Impacts of climate change on vegetation and geomorphologic

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IMPACTS OF CLIMATE CHANGE ON VEGETATION AND
GEOMORPHOLOGIC HAZARDS IN THE CHIC-CHOCS RANGE,
EASTERN CANADA
Daniel Germain1 & Guillaume Fortin2
1
Department of Geography
Université du Québec à Montréal
Case Postale 8888 Succursale Centre-Ville
Montréal (Québec) H3C 3P8 CANADA
Tél. + 1-514-987-3000 poste 7096
[email protected]
2
Department of Geography
Université de Moncton
165 avenue Massey,
Moncton (Nouveau-Brunswick) E1A 3E9 CANADA
Tél. + 1-506-858-4364
[email protected]
ABSTRACT
Mountain environments are very sensitive to climatic and environmental fluctuations. In view of
the ongoing and anticipated impacts of global climate change on mountain systems, we must
develop valid forecasting models for sediment flow patterns, notably for high-magnitude
geomorphic events which can represent a risk and a major disturbance for mountain geoecosystems. This study proposes a retrospective approach based on the history of high-magnitude
events in the Chic-Chocs Range in the northern Gaspé Peninsula of eastern Canada.
Dendrogeomorphologic techniques have allowed us to retrace the sedimentary dynamics of the
last century with an annual resolution. The results obtained shed new light on the impact of the
frequency and intensity of different geomorphologic processes interacting with the forest cover
on short and medium timescales.
INTRODUCTION
Global climate change is expected to have an important effect on mountain geo-ecosystem processes.
Despite widely diverging projections from different global circulation models, there is a consistent
trend towards an increase in temperature. Future projections of the precipitation regime are more
variable, but greater precipitation variability is often expected, resulting in longer and/or stronger dry
seasons and a higher frequency of freeze-thaw cycles, which impact the precipitation type (rain or
snow) and therefore the type of observed geomorphologic hazards (dry or wet snow avalanches, slush
flows, debris flows, etc.).
Mountain forests undergo vertical shifts in their range, upward or downward and of greater or lesser
amplitude, following climatic variations (Walsh et al. 2003). However it seems that a certain delay is
necessary to allow the vegetation to adapt to the new climate conditions. In the absence of major
perturbations, the structural properties of the forest cover create an inertia effect allowing mountain
forests to hold up against changing climatic conditions for some time. The work of Camarero et al.
(2000) in the Spanish Pyrenees have revealed the dominating influence of abiotic factors (strong winds,
avalanches) on the spatial structure of the tree line.
Contrary to the dynamic of mountain forest ecosystems, geomorphologic activity can change rapidly
during periods of changing climate (Germain et al., 2009), increasing in frequency and intensity as was
the case during the Little Ice Age. An increase in climate variability (stronger storms and blizzards,
more frequent thawing) could also have severe impacts on the frequency of geomorphologic processes
with a strong destructive capacity, such as snow avalanches and debris flows. Because they are often
associated with specific meteorological phenomena, these geomorphologic processes could be more
sensitive to climate changes than the forest systems that they in turn affect.
In order to develop a valid forecasting basis for hillslope dynamics and mountain ecosystems in
response to climate change, the analysis of past geomorphic events over medium and long timescales is
required, rather than solely analyses on short timescales. However, linking geomorphic activity and
weather conditions using standard forecasting methods is a difficult task in the Chic-Chocs Range of
eastern Canada, because of the lack of surveys and of weather stations providing long-term records,
both at low and high elevations. However tree-ring data from several sites spread over large areas may
improve our understanding of the spatiotemporal variations of natural hazards (Germain et al., 2010).
The natural environment has in fact provided us with two fairly recent reference models that take
advantage of the annual resolution provided by dendrochronology. These models are: i) the warming of
the late nineteenth and twentieth centuries that followed the Little Ice Age and ii) the period of
accelerated warming, unprecedented at the scale of the last millennium, which has been ongoing since
the late 1980s. Our main objective was to study, quantify and strengthen the knowledge base of the
interactions between geomorphological hazards, the mountain forest environment, and climate change.
Our analyses focused on: i) climate variability for the period 1970-2010; ii) the recent tree line shift
and the acceleration of tree colonization rates in the alpine belt; iii) the frequency-magnitude
relationship of geomorphological hazards; and iv) the synergistic influence of climate, ecological
parameters, and geomorphic processes in the general response of mountain ecosystems to the climate
warming occurring since the end of the Little Ice Age.
