1).
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Figure 1. Growth curve for R. geographicum developed for the Illecillewaet glacier.
After age of plots was extrapolated from lichenometry and dendrochronology data,
anomalous results were assessed and subsequently omitted from the development of the
retreat timeline. The retreat timeline was created using dendrochronology data and
historical photographical evidence. Plot 1 (the terminal moraine), was dated using a
historical photograph, and plots 3 to 7 were dated using tree ring counts. Dates for plots 2
as well as plots 8 to 14 were extrapolated using Microsoft Excel linear regression.
Plant Community Analysis
At each 5m x 5m plot along the transect, plant community composition was
assessed. Species present in each plot were identified using Plants of Coastal British
Columbia and the Handbook of the Canadian Rockies (Gadd, 1996; Mackinnon and
Pojar, 2005). The presence or absence of species was used to determine species richness
(number of species) at each site. Species richness values at each plot along the transect
were compared in order to assess how species richness changes as a function of time.
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Percent cover of each species was evaluated and recorded, as well as overall bryophyte
and debris cover for each plot. Using a Bray-Curtis Similarity Analysis, plant percent
cover data was used to assess similarity of plant community composition between the
sites (PRIMER 6). In order to demonstrate these relationships, percent similarities
between adjacent plots were plotted on a multidimensional scaling (MDS) plot. In
addition, the percent cover data was used to create a cross-correlation matrix (SPSS
Statistics). This cross-correlation matrix was constructed in order to determine the
relationship between specific plant species found within the plots. Cross-correlation
involves assessing the influence of one species on the presence or absence of another
species. Species indicating early succession were chosen using Indicator Species of
British Columbia and the Plant Indicator Guide for Northern British Columbia (Klinka et
al., 1989; Beaudry et al., 2002). Compositions of these indicators among the plots were
compared to illustrate species-specific relationships, which were used to extrapolate the
successional stage of the landscape.
Plant community analysis was combined with dendrochoronology and
lichenometry data in order to develop an idea of succession as a function of time. Factors
recorded at each site such as debris cover and substrate were considered in this process.
Results
Dendrochronology
The general trend of the dendrochronology data moving from the terminal
moraine towards the glacial nose, shows a decrease in age, as seen in Figure 2.
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Figure 2. Tree ring counts along the transect, with anomalous data
points circled in red
This trend demonstrates the linear proportional relationship predicted between
substrate age and tree growth. This is the case for all the study plots, except plots one and
two, which fail to conform to the trend. The oldest tree cored at plot 1 was 89 (+/- 2)
years old, while the oldest tree cored at plot two was 98 (+/- 2) years old. This suggests
that tree age decreases with increased distance from the glacier. In addition, an ecesis
interval of 50-60 years was developed from the known dates on the transect. Using this
ecesis, plot 2 dates to 1857, which is older than the known terminal moraine date of 1889.
Lichenometry
The general trend of the lichenometry data, moving from the terminal moraine
towards the glacial nose, was a decrease in lichen diameter (Figure 3).
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Figure 3. Lichen diameters along the transect, with anomalous data points circled in red
This trend demonstrates the proportional relationship predicted between substrate
age and lichen growth (indicated by size). This is the case for all the study plots except
plots 1, 9, and 11. The lichen diameters from plots 9 and 11 are both larger than the
lichen diameter from plot 7, suggesting that these plots are older than plot 7, which is
unlikely. Secondly, the largest lichen diameter measured at plot 1 was smaller than the
largest diameter found at plot 2. Therefore, data points from plots 1, 9, and 11 were
deemed anomalous.
The retreat dates given from the lichnometry data are older than those suggested
by the dendrochronology data (Figure 4). As the distance from the glacial tongue
increases, the difference between suggested dates from each method increases in value. A
correlation between the data from both methods is apparent.
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Figure 4. A comparison of dates of retreat determined from both dendrochronology and
lichenometry data.
The timeline that was developed for the recession of the Asulkan glacier ranges
over 81 years (Figure 5). The terminal moraine was dated to 1889, and plot 14 at 1300 m
from the terminal moraine was dated to1970. The average rate of retreat is 16 m per year.
Figure 5. Timeline for the retreat of the Asulkan glacier
Plant Community Analysis
The plant communities changed along the transect from the terminal moraine to
the tongue of the glacier, exposed from glacial recession in 1889 and 1958 respectively.
