Snow avalanche activity after fire and logging disturbances, northern ...

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Snow avalanche activity after fire and logging disturbances, northern Gaspé Peninsula, Quebec, Canada Daniel Germain, Louise Filion, and Bernard Hétu

Abstract: In mountainous areas, ecological disturbances causing forest fragmentation may influence the pattern and regime of snow avalanches. In the northern Gaspé Peninsula (Quebec), at two sites located on treed slopes of a south– north oriented valley, tree removal by fire and logging operations was the precursor factor for avalanche activity. Years of high-magnitude snow avalanches were identified based on tree-ring techniques; these avalanches were different from those identified by Dubé et al. (2004) for three undisturbed scree-slope sites in the same area. The lack of synchronicity in avalanche occurrences between disturbed and undisturbed sites suggests a strong influence of local factors (e.g., disturbance, local topography, slope aspect, vegetation). The first avalanche years were recorded in 1941 and 1988, after a fire in 1938 and logging operations in 1986–1987, respectively. Both of these years had above average snowfalls; this indicates that climate (total annual snowfall) was also a contributing factor for avalanche activity. The avalanche window in the post-logging site was shorter (four years) than that of the post-fire site (15–20 years). This is an indication that avalanche activity after tree removal largely depends on the capacity of woody vegetation to reach heights sufficient to control snow drifting and thus avalanche activity. Résumé : Avalanche de neige après feu et coupes forestières, Gaspésie septentrionale, Québec, Canada. Dans les régions montagneuses, les perturbations écologiques causant une fragmentation du couvert forestier peuvent influencer la répartition et le régime des avalanches de neige. En Gaspésie septentrionale, dans deux sites situés sur des versants forestiers dans une vallée d’orientation sud-nord, le déboisement par le feu et l’exploitation forestière a été le facteur précurseur d’une activité avalancheuse. Les années de grosses avalanches identifiées par des méthodes dendrochronologiques diffèrent de celles identifiées par Dubé et al. (2004) dans trois talus d’éboulis non perturbés, dans la même région. L’absence de synchronisme entre les sites perturbés et les sites non perturbés suggère une influence déterminante de certains facteurs locaux (perturbation, topographie, orientation des versants, caractéristiques du couvert végétal). Les premières années avalancheuses suivant le feu de 1938 ainsi que la coupe forestière de 1986–1987 (1941 et 1988) ont été des années durant lesquelles la précipitation en neige fut supérieure à la moyenne, une indication que le climat est aussi un facteur important dans la formation de ces avalanches. La fenêtre d’activité avalancheuse après la coupe forestière fut plus courte (4 ans) que celle après feu (15–20 ans). La formation des avalanches suivant le déboisement dépend donc, dans une grande mesure, de la capacité de la végétation ligneuse à atteindre une hauteur suffisante pour restreindre la redistribution de la neige par le vent et l’activité avalancheuse. Germain et al.

Introduction Because of their impact on woody vegetation in mountain areas, snow avalanches have been recognized as a major disturbance factor (Khapayev 1978; Johnson 1987; Veblen et al. 1994; Larocque et al. 2001). Despite considerable data on snow avalanche occurrence and frequency, especially in alpine sites where historical records are available, the ecological impact of avalanches on treed slopes remain poorly documented. Nevertheless, the structure and diversity of plant

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communities in avalanche paths and vegetation patchwork on steep slopes are largely influenced by the variability in space and time of avalanche occurrences (Erschbamer 1989; Patten and Knight 1994; Walsh et al. 1994; Camarero et al. 2000). Disturbances, on the other hand, especially those causing deforestation, can influence both the distributional pattern and regime of avalanches. Tree removal caused by forest disturbances, e.g., fire or logging, may contribute to increased avalanche activity (Butler et al. 1992; McClung 2001; Weir 2002).

Received 29 July 2004. Accepted 31 August 2005. Published on the NRC Research Press Web site at http://cjes.nrc.ca on 7 February 2006. Paper handled by Associate Editor R. Gilbert. D. Germain1 and L. Filion. Centre d’études nordiques and Département de géographie, Université Laval, Québec QC G1K 7P4, Canada. B. Hétu. Centre d’études nordiques and Module de géographie, Université du Québec à Rimouski, QC G5L 3A1, Canada. 1

Corresponding author (e-mail: [email protected]).