STUDY AREA
The Chic-Chocs highlands are located in the northern Gaspé Peninsula, at the northeastern limit of the
folded sedimentary Appalachian system. The topography, which ranges from the sea level to
1,200 m.a.s.l., is dominated by three plateau levels strongly dissected by glacial valleys (Hétu and
Gray, 1980). In these mountains, numerous active geomorphological processes have already been
identified (snow and ice avalanches, slush flows, debris flows, hyperconcentrated flows, etc.) (Hétu and
Gray, 2000).
The climate of the Chic-Chocs Mountains is complex, largely due to orographic effects and the
influence of oceanic air masses on precipitation and temperature, which results in frequent thermal
inversions and high cloudiness (≥ 70%, Gagnon, 1970). Because of the alternating periods of warm and
cold conditions lasting from days to weeks which are associated with synoptic weather changes, rain
can occur in any month during the snow season, even on the highest summits. The mean annual
temperature ranges from +3ºC at sea level to –4ºC at 1,200 m.a.s.l. (Gagnon, 1970). The annual
precipitation ranges from 800 mm at sea level to 1,600 mm in the mountains, with 40% falling as snow.
The altitudinal climatic gradient is responsible for the change in vegetation from mixed balsam fir
(Abies balsamea (L.) Mill.) – white birch (Betula papyrifera Marsh.) forests at lower elevations (<600
m) to a subalpine forest belt (600 to 1,000 m.a.s.l.) dominated by white spruce (Picea glauca (Moench)
Voss), black spruce (Picea mariana (Mill.) BSP), and balsam fir (Richard and Labelle, 1989). Alpine
tundra is found at elevations greater than 1,000 m.a.s.l.
METHODS
Climatic Data Analysis
Recently a joint temperature/precipitation modes approach have been used for different regions in the
Alps to detect trends based on four climatic scenarios : Cold/Wet (CW), Cold/Dry (CD),
Warm/Wet(WW) and Warm/Dry (WD) (Beniston, 2009; Beniston et al., 2010; Lopèz-Moreno et al.,
2011). One advantage of this method is to relate the snow cover, which is greatly affected by the
climate change, with the four scenarios. We have used a similar approach for the data coming from six
weather stations located in the Gaspé Peninsula over the last forty years. Other statistical tests (Mann
Kendall test) have been applied to our data, particularly for the winter months (December to April) in
order to detect any climatic trend for the period 1970 to 2010 (i.e. see Fortin and Hétu (2011) for more
details).
Diachronic Treeline Monitoring
Since alpine vegetation is a reliable indicator of climatic changes, we have used the data of Fortin and
Pilote (2008), which have analyzed diachronic panchromatic aerial photographs (scale 1:15,000) for the
period 1973 to 2004 in order to characterize the spatial evolution (expansion or retreat) of the
vegetation. The aerial photographs were digitized and orthorectified using GIS (ArcInfo).
Supplementary field measurements were made using a Differential Global Positioning System (DGPS).
Dendrogeomorphology
Site selection and mapping
Site selection was made partially based on accessibility given the low density of roads in the region,
and also with respect to the presence of trees providing a relatively high potential for
dendrochronological reconstruction of past events. Woody vegetation in zones impacted by the transit
and arrival of geomorphologic processes (snow avalanches, debris flows, hyperconcentrated flows)
should show obvious signs of impact caused, a priori, by the geomorphologic activity. The absence of
apparent perturbation by other factors (fire, forest cuts) which could influence the frequency and
intensity of geomorphologic processes was also considered during site selection. The sites were first
surveyed to record, map and measure main geomorphologic attributes which were then entered in
ArcMap and supplemented with various morphometric parameters such as area, length, gradient, relief,
etc. Key sampling sites were also identified.
Tree sampling
Given the absence of historical data, the dendrochronological analyses of tree damage caused by snow
avalanches and debris flows is particularly useful to reconstruct the chronology and spatial extent of
geomorphologic activity, and even the intensity of such activity. The presence of reaction wood and
impact or abrasion scars provides the best means to date past events. The scars are formed upon impact
on the tree trunk by rocky material and broken tree trunks transported by the flow, causing abrasion of
the crust and part of the cambium. The subsequent formation of scar tissue can be dated by the number
of growth rings.
For each of the study sites, transversal trunk sections were taken from a population varying from 10
to 243 trees. The trees sampled were located for the most part in the track and runout zones or in the
forest edges adjacent to the avalanche path (trimline). Sampling was mainly carried out on trees
showing heightened information potential (trees with multiple scars). This reduced the total necessary
number of sampled trees, particularly important given the location of certain sites within the Gaspésie
National Park. At each of the sites, sampled trees were positioned to determine the spatial extension of
reconstructed events. All trees were live at the time of sampling.