Table 1 indicates the presence of plant species found in plots every 100m across the
glacial forefield. There is a wide range of plants present in all the plots, however, some
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species are exclusively present in plots closer to the terminal moraine while others are
only found closer to the glacial tongue. In addition, species such as Epilobium latifolium
were present in almost all plots across the transect. The plots closer to the terminal
moraine are characterized by higher species richness and an increase in the presence of
unique species (Table 1, Figure 5). Along the transect, species richness decreases closer
to the glacial tongue. At 600m there is a dramatic decrease, this corresponds to the
sudden change in substrate; a higher proportion of bedrock and debris cover are present
towards to glacial tongue (Figure 5 and 6).
Figure 5. Species richness of vegetation plots along the transect from the terminal
moraine to the tongue of the Asulkan glacier.
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Table 1. Presence of plants found in plots from the terminal moraine to the tongue of the
Asulkan glacier. X= plant presence,
= unique species,
= 1st discovery,
=
final appearance.
0
Species
Luetkea pectinata
Unidentified sp. 2
Sorbus sp.
Veratrum viride
Vaccinium ovalifolium
Thuja plicata
Lycopodium complanatum
Pedicularis bracteosa
Unidentified sp. 3
Cryptogramma sterlleri
Gaultheria procumbens
Senecio Canus
Asplenium viride
Hieracium sp.
Cryptogramma crispa
Antennaria neglecta
Aquilegia flavescens
Erigeron sp.
Lycopodium selago
Lomatium sp.
Adiantum pedatum
Botrychium lunaria
Unidentified sp. 1
Athyrium felix-femina
Alnus crispa
Vaccinium vitis-idaea
Leptarrhena pyrolifolia
Parnassia fimbriata
Menziesia ferruginea
Tsuga mertensiana
Phyllodoce sp.
Vaccinium membranaceum
Arnica augustifolia
Salix sp.
Petasites sp.
Cassiope blanche
Abies lasiocarpa
Senecia triangularis
Epilobium latifolium
Castilleja sp.
Picea sp.
Polystichum lonchitis
Antennaria alpina
Arnica amplexicalis
Oxyria digyna
Woodsia scopulina
Saxifraga lyalli
Dryopteris expansa
Gymnocarpium dryopteris
Veronica wormskjoldii
Stellaria calycantha
Unidentified sp. 4
Species Richness
X
X
X
Distance from Terminal Moraine
100 200 300 400 500 600 700 800 900 1100 1200 1300
X
X
X
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13
24
17
26
23
16
14
9
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7
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16
2
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Figure 6. Percent cover of bryophyte and debris in plots from the terminal moraine to the
glacial tongue.
Upon comparison, the plant communities exposed earlier (closer to the terminal
moraine) were far more similar to each other than those exposed more recently (Figure 7
and 8). Community composition differs between each sample site, although there are
greater differences between sites closer to the glacial tongue (Figure 7, R= 0.449, p=
0.023, ANOSIM). Figure 7 illustrates the similarity of community composition for sites
located between 0-600m in the glacial forefield. For example, plots located at 0m and
100m were more similar (61%) that those located at 1100m and 1200m (29%); (Figure 8,
SIMPER).
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Figure 7. MDS plot of percent cover site similarity between sites along the transect of
the Asulkan glacier from the terminal moraine (0m) to the glacial tongue (1300m).
Figure 8. Percent similarity of adjacent vegetation community composition from the
terminal moraine to the tongue of the glacier.
Next, Figure 9 and 10 illustrate the cross-correlation of indicator species. There is
a relationship in percent cover across the transect between early successional species:
Salix sp., Epilobium latifolium and Leptarrhena pyrolifolia (Figure 9). The abundance of
one species is correlated to the abundance of another, and all species have a higher
percent cover composition in plots closer to the terminal moraine. Furthermore,
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Epilobium latifolium is present in every plot (with one exception) up to the glacial
tongue. In addition, there is a similar correlation between the presence of trees (Tsuga
mertensiana) and other plant species such as: Phyllodoce sp. and Vaccinium
membranaceum; species requiring soils with higher nutrient concentrations (Figure 10).
All species are present with correlated compositions towards the terminal moraine, and
rarely appear in plots past 600m.