Can. J. Earth Sci. 42: 2103–2116 (2005)

doi: 10.1139/E05-087

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Fig. 1. Location of the study area and sites T-9 and T-10 in the Mount St. Pierre Valley, northern Gaspé Peninsula. Arrows identify snow avalanche paths (C-1–C-4). Figure used with permission.

Tree-ring techniques have been particularly useful in the assessment of past avalanche activity (e.g., Potter 1969; Butler 1979; Carrara 1979; Rayback 1998). In the Gaspé Peninsula (Fig. 1), the ecological impact of snow avalanches were recently documented at subalpine (Larocque et al. 2001; Boucher et al. 2003) and coastal forest sites (Dubé et al. 2004). Dubé et al. showed that high-magnitude avalanches have no direct geomorphic impact on treed scree-slope sites in the valley where this study was conducted. However, large avalanches can severely damage woody vegetation along their paths and cause local treeline recession. It was also shown that most high-magnitude avalanches occurred during snowy winters. Although past large-scale disturbances, such as wildfire and abiotic or biotic factors causing forest opening (e.g., windthrow, insect outbreaks, snow avalanches) can be dendrochronologically reconstructed, quantitative data describing interactions among these disturbances are still scarce (Veblen et al. 1994). However, data on past logging activity can be traced easily, and logging is a major issue in snow hazard management in montane environments because of its effects on forest fragmentation (McClung 2003; Canadian Avalanche Association 2002). In British Columbia, surveys of snow avalanche paths over the last three decades indicated that avalanches occurred in -10 000 clearcut areas (McClung

2001). In this paper, we document snow avalanche activity at two disturbed sites (T-9 and T-10) on treed slopes of the Mount St. Pierre Valley in the northern Gaspé Peninsula (Fig. 1). Aerial photographs showed that tree removal by fire and cutting operations took place occasionally during the 20th century. We thus hypothesized that snow avalanche activity was possibly initiated after fire and logging disturbances in an area where high-magnitude avalanches were known to occur on active scree slopes only (Hétu and Vandelac 1989; Hétu and Gray 2000; Dubé et al. 2004). The objectives of this research were twofold: (1) to provide a tree-ring-based chronology of snow avalanches for two disturbed sites originating from fire and logging, respectively, and (2) to evaluate the impact of tree removal (through fire and logging) as a precursor factor for avalanche activity, along with the influence of site and climate (snowfall) factors in avalanche initiation. The recolonization patterns of the two avalanche-disturbed sites by trees were also examined. Our data were compared with those from undisturbed, scree-slope sites from the study area.

Methods Study area In the northern Gaspé Peninsula, the coastal plateau sur© 2005 NRC Canada

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rounding the Mount St. Pierre Valley is largely composed of Cambrian–Ordovician folded rocks of the Cloridorme Formation (Enos 1969; De Römer 1977). The surface of the plateau (-400–600 m above sea level) is deeply dissected by valleys. Several scree slopes, which developed throughout the postglacial period, were stabilized by vegetation sometime during the Holocene (Hétu and Gray 2000; Dubé et al. 2004). The mean annual temperature is about 3.2 °C and monthly minimum (January) and maximum (July) temperatures averaged –14.5 °C and 20.7 °C, respectively. The annual precipitation totals about 825 mm, with total snowfalls averaging 333 cm. Because of the strong maritime influence, the climate is characterized by alternating periods of warm and cold conditions lasting from days to weeks, which are associated with synoptic weather changes. Prevailing winds (35% frequency) blow from the northwest (Gagnon 1970). As a result, slopes on the west valley side (lee side) accumulate larger amounts of snow than those on the east side; the slopes are largely exposed to strong winds from the northwest across the St. Lawrence Estuary. Because of high turbulence close to rockwalls and effective snow drifting, active scree slope can remain free of snow until the end of January (Hétu and Vandelac 1989; Hétu and Bergeron 2004). The altitudinal gradient from the valley to the plateau is responsible for a shift from deciduous to coniferous forests. At lower levels ( 5 cm, which were piled up in runout zones, were measured (T-9, number of diameters measured (n) = 87; T-10, n = 172). Diameters of normal standing trees were also measured at breast height in undisturbed mature forests adjacent to runout zones (T-9, n = 78; T-10, n = 66). The orientation of each leaning stem was determined and inclination was measured with an inclinometer. Six Schmidt stereograms were built for runout zones: four for T-9 and two for T-10. The following statistics were calculated: (1) the vector magnitude L, which reflects the strength of any preferred orientation. According to Curray (1956), this parameter can be combined with the Rayleigh test to calculate the probability (p) to obtain a vector magnitude larger than L by pure chance combination of n random orientation vectors. A p > 0.05 indicates that randomness of orientations cannot be rejected; (2) the normalized eigenvalues E1, E2, and E3, and the eigenvalue ratios, where r1 = [ln (E1/E2)] and r2 = [ln (E2/E3)], for discriminating cluster and girdle fabric shapes; (3) the K ratio, where K = r1/r2 as per Davis (1986) and Woodcock’s notation (1977); and (4) the C index, where C = [ln (E1/E3)] with high index values corresponding to high fabric strength. The spherical © 2005 NRC Canada