Tree-ring reconstruction of high-magnitude events
The transversal wood sections were dried and sanded for observation under a binocular magnifying
glass to determine the age of each section and to date the impact and abrasion scars, breaks, and the
sequence of reaction wood using the number of growth rings. Some diagnostic rings were also used to
date the oldest trees. The first 20 growth rings from sections taken at the soil surface were not
considered due to their small stem diameter, and the possibility that their deformation is related to snow
conditions.
The identification of years of strong geomorphic activity relies on the temporal and spatial analysis of
dendrochronologic indicators. The distinction between years of intense geomorphic activity and years
of normal geomorphic activity was determined by the response rate (number of damaged trees/number
of living trees) for each year. The fitting of the General Pareto Distribution (GPD) to the extreme
values of the dataset was used to identify a critical response rate threshold (Embrechts et al., 1997).
RESULTS AND DISCUSSION
Climate and Alpine Vegetation Dynamics
The results of our statistical analysis of seven weather stations in the area showed no obvious trend in
air temperature, precipitation regime, or any of the four modes of climate (CW, CD, WW, WD). These
observations are in accordance with the results of Fortin and Hétu (2010), which showed no evidence
of statistically significant changes for snow depth, snow density and snow water equivalent for the
period of 1980-2009 in the Gaspé Peninsula. However, the analysis of diachronic aerial photographs
revealed an important increase in the vegetation cover in the alpine belt, particularly on flat areas and
very gentle slopes. We observe on the one hand a significant decrease in meadows/alpine grasslands
and rocky surfaces (Table 1) and on the other hand a significant increase in shrubs (Table 1) by infilling of shrubs patches and colonization of new areas for the period from 1973 to 2004. These results
are in accordance with the observations Tape et al. (2006) in Alaska (USA).
Classes
Zone 1
-6.11
Meadows
20.04
Shrubs
-6.24
Rocks
+ area increase; – area decrease
Area changes %
Zone 2
Zone 3
Zone 4
10.22
-17.02
-0.35
2.75
12.45
1.32
-12.63
-14.98
0
Modified from Fortin and Pilote, (2008).
Table 1. Changes of the land cover in four different sites on the plateau of the Mount Jacques Cartier
(1268 m a.s.l.) for the period 1973-2004.
Tree-Ring based chronology of high-magnitude geomorphic events
Snow and slush avalanche activity
Since the mid- 20th century the number of large snow avalanches has increased dramatically, from less
than 10 total occurring in the period between 1900 and 1950 to an average of 30 per decade between
1950 and 2000. We also note that the number of large avalanches has doubled in number between
1950-1970 and 1970-1990, reaching a peak during 1990-2000 with 45 recorded events (Figure 1). The
average return period of large snow avalanches on the flanks of the high plateaus of the central
Gaspésie is 5.3 years. The minimum and maximum return periods are 2.3 and 10.5 years respectively.
With the annual probability of avalanche greater than 15% in all corridors, the subalpine setting is
characterized by greater probability of avalanche (21% on average) than the coastal mountain
environment (12% on average but with substantial differences between coastal sites).
In total, 19 years of high avalanche activity between 1895-2000 have been identified on the basis of a
regional avalanche activity index (R2 = 0,67; p <0,0001). Fifteen (79%) of these 19 years are after 1950
(Figure 1), which is associated with a shorter return period of 3.3 years (15 over 50 years), underlining
again the revival of avalanche activity during the second half of the 20th century (Figure 1).
Figure 1. Avalanche activity and chronology of widespread snow avalanche cycles (grey bars). The
zero represents the average avalanche activity for the last century.
These low frequency large avalanches with high destructive capacity (associated with an average of
65% of observed tree damage) are responsible for the dynamic equilibrium between forest vegetation
and the avalanche runout zone. These avalanches play a crucial role in shaping the forest landscape of
the central Gaspésie by causing the fragmentation of the forest area in parallel bands from higher to
lower on the slopes. The forest mosaic is characterized by a lowering of the altitudinal limit of the
forest at the locations of avalanche corridors.
Given the anticipated climate changes, particularly an increase in climatic variability on diverse spatial
scales, we can expect an increased fragmentation of mountain forest landscapes characterized by steep
slopes. Snow avalanches seem more sensitive to climate change than the forest, or partially-forested
systems that they affect. However recently acquired data on large snow avalanche regimes in coastal
regions (Dubé et al., 2004; Germain et al., 2005) demonstrates an important disparity with the
avalanche regime prevalent on the highlands of central Gaspésie. In fact only three (1898, 1939 and
1977) of the 19 years of high avalanche activity identified in the subalpine setting of the current study
were also identified in the avalanche chronology of the coastal areas.