Figure 9. Percent cover composition of correlated species from the terminal moraine to
the glacial tongue of the Asulkan glacier: Salix sp., Epilobium latifolium,
Leptarrhena pyrolifolia
Figure 10. Percent cover composition of correlated species from the terminal moraine to
the glacial tongue of the Asulkan glacier: Tsuga mertensiana, Phyllodoce sp. and
Vaccinium membranaceum.
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Discussion:
Dendrochronology
The trend in tree-ring data shows the two anomalous data points (Figure 2).
Possible explanations for this discrepancy are either the oldest tree at plot 1 was not
cored, or the tree that was cored was secondary growth. The presence of secondary
growth trees at plot 1 could be due to two factors. Firstly, the first generation trees may
have died from natural causes. Subalpine fir naturally completes its life history at around
100 years of age (Aplet et al., 1988). Alternatively, a disturbance event such as a
landslide, rockfall, or fire could have wiped out the first growth of trees at the site. Plot 1
was at the base of a large scree slope with evidence of a rock glacier, which is suggestive
that the latter cause was probable.
Plot 2, the second anomalous data point, dates older than the terminal moraine
and represents a discrepancy in the ecesis interval. One explanation is that the glacier did
not scrape this area to bedrock, and an isolated pocket of soil was present after retreat
allowing for more rapid plant colonization. Alternatively, microhabitats and
environmental variation could have encouraged a faster accumulation of soil at that site,
reducing the ecesis interval. Characteristics of retreat and advance of this glacier indicate
that this site may have had a different environmental history than further up the transect
(Gerlib, 1961). However, further data collection would be required in order to develop a
more robust theory. For the purposes of this study, the plot 2 data was simply assumed to
show an anomaly in the ecesis interval, and was omitted from the development of the
retreat timeline.
The ecesis interval that was developed for the Asulkan glacier was a period of 50
20
to 60 years. This interval is comparable to the ecesis developed by McCarthy for the
neighbouring Illicellewaet glacier (McCarthy, 2003). Furthermore, previous Geography
477 dendrochronology studies illustrated similar eceses for the Asulkan (Bingham and
Pitman, 2008; Anastasiades et al., 2007). Using the ecesis interval of 50 to 60 years, the
plots for the first 600 m of the transect were dated from 1989 at the terminal moraine, to
1918 at 700 m. These dates are similar to dates of glacial retreat for the Asulkan
determined by Anastasiades et al. (2007).
Lichenometry
The trend in the lichenometry data shows three anomalous data points (Figure 3).
Firstly, the abnormally large lichen diameters at plots 7 and 9 can be explained by diverse
environmental factors that facilitate more rapid establishment and growth for the lichen in
these locations. Environmental factors greatly affect the eceses developed through
lichenometry. McCarthy found that lichen eceses on the Illecillewaet were site-specific,
ranging from 35 to 135 years (2003). Due to proximity, one can assume similar variation
in the growth environment at the Asulkan. Additional sources of error include data
collection irregularities such as the measurement of two separate lichen thali that were
growing together. This error cannot be attributed to miscalibration of the calipers, as a
ruler was used for data collection at these plots.
Secondly, the largest lichen diameter measured at plot 1 was smaller than the
largest diameter found at plot 2. This contradicts the proportional relationship predicted
between substrate age and lichen size. In this case, sources of error may have arisen from
either data collection or variation of environmental factors. It is possible that the largest
21
lichen at the site was not measured or environmental conditions at plot 1 allowed for a
shorter lichen ecesis interval. In addition, a disturbance event like a rockfall could have
eliminated the first growth lichen. The latter is likely the case. The site in question was
located at the base of a scree slope showing evidence of rockfalls and a moving rock
glacier. A rockfall could have led to anomalous data in two ways: data may have been
collected from lichen that began growing after the event, or data may have been collected
from lichen that was already established on a rock that originated from somewhere else.
The trend in lichenometry data for plots 1 and 2 correspond with the dendrochronology
data for the same plots (Figure 4).
The plot exposure dates that were extrapolated from the lichenometry data range
from 1740 at 100 m to 1920 at 1200 m, and show a non-linear glacial retreat timeline.
These dates are up to 88 years older than retreat dates given from Bingham and Pitman
(2008), and up to 110 years older than the dates extrapolated from our dendrochronology
data. Furthermore, the lichenometry dates do not correlate with the historical photographs
used in the establishment of a dendrochronology ecesis. For these reasons, the
lichenometry dates were omitted from the development of a glacial retreat timeline.