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Fig. 2. Sites T-9 and T-10 in the Mount St. Pierre Valley. (A) Extent of the 1938 fire (shaded) and location of the avalanche path studied. Also shown are the positions of the two 250 m2 quadrats used for the study of tree regeneration (size and age structures). (B) General view to the northewest showing the area burned during the fire in the upper part of the slope and the avalanche path penetrating into the forest (April 2001). (C) and (D) Aerial photographs of site T-10 before (C: 1986) and after (D: 1992) logging activity in 1986–1987 on the plateau. The 1992 photo shows several avalanche paths on the steep east-facing slope. Also shown is the position of the quadrat used for the study of post-logging regeneration (size and age structures). Figure used with permission.

variance (Svar) of fabrics was also calculated, high values reflecting high dispersion or low fabric strength. Tree regeneration in the post-fire, avalanche-disturbed site: T-9 Post-disturbance balsam fir regeneration was studied (fire on the plateau and avalanche on the slope). Although white cedar was also found in the avalanche path, we did not consider this species because it is confined to scree substrates. Fir stems were surveyed in two 250 m2 (50 m × 5 m) quadrats:

one on the plateau and one midslope in the avalanche track (Fig. 2A). The first quadrat was positioned perpendicular to the slope, at the outer edge of the plateau surrounding the avalanche track. The second quadrat was also positioned perpendicular to the slope, within the lower avalanche track, but below the lower limit of the 1938 fire because of the scarcity of vegetation in the upper avalanche track and the starting zone. Fir stems with a dbh < 15 cm were cut at ground level, whereas those with a dbh > 15 cm were cored with a Pressler probe 30 cm above ground level. All trees © 2005 NRC Canada

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Fig. 3. Longitudinal profile of the slope at site T-9 showing the different zones within the avalanche paths. Slope angles were averaged over 50 m segments. The two 250 m2 quadrats used for the study of post-fire regeneration (size and age structures) were positioned along the lower limit of the 1938 fire. Black dots on the map indicate damaged trees used for the reconstruction of past avalanche activity. Also shown in (A) are the diameters of broken trees as a function of standard deviations from the mean diameter, in fully preserved (䉭) and partly decomposed (䉬) tree stems, and fabrics (circles in B to E) from broken trees. The bottom of the circles indicates the downslope direction.