From a meteorological point of view, heavy snowfalls create favourable conditions for snow avalanche
formation, but this alone is not enough to explain the 19 identified years of high activity in the
subalpine settings. Snow and weather conditions during these 19 years can be summarized as follows:
i) 16% (3 years) are associated with exceptionally heavy snow fall conditions; ii) 11% (2 years) with an
increased frequency of snowstorms; iii) 21% (4 years) with heavy winter rains (precipitation overload
and formation of a melt-freeze crust); iv) 21% (4 years) with episodes of freezing rain and strong
winds; v) 16% (3 years) with the probable formation of facets and depth hoar at the base of the snow
cover. Sixteen percent (3 years) remain inexplicable due to the absence of valid meteorological data.
Although much less common in this landscape, slushflows also represent a geomorphologic risk given
their very high capacity for destruction. Defined by Washburn and Goldthwait (1958) as a mudflowlike flowage of water-saturated snow, slushflows are generally taking place on weak slopes. On
January 14th, 2006, the Coulée des Mélèzes (Mont Albert), a mountain torrent of a first order with an
11° slope, was swept by a spectacular slushflow of more than 1 kilometer in length. The source of the
Coulée des Mélèzes is a funnel-shaped glacial cirque where snow avalanches frequently occur on the
slopes. Around mid-January, the head of the gully that drains the glacial cirque was obstructed by a
compact snow cover of a few meters thick, formed by snow avalanche deposits and blowing snow from
strong winds that swept the alpine plateau.
The slushflow of January 14th was triggered by the combined effects of heavy winter rain (72 mm) and
accelerated melting caused by a strong increase in temperatures that reached 11.3°C during the day.
The tunnel-like morphology of the small basin caused stream flow to concentrate at the head of the
gully, leading to the saturation of the snow cover. The rupture of the waterlogged snow took place at
the foot of the cirque slopes, in an area where the slope decreased sharply from more than 30° to about
15°. After flowing through the entire length of the mountain torrent (approximately 1 kilometer), the
slushflow spread throughout the southern half of accumulation zone, leaving behind a huge deposit of
mineral and organic debris (broken and uprooted trees).
Debris flow and hyperconcentrated flow dynamics
The event-response histogram derived from 165 tree samples on an alluvial fan allowed us to
reconstruct a chronology for a 129-year period (1880-2009) (Ouellet and Germain, 2010). However
1934 marks the true beginning of the chronology, when a significant minimum of 10 living trees is
reached. The GPD method, used to distinguish between high-magnitude and normal years, determined
a critical response threshold of 8.5 %. Between 1934 and 2009, eight years show a response rate higher
than this threshold (1969, 1975, 1980, 1988, 1996, 2000, 2002, and 2003). This information was used
to calculate a minimum return period for high-magnitude flow events of 9.25 years and a corresponding
annual probability of 11%. As in the case of the snow avalanches, the geomorphic activity associated
with hyperconcentrated flows seems to have been absent or of very weak intensity during the 19341969 period.
Like the sedimentary dynamic of the alluvial fan studied, coastal scree slopes, on which debris flows
occasionally occur, are characterized by sustained plant colonization between 1850 and 1940, with a
low sedimentation rate. The second half of the 20th century was then characterized by strong
geomorphic activity associated with the occurrence of debris flows, notably during the summers of
1979, 1987, and 2009, which also contributed to the one-time lowering of the tree line by several dozen
meters.
CONCLUSION
Geomorphologic hazards in the Chic-Chocs Range, such as snow avalanches, debris flows and
hyperconcentrated flow events increased significantly over the course of the 20th century, particularly
during its second half. Based on the observations of this study, the interactions between geomorphic
activity, climate and forest cover over time need to be more thoroughly documented. An important
question is whether the opening of the forest cover occurs as a result of debris flows which favour the
occurrence of avalanches. This issue currently seems to be prerequisite to an improved understanding
of mountain environments generally. It will allow the validation of the assumption that the occurrence
of external factors influences the frequency-magnitude relationship of geomorphological hazards due to
changes in environmental conditions at a local scale. Should this assumption be proven true, the fact
that geomorphologic hazards and mountain geosystems and ecosystems will not respond in a linear
manner to future climate change will need to be taken into account.
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ACKNOWLEDGEMENT
This research was financially supported by the Natural Sciences and Engineering Research Council of
Canada (NSERC), the Fonds québécois de recherche sur la nature et les technologies (FQRNT),
Emergency Preparedness Canada, and Quebec-New-Brunswick Scientific Cooperation. We are also
indebted to the Gaspé National Park for providing access to snow avalanche paths and permission to
sample trees.
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