The most probable reason for the uncorrelated lichenometry and
dendrochronology data was the use of an inappropriate lichen growth curve. The
previously mentioned growth curve was developed by McCarthy for the Illecillewaet
glacier. It is possible that there were enough environmental differences between these
two sites to result in differing lichen growth rates. Moreover, when plotted together, the
dates extrapolated from both the lichenometry and dendrochronology data show
correlations in trends over distance (Figure 4). This correlation further suggests that the
22
error lies not in the data, but rather the extrapolation of dates from the growth curve. The
validity of the lichenometry data is reinforced by the accurate lichenometric dating of the
exposure of the Glacier House foundation. The lichenometrically derived date of 1925
(+/- 10 years) correlates positively with the known date of 1927. The most probable
environmental condition that would have resulted in a faster growth rate for the lichen at
the Asulkan site is the high local moisture content. Moisture has been found to be a
limiting factor for lichen growth, with high-moisture environments conducive to faster
growth (Armstrong, 1983). The transect in this study followed a braided river
environment for the first 600 m, and small bedrock canyons for the next 400 m. Further
evidence of these high-moisture conditions is indicated by the presence of hydrophilic
species (ferns, mosses) along the transect.
Developing a timeline of retreat
Using the dendrochronology data, a timeline was developed for the retreat of the
Asulkan glacier from the terminal moraine to the nose of the glacier (Figure 5). The
timeline for the glacier’s recession along the transect ranges over 81 years, from 1889 to
1970. The average retreat rate is 16 m per year, which is comparative to rates of retreat of
the Illecillewaet glacier compiled by Champoux and Ommanny (1986).
This study demonstrates the advantages and limitations of using
dendrochronology and lichenometry as dating tools. The main advantage of
dendrochronology is the ability to accurately date the age of a tree from raw data, which
helps to minimize error in estimating the substrate age. The potential disadvantages of
23
dendrochronology include not coring the oldest tree at the site, and tree mortality due to
environmental factors. These sources of error can be minimized by collecting a larger
sample size at each site, and by ensuring that the dates found at a given site correlate with
local trends.
Alternatively, the main advantage of using lichenometry as a dating tool is the
widespread presence of R. geographicum, which allows for the dating of sites in which
tree growth is not favorable. However, there are two main disadvantages of lichenometry.
Firstly, as with dendrochronology, there is the possibility that the oldest sample at the site
was not measured, or that measurements were taken from second growth lichen.
Secondly, the age of the lichen cannot be determined from raw data, and a pre-established
growth curve must be used for the determination of the substrate age. As a result, there is
an intrinsic increase in error in the resulting age approximations. Both dendrochronology
and lichenometry are limited by the life spans of the trees and lichens being studied
(Aplet et al., 1988; Calkin and Ellis, 1980). Although useful in the determination of
absolute dates, they can be used with more confidence as tools for relative dating.
Plant Community Analysis
The Asulkan glacier is an alpine environment characterized by plants adapted to
harsh growing conditions such as cold weather, a short growing season and low nutrient
availability. In such a severe environment, facilitation, plant life history traits and seed
dispersal ability are the major mechanisms that cause successional change (Matthews,
1992; Chapin et al., 1994). Colonizing plants supplement the available resources to
levels required for subsequent plants to survive and thus facilitate community change
24
(Baumeister and Callaway, 2006; Callaway and Walker, 1997; Chapin et al., 1994). This
facilitative effect is derived primarily from the addition of organic matter and the
associated nutrients from pioneer plant species (Chapin, 1994; Ohtonen, et al., 1999). In
addition, long-distance seed dispersal ability, short generation time and ability to survive
harsh environments, allow for plants to colonize disturbed environments (Fastie, 1995).
The Asulkan glacier began its current recession from the terminal moraine in 1889 and
has continued the retreat to its present-day position. As the glacier recedes, these
successional mechanisms ultimately determine the plant community composition of the
glacial forefield. Through plant community analysis, this dynamic composition of glacial
landscape and alpine ecosystems can be observed over time.