sampled were cored to the pith. Cored trees represented 9% of all fir sampled on the plateau, whereas no trees with a dbh > 15 cm were found in the avalanche track. The diameter was measured at ground level for each living fir stem. Tree rings were counted on finely sanded cross-sections and cores. No correction was applied to the age of the trees. Age–diameter relationships were evaluated for both balsam fir populations (plateau and avalanche track). The reference year for age structures was 2000. Tree regeneration in the post-logging, avalanche-disturbed site: T-10 At site T-10, tree stems established after logging were surveyed within 25 m2 (5 m × 5 m) plots at the four corners of a 400 m2 (20 m × 20 m) quadrat, which was positioned at random at the outer edge of the plateau overlooking the avalanche paths (Fig. 2D). Stem diameter at ground level and total height were measured for each living conifer (i.e., balsam fir and white spruce) and deciduous (yellow birch) tree species. Tree stems were cut at ground level and age was determined by counting the number of growth rings on

cross-sections. Stem density was calculated for each species within each 25 m2 plot. Because of its scarcity in all four plots ( 3, and high E1 and r1 values except for E (sample E, stand T-9), which had a lower C index as well as lower E1 and r1 values (Table 3). Stereograms indicate that stems tended to be oriented parallel to the local slope, either laterally (Figs. 3B, 3C, 3E) or downslope (Fig. 3D). Because of the low p values (Rayleigh test), the hypothesis of a random distribution of the orientations at a 0.05 level was rejected, except for B (Table 3). Fabric shapes reflected by K ratios showed moderately developed girdles to well-developed clusters. Post-disturbance tree regeneration The two balsam fir age structures, one from the plateau where forests were destroyed by the 1938 fire and one from the lower avalanche track (i.e., non-burned zone of the avalanche path), are both multi-modal (Figs. 4A, 4C). On

the plateau, fir > 65 years of age likely survived the 1938 fire and recruitment was continuous over the last 60 years (Fig. 4A). Stem diameter increased with age, but scattering data among the larger (dbh > 15 cm) and older (> 70 years of age) trees indicated differential conditions for tree growth in this second-growth forest (Fig. 4B). In the avalanche path, balsam fir peaked in the 45–50 to 55–60 years of age classes (between -1940 and 1955) (Fig. 4C), but stem diameters did not exceed 8 cm (Fig. 4D). Small tree diameters likely reflect poor growth conditions compared with those on the plateau. Past avalanche activity At site T-9, years with site total response scores above threshold values were 1941, 1945, 1955, and 1988–1995 (Fig. 5A, Table 4). The two groups of broken or uprooted trees in the avalanche track, based on stem preservation (Fig. 3A), likely reflect the two periods of avalanche activity, i.e., 1941–1955 and 1988–1995. Forty-two of the 78 living trees (54%) were sampled along the avalanche path to reconstruct the avalanche activity, which was established to be after 1955 but before the 1988–1995 events; this indicates that the recent avalanches were of lower magnitude than those postdating the 1938 fire. The lower magnitude of recent avalanches was also confirmed by the extent of past runout zones as determined from the distribution of damaged trees (Fig. 5B). Despite similar runout distances (-675–700 m), after the avalanches of the 1940s and 1950s the runout zone was -20 m wider than that of the 1988–1995 events. The height of impact scars varied from 0.16 to 6.35 m above ground level. Maximum heights of scars likely reflect the impact of avalanche-transported stems on standing trees rather than the thickness of the snow mass (Mears 1975). No correlation was found between distances from the avalanche starting zone and scar heights, nor between scar heights and ages. The post-logging, avalanche-disturbed site: T-10 Site characteristics and avalanche damage On the plateau, logging operations (i.e., clear cut) were carried out in the mid-1980s when -2250 ha of forest were harvested (Figs. 2C, 2D). Avalanche activity started sometime after logging, but before 1992 when 5 ha of mature forest were destroyed. Indeed, the 1992 aerial photographs showed well-delineated avalanche paths on this steep slope (Fig. 2D). The mean diameters of broken trees from two avalanche © 2005 NRC Canada

2110 Fig. 4. (A) Age structure of balsam fir at site T-9 on the plateau. Reference year is 2000. (B) Age–diameter relationships in balsam fir on the plateau. (C) Age structure of balsam fir at T-9 in the lower part of the avalanche track. (D) Age–diameter relationships in balsam fir in the lower part of the avalanche track.

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orientation according to the Rayleigh test (p ≤0.05), and C indices close to 4. The normalized eigenvalue E1 and eigenvalue ratio r1 were also high (Table 3). Stereograms showed a prominent vertical position of avalanche-transported stems (Figs. 6B, 6C) and reflected a cluster-like fabric shape. At site T-10, vegetation in the four avalanche paths reflects a gradient in avalanche disturbance. In the starting zone and upper avalanche track, the cover is scarce and dominated by small conifers (mainly balsam fir, white cedar, and white spruce) and yellow birch (Table 2), whereas paper birch clearly dominated at T-9. The lower avalanche track and runout zones were occupied by mature sugar maple – yellow birch stands. Because wood debris was widespread in runout zones, mountain maple was abundant, with coverage ≥ 40% in the four avalanche paths (Table 2). Post-logging tree regeneration on the plateau At site T-10, balsam fir (n = 699) and yellow birch (n = 220) accounted for 76% and 24%, respectively, of total tree stems surveyed within the four 25 m2 plots (100 m2). Stem density of balsam fir and yellow birch was about 6.9 × 104 and 2.2 × 104 stems ha–1. Both species showed unimodal age structures with peaks in 13–16-year age classes (-1984-1987) for balsam fir, and 13–14-year age classes (-1986 and 1987) for yellow birch (Fig. 7A), which indicates that recruitment likely started during logging from seedlings (yellow birch) and (or) advance regeneration (balsam fir). Mean stem diameters and heights were 14 ± 9 mm and 101 ± 55 cm for balsam fir, and 22 ± 6 mm and 175 ± 43 cm for yellow birch. The diameter–height relationship in the balsam fir (R2 = 0.86) differed slightly from that of yellow birch (R2 = 0.54); the light-demanding birch species showed faster radial and vertical growth than balsam fir after tree removal (Figs. 7B, 7C). Mean sapling heights of both balsam fir and yellow birch responded to a linear fit model, but showed more variations before the 1986 clearcut and after avalanche occurrences, i.e., after 1995 (Figs. 7D, 7E). Past avalanche activity At site T-10, in avalanche paths C-1 and C-2, years with site total response scores above threshold values were 1988– 1992 and 1995 (Figs. 8A, 8B; Table 4). Four of the six years (1988–1990, and 1995) were recorded in both paths. Complementary data from trees sampled in the runout zones of avalanche paths C-3 and C-4 also indicated avalanche activity in 1988, 1989, and 1990. No damage was found on standing trees outside the four avalanche paths in adjacent mature forests, where the oldest trees dated back to the late 18th and early 19th centuries. Present trimlines likely correspond to the maximum extent of snow avalanches over the last century.

paths (C-1 and C-2) and of standing trees from an adjacent undisturbed site were 24.9 ± 10.6 cm (n = 172) and 20.4 ± 6.4 cm (n = 66), respectively. The similarity in the distribution of stem diameters for the two groups of trees indicates that avalanche paths were created recently, i.e., after tree removal on the plateau (Fig. 6A). Tree stems piled up in the runout zone of the C-1 and C-2 avalanche paths revealed very high fabric strength, with L values of 66% and 42% showing a significant preferred

Snow avalanche occurrences and snowfalls Two of the 11 years of avalanche occurrences identified in this study (1955 and 1995) had above-average + 1 standard deviation (SD) snowfalls and five were rather snowy (above average), including 1988, which marked avalanche reactivation at the post-fire site (Fig. 9; Table 4). Four years had below average snowfalls (1989, 1990, 1992, and 1993). The year with the lowest total snowfalls (1990: 215 cm) had the highest total response scores for avalanche activity at both stands (Figs. 5A, 8). © 2005 NRC Canada

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Fig. 5. (A) Total tree response scores for avalanche activity at site T-9. Tree damage revealed high-magnitude events (black bars) in 1941, 1945, and 1955, and between 1988 and 1995 (see under Methods section in the text for the criteria). (B) Extent of the 1940s–1950s (䊉 and heavy line) and the 1980s–1990s (䊊 and light line) avalanches established from the distribution of damaged trees at T-9.