Similar studies have demonstrated that communities become more complex over
time, characterized by higher biodiversity and an increase in species-specific
relationships (Connell and Slatyer, 1977; Chapin et al., 1994; Fastie, 1995). Biodiversity
can be described as the variation of life within a particular ecosystem and is often used to
evaluate the health of a community (Harrington et al, 2010). Species richness and the
number of unique species are commonly used as a biodiversity index, as they contribute
to this variation (Harrington et al., 2010). Landscape closer to the terminal moraine of the
Asulkan glacier demonstrated increased species richness and a higher number of unique
species (Table 1, Figure 5). This increased biodiversity can be attributed to close
proximity to the seed source, as well as the increased time elapsed (since glacial
recession) for facilitation to occur. This facilitation creates favorable soils by increasing
soil biomass and nutrients, thus enabling further species to colonize (Ohtonen, et al.,
1999). Non-vegetated soils slowly accumulate biomass as the microbial community
25
changes to effectively manipulate the newly available resources exposed by glacial
retreat (Ohtonen, et al., 1999). In addition, the microbial community shifts from
bacterial-dominated to fungal-dominated communities, which further increases the
uptake of substrate into soil, and helps facilitate plants through mutualistic associations
such as with mycorrhizal fungus (Ohtonen, et al., 1999). This improvement of
environmental conditions, allows for higher biodiversity a wider range of species are able
to survive.
The community composition was first analyzed by comparing the presence or
absence of plant species across the transect (Table 1). Some species present are widely
distributed across the plots, while others are only present closer to the terminal moraine
or the tongue of the glacier. This distribution can be observed to see how plant
communities change across the landscape. For each plot, the plant community
composition is different in respect to the other plots (Figure 6 and 7). In addition, plant
communities closer to the terminal moraine were more similar to each other than those
after 600m; closer to the glacial tongue. Plant communities close to the terminal moraine
had similar bryophyte and debris cover facilitating a similar plant community
composition (Figure 5). The similarity of environments creates the opportunity for a more
uniform pattern of vegetation across the landscape. In comparison, plant communities
closer to the glacial tongue were characterized by increased topographic diversity. At
600m, a change in debris cover with a higher proportion of bedrock existed. This change
continued to the glacial tongue. Varying factors such as slope, moisture availability, cliffs
and boulders create a diverse array of microhabitats (Kruckeberg 2002). These different
habitats ultimately contribute to the dissimilarity of plant communities discovered near
26
the glacial tongue. In the future, progressive replacement of plant communities near the
glacial tongue by those characterized by increased similarity in topography and
biodiversity may occur. These communities represent a more mature ecosystem and later
successional stage.
The change in topography at 600m could potentially impact plant succession and
community development by decreasing the chances of plant survival, caused by poor soil
and nutrient availability (Matthews, 1992). On the other hand, plant pioneer species are
shown to increase soil biomass and nutrients and create a more favorable environment
over time for successive plants to colonize (Ohtonen, et al., 1999). More research should
be performed to determine whether bedrock is shaping plant communities in the Asulkan
glacier or vice versa. Both initial site conditions and facilitative effects of plant
colonization have been shown to influence the rate of succession (Chapin, 1994). In
addition, this study did not evaluate the effect of slope on plant communities. The effect
of substrate and slope could be important mechanisms in the succession of these alpine
communities.
Indicator plants can be used to extrapolate about environmental conditions and
successional stages. Salix sp., Epilobium latifolium and Leptarrhena pyrolifolia are early
successional indicator species, present in environments characterized by full sunlight and
moderate soil nutrients (Klinka et al.,1989; Beaudry at al. 2003). The composition of
these plants was positively correlated (Figure 8); the presence of one influencing the
presence of the other. In addition, factors such as proximity to an available seed source
(surrounding vegetation), length of life histories and dispersal mechanisms contribute to
their presence in these disturbed sites. Epilobium latifolium increases in abundance in
27
early pioneer seral stages and thus should be present near the tongue of the glacier. In our
study, Epilobium latifolium was found continually from the terminal moraine to the
tongue of the glacier, which supports its presence as an indicator species for pioneer seral
stages. Although the recently deglaciated landscape appears inhospitable, often
characterized by a high pH and low nutrients, these small concentrations of nutrients and
available substrate are enough to support such pioneer species (Matthews, 1992). Such
pioneer seral stages favor plants such as Epilobium latifolium that have smaller, easily
dispersed seeds and faster life histories (Whittaker, 1993; Chapin et al., 1994). In
comparison, species such as Salix sp., with a slower lifecycle fail to demonstrate the same
response as shown by pioneer species. Provided with a close seed supply and adequate
moisture these species can be found in the early successional stages instead of the initial
pioneer stages (Whittaker, 1993).