Discussion and conclusion Causal factors of snow avalanches We found no indication from damaged trees that snow avalanches had occurred at the two disturbed sites prior to the 1938 fire and the 1986-1987 logging operations. Snow avalanches, which occurred after these two disturbances, were either specific to site T-9 (post-fire site), i.e., 1941, 1945, 1955, 1993, and 1994, or common to both sites, i.e., 1988– 1992 and 1995 (Table 4). However for the period common to both studies (1939–1997), none of these high-magnitude avalanche events corresponded to those identified by Dubé et al. (2004) for three undisturbed scree-slope sites in the Mount St. Pierre Valley and an adjacent valley. The lack of synchronicity in avalanche occurrences among our sites and undisturbed sites (Dubé et al. 2004) suggests a strong influence of local factors, i.e., forest opening on the plateau overlooking the rock walls that created conditions conducive to snow drifting and formation of cornices. On the other hand, winters during the avalanche years 1988– 1995 were mainly characterized by freezing rain and strong wind events, i.e., 1988, 1990, 1991, and 1992 (Germain, 2005). At our study sites, these conditions were responsible for increased avalanche frequencies, as opposed to undisturbed, scree-slope sites where avalanche activity ceased between 1988 and 1995. Strong winds blowing on active scree slopes often result in snow accumulation in forest fringes, thus reducing the probability of an avalanche to release (Hétu and Bergeron 2004). The sequence of events from tree removal by fire (site T-9) and by logging operations (site T10) to avalanche formation at our sites is summarized in the following text.

Post-fire, avalanche-disturbed site: T-9 (1) The 1938 fire created local conditions conducive to snow drifting and accumulation of large amounts of snow in the burned, vegetation-free area in the upper steep slope. The burned area was clearly visible on the 1963 aerial photographs. (2) High-magnitude avalanches occurred in the 1940s and 1950s (Fig. 5A). The post-fire avalanche window (1939– 1955) thus lasted 15–20 years, which corresponded to the period of maximum recruitment of balsam fir on the plateau (Fig. 4). Large trees (dbh >15 cm) that survived the 1938 fire or were pre-established (advance regeneration which is common in balsam fir stands) contributed to reduced, if any, snow drifting on the plateau and avalanche activity on the slope after 1955. (3) Our tree-ring-based avalanche record indicated no highmagnitude events occurred between 1955 and 1988 (Fig. 5A), although low-magnitude avalanches close to the rockwall likely occurred during this -30–35-year period. (4) Avalanches returned to T-9 in 1988. In January 1988, after a period of very strong winds (>100 km h–1, 5–6 January) and major snowfall (>50 cm, 26 January), avalanche activity was also recorded at our post-logging site T-10 and the undisturbed scree-slope site T-3 studied by Dubé et al. (2004) in the Mount St. Pierre Valley. Many snow avalanches were also observed on several scree slopes along the coast (Hétu and Bergeron 2004). The total amount of snowfall for January (above-average + 2 SD) was probably the main causal factor for high-magnitude avalanche occurrence. Blizzards and snowstorms likely favoured large accumulations of snow down the wind© 2005 NRC Canada

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Can. J. Earth Sci. Vol. 42, 2005 Table 4. Avalanche occurrences and total annual snowfalls from Cap-Madeleine and Grande-Vallée weather stations at our study sites and sites studied by Dubé et al. (2004). This study

Dubé et al. (2004)

T-9

T-5

T-10 C-1

T-3

This study

Dubé et al. (2004)

Snowfall (cm)

Snowfall (cm) — 289.0 —

RC

C-2 1871 1898 1939

1941

421.7 1942

1945

1952

1947 1950 1952

1955 1956 1966

1988 1989 1990 1991 1992 1993 1994 1995

415.6 362.2

1988 1989 1990 1991

1956 1958 1966 1972 1975 1977

1988 1989 1990

442.7

1966

1977 368.0 253.0 215.4 373.8 267.0 271.6 364.4 498.6

1992

1995

1952 1955 1956

1995 1996

1996

— — 695.9 442.7 551.8 420.9 412.5 366.1 281.1 363.9 368.0

432.6

Note: Mean and standard deviation for 1885–2000, 94 years with snowfall data: 332.7±101.7. Underlined years are those in high-magnitude avalanches are identified by Dubé et al. (2004). Bold indicates above-average total annual snowfalls.

exposed rockwall where avalanche activity was initiated in the upper, sparsely vegetated slope. Because of its ability to resprout after stem tilting or breakage, paper birch is widespread in avalanche-starting zones (Larocque et al. 2001). (5) After the 1988 event, high-magnitude avalanches occurred each year until 1995, as evidenced by our tree-ring data (Fig. 5A, Table 4). Any factors likely to increase the vegetation-free area in avalanche-starting zones and decrease surface roughness should be carefully examined in the evaluation of avalanche risks on steep, treed slopes. The long-term influence of wildfires may result in slow tree regeneration and vegetation recovery or in the depletion of tree populations and a shift to a treeless environment, particularly in exposed sites, such as upper scree slopes where strong winds and turbulence close to rockwalls are common. Post-logging, avalanche-disturbed site: T-10 (1) Avalanche activity at T-10 started in 1988 and opened up new paths in formerly intact forests; this suggests the influence of factors common to T-9. Windstorm and snowfalls during January, as well as slope aspects (east

and southeast), were key variables in avalanche initiation. At this site (Fig. 1), avalanche-starting zones and slopes are on the lee side of winds blowing from the northwest. After tree removal on the plateau in 1986 and 1987, large amounts of drifting snow started to accumulate in upper slopes as of the winter of 1987–1988. (2) After the major event in 1988, avalanches occurred each year until 1992, during a period that included winters with below-average snowfalls (1989, 1990, and 1992). Avalanches reoccurred in 1995, the snowiest winter of all avalanche years recorded. Tree removal after logging operations appeared to be the precursor factor for avalanche activity on these steep slopes where local topography and slope aspects also created favourable conditions for large accumulations of snow. (3) All avalanches recorded between 1988 and 1995 showed similar runout distances despite contrasting conditions during the avalanche years in total snowfalls (e.g., 215 cm in 1991 versus 498 cm in 1995) or surface roughness (e.g., tree regeneration < 50 cm in 1988 and above 1.0 m in 1995; see Figs. 7D and 7E). Similarly, McClung (2001) found no relationship between avalanche size and mean maximum annual snowfall in the Canadian Rockies. These © 2005 NRC Canada

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Fig. 6. Longitudinal profile of the slope at site T-10 showing the different zones within the avalanche path. Slope angles were averaged over 50 m segments. Black dots on the map position damaged trees used for the reconstruction of past avalanche activity in the four paths studied (C-1 to C-4). Also shown are (A) the distribution of stem diameters as a function of standard deviations from mean diameter from broken trees in avalanche paths C-1 and C-2 (open diamond) and standing trees in an adjacent, undisturbed site (filled diamond), and fabrics from broken trees (circles in B and C).

aspects pose problems in modelling avalanche behaviour on steep, treed slopes. Only two avalanche years (1955 and 1995) had well-aboveaverage snowfalls (1885–2000, mean + 1 SD, > 434 cm) at our sites as well as at Dubé’s sites (Dubé et al. 2004). At our sites, 64% of all avalanche years recorded (n = 11) had above-average snowfalls (332.7 cm) compared to 83% (n = 12) for undisturbed scree slopes (Dubé et al. 2004), for a similar timeframe (1939–1997, i.e., the period with quasi-continuous snowfall data). In the Mount St. Pierre Valley, tree removal through forest disturbances (fire and logging) was the precursor factor for avalanche activity at our two study sites, whereas local topography (rock walls, steep slopes) and slope aspects (exposure to northwest winds at T-9, lee side with respect to northwest winds at T-10) were predisposing factors. However, initiation of avalanche activity after disturbances was largely controlled by the amount of snow available at each site, which in turn is related to winter climatic conditions. The first avalanche years that were recorded after the 1938 fire at site T-9 (1941) and the 1986–1987 logging operations at T-10 (1988) were years with above-average snowfalls; this indicates

that disturbance by either fire or logging alone is not sufficient to alter the avalanche regime. Vegetation parameters and snow avalanche activity Fire at site T-9 and logging at T-10 were precursor factors for cascading changes on treed scree slopes, from tree removal to snow redistribution and avalanche activity. In mountainous areas with steep slopes, ecological disturbances causing deforestation and resulting in landscape heterogeneity should be more carefully considered in the study of snow avalanche dynamics (Veblen et al. 1994; Villalba and Veblen 1998; Camarero et al. 2000; Walsh et al. 2003). Avalanche activity also affects slope dynamics on treed scree slopes along with a variety of processes, notably debris-transfer processes that can be reactivated after tree removal (Hétu and Gray 2000; Dubé et al. 2004). Vegetation recovery also helps create a mosaic of plant communities that reflect a magnitude– frequency gradient in snow avalanche occurrences. At both sites, numerous tree stems in runout zones attested to the great destructive capacity of snow masses loaded with wood from upslope. At site T-9, broken and uprooted trees © 2005 NRC Canada