Trees require increased soil biomass and nutrients to survive, therefore they are
usually found in early to secondary successional stages (Klinka et al.,1989; Beaudry at
al. 2003). Furthermore, they are usually found in conjunction with higher plant diversity,
as the conditions able to support tree species are usually favorable for a whole host of
shrubs and herbaceous plants. In the forefield of the Asulkan glacier, Tsuga mertensiana
increased in abundance in plots closer to the terminal moraine (Figure 10). This
corresponds to the presence of Phyllodoce sp. and Vaccinium membranaceumi, two high
elevation herbaceous plants found in nutrient medium soils. Increased abundance of these
herb and tree species increases habitat complexity and provides resources important for
animals such as birds, bears and small mammals (Fastie, 1995; Beaudry at al. 2003).
Specifically, Phyllodoce sp. does not seem to develop healthy, locally regenerating
28
populations until terrain is in excess of 50 years of age (Whittaker, 1993). The correlation
of these plants indicate that closer to the terminal moraine, the glacial forefield is in early
to secondary successional stages. The increased biodiversity found within these plots is
indicative of a more advanced successional stage. When compared to the timeline of the
glacier, it can be said that such changes arose approximately over 100 years after glacial
recession. In comparison, the landscape exposed for less than 100 years, closer to the
glacial tongue, remains in the pioneer seral stage. After 600m, biodiversity decreases,
plants requiring more soil nutrients disappear and pioneer species such as Epilobium
latifolium persist. In Glacier Bay, Alaska, Chapin et al. (1994), assessed succession in
glacial forelands. This study found similar results in regards to succesional change over
time. Study sites showed progress from pioneer seral stages to more advanced
communities from 50 to 100 years after glacial recession. After 100 years, early
successional species (Alnus crispa) began to be overtaken by mature successional species
(Picea sp.)
The plant communities present across the forefield of the Asulkan glacier are part
of a dynamic succession and will eventually change. Pioneer plants such as Epilobium
latifolium and Alnus crispa will eventually be shaded out by later successional plants
such as Picea sp. The time line created with dendrochronology and lichenometry, can be
used to approximate plant community composition and successional seral stage following
disturbance. When combined with these dating techniques, the rates of plant succession
could be measured and applied to changing ecosystems.
29
Conclusion
Succession is the process in which a community of organisms changes over time
after a disturbance such as the retreat of a glacier. Species richness and indicator species
were used to assess the dynamic succession of plant communities along the forefield of
the Asulkan glacier. Progression of succession is accompanied by increasing biodiversity.
It was found that communities close to the terminal moraine had higher biodiversity than
those close to the glacial tongue, indicating later successional stages. Furthermore, the
presence of nitrogen-rich soil indicators, such as Tsuga mertensiana, in communities
closer to the terminal moraine were indicative of later successional stages. As seen in
communities closer to the glacial tongue, presence of nitrogen-poor soil indicators, such
as Epilobium angustifolium, are indicative of earlier seral stages. It was found that those
plots from the terminal moraine to 600 m had been exposed for over 100 years and were
in early to mature successional stage. Those plots from 600 m to the glacial tongue were
found to have been exposed for less than 100 years and were in pioneer successional
stage. The retreat of the Asulkan glacier began in 1889 and progressed to its current
position at an average rate of 16m per year. Severe environments, such as the forefield of
the Asulkan glacier, employ many mechanisms that contribute to the succession of plant
communities over time. Diverse topography, seed-source proximity, and soil facilitation
all influences ecosystem change. In a world that is destined to continually undergo
natural disturbances, such as glacial recession, understanding succession enables us to
predict the future of dynamic ecosystems.
30
Acknowledgements
We would like to extend a big thank you to Miss Kara Pitman for all her help and support
throughout this course. Thank you to Jim Gardner for sharing your remarkable stories
about the mountains with us while braving the wind and rain in the field, we couldn’t
have kept such a positive attitude without you. Dr. Dan Smith, many thanks for such a
wonderful and inspiring course, the mountains of Glacier National Park truly captured
our imaginations. We also send our thanks to our wonderful friends who made Geog 477
such a fun experience, we hope to meet you in the mountains again someday soon!
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