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Can. J. Earth Sci. Vol. 42, 2005 Fig. 7. A) Age structures of balsam fir (black bars) and yellow birch (light bars) in the logged area at site T-10. Reference year is 2000. (B) Diameter–height relationships in balsam fir. (C) Diameter–height relationships in yellow birch. (D) Mean sapling heights (dotted line) of balsam fir over time. (E) Mean sapling heights (dotted line) of yellow birch over time. Also shown in D and E are the linear fit model (full line) and 95% confidence curves (dashed line). Fig. 8. Total tree response score for avalanche activity evaluated at site T-10. Tree damage revealed high-magnitude events (black bars) between 1988 and 1991 and in 1995 in avalanche path C-1 (A), and between 1988 and 1990 and in 1992 and 1995 in avalanche path C-2 (B).

were oriented downslope, probably because of sliding before stabilization (Mears 1992). At T-10, avalanche-transported stems piled up in runout zones were chiefly vertical, reflecting the high-impact pressure of snow avalanches penetrating into mature forests. Although steeper surface terrain may mean a higher probability of a snow avalanche, avalanche activity on treed slopes largely depends on vegetation parameters (stem height, diameter, and density) in avalanche-starting zones. The post-logging avalanche window at site T-10 was shorter (four years) than the post-fire avalanche window (15–20 years) and the post© 2005 NRC Canada

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Fig. 9. Departures (solid bars) from average snowfalls (332.7 cm) at weather stations Cap-Madeleine (1885–1994) and Grande-Vallée (1995 to 2000), modified from Dubé et al. (2004). Minimum departure values (open bars) derived from incomplete monthly snowfall records ( 1 month in the snow season. The years indicate snow avalanche occurrences identified from tree-ring records at the two disturbed sites T-9 and T-10. The years in parentheses indicate snow avalanche occurrences at the undisturbed scree-slope sites from Dubé et al. (2004).

1988 avalanche event (seven years) at T-9. At T-10, the fast vertical growth of balsam fir and yellow birch, which had reached 1.0–1.5 m in height 10 years after the 1986–1987 clearcut on the plateau (Fig. 7), soon stopped avalanche activity. Tree stems helped reduce snow drifting from the northwest and the probability of avalanche. McClung (2001) mentioned -2.0 m as the stem height required to control snow redistribution and avalanche activity in similar conditions in western Canada. At T-9, vegetation recovery in the upper slope was slow where both high- and low-magnitude avalanches originated. Vegetation cover in the starting zone is probably the most important factor for avalanche release on treed slopes (Bebi et al. 2001). Because of increased deforestation owing to tree harvesting, snow avalanche activity will become a common source of vegetation disturbance in mountain areas. Rapid tree regeneration and vegetation recovery is likely to mitigate the effect of tree removal on avalanche activity. However, it was not possible with the current research design to associate differences in the tree-ring-based avalanche records from T-9 and T-10 to the magnitude of the initial disturbance or to the stand capacity and speed to recover.

Acknowledgments This research was financially supported by the Natural Sciences and Engineering Research Council of Canada and the Fonds québécois de recherche sur la nature et les technologies (FQRNT) with grants to Louise Filion and Bernard Hétu. Daniel Germain was granted a scholarship by Emergency Preparedness Canada. We thank Stéphane Babin and Chantal Lemieux for field and laboratory assistance, and Céline Meunier, Violaine Lafortune, and Vincent Jomelli for comments during the preparation of the manuscript. The authors also acknowledge thoughtful comments from anonymous reviewers and R. Gilbert, Associate Editor.

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