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Caribou-induced changes in species dominance of lichen woodlands: an analysis ofplant remains¹

Centre d'études nordiques and Département de biologie, Université Laval, Québec City, Canada, G1K 7P4

Received for publication June 5, 2003. Accepted for publication October 30, 2003.

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIAL AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
Plant communities in northern Quebec-Labrador, Canada have been severely grazed and trampled since the early 1980s by the increasingly large George River caribou herd (GRCH). To evaluate changes in species dominance associated with caribou disturbance, we compared past and present ground vegetation from 14 lichen woodlands. Plant remains from superficial organic horizons indicate that ground vegetation was largely dominated by lichens (especially Cladina) before the onset of caribou disturbance. In enlargments of aerial photos taken before 1975 (i.e., prior to maximum size of the GRCH), all sites were free of caribou trails and were dominated by a continuous lichen (Cladina) carpet. Principal components analysis showed that partial or complete destruction of the Cladina-dominated lichen carpet was the most striking change in ground vegetation. Severe trampling degraded superficial organic horizons, subsequently exposing mineral soil in heavily used sites. With reduced caribou activity in the 1990s, exposed ground was colonized by crustose lichens and Cladonia. Sites that faced severe grazing but light trampling were recolonized mainly by small podetia of Cladina stellaris sprouting from the lichen litter. However, patterns of post-caribou disturbance lichen succession differed from those of post-fire succession, because species from different successional stages are present at the same time in a stand and also because caribou can modify the successional trajectory at any time.

Key Words: aerial photos • caribou disturbance • Cladina • George River caribou herd • grazing • lichen succession • plant remains • trampling

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIAL AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
Through selective grazing, herbivores are one of the most important factors shaping plant communities in tropical as well as in temperate and cold regions (McNaughton, 1983 ; Oksanen and Oksanen, 1989 ; Oksanen and Virtanen, 1995 ; Oksanen et al., 1995 ; Augustine and McNaughton, 1998 ; Suominen and Olofsson, 2000 ). Species composition, biomass, and plant architecture are among the main components of plant communities that can be affected by herbivory (McNaughton, 1984 ). Although of significance in several instances, herbivore trampling has received much less attention. Trampling from large herds of migratory caribou seems to be a major disturbance factor in the Arctic and the Subarctic. Indeed, lichen woodlands, a dominant vegetation type of the boreal forest (Kershaw, 1978 ), are particularly vulnerable to such disturbance (Ukkola, 1995 ). Repeated use of the same path by caribou results in trail formation as trampling rapidly destroys the lichen carpet. When trampling and grazing effects are combined, thick lichen carpets may disappear almost completely in a relatively short period (Morneau and Payette, 1998 ).

The George River caribou herd (GRCH) experienced a rapid demographic growth between the early 1960s and the mid-1980s (Messier et al., 1988 ), when it reached a relative stability at approximately 700 000 individuals (Couturier et al., 1996 ) before declining during the 1990s (Boudreau et al., 2003 ). With the increasing number of caribou in the summer habitat of the GRCH, lichen woodlands faced sustained herbivore pressure. Extensive surveys of the summer habitat of the GRCH indicated that all lichen woodlands showed signs of caribou disturbance as revealed by dense trail networks and degradation of the lichen carpet (Fig. 1a, b, c).

View larger version (88K): [in this window] [in a new window]    Fig. 1. Lichen woodlands of the summer habitat of the George River caribou herd (GRCH), northeastern Québec-Labrador, Canada (a) undisturbed, (b) with well-developed trail network, (c) after destruction of lichen carpet, (d) sampling scheme along trail, (e) caribou trails in fen peatland, site 118, (f) site 33

Several studies on the impact of caribou on vegetation have compared sites with different herbivory histories: ungrazed vs. grazed sites (Moser et al., 1979 ; Klein, 1987 ; Henry and Gunn, 1991 ; Manseau et al., 1996 ) and exclosure sites vs. grazed sites (Ouellet et al., 1993 ; Väre et al., 1995 , 1996 ). Such studies provide useful information on plant–caribou relationships but some limitations are apparent. Comparisons between ungrazed and grazed sites are based on the assumption that vegetation composition of the sites being compared was initially the same, an assumption generally difficult to assess. Such comparisons can be misleading when trying to identify vegetation change through time.

Exclosure studies are the best way to evaluate change in vegetation composition (or recovery patterns) associated with herbivore removal (Watkinson et al., 2001 ). However, processes involved in vegetation degradation caused by herbivores cannot be studied directly using this approach. Furthermore, exclosure studies may not be ideal when working with slow-growing species like lichens. A more straightforward approach to identify changes in species dominance is to use vegetation composition of the same site at two different periods, before and after herbivore disturbance.

In this study, we analyzed plant remains to identify the relative abundance of ground species prior to caribou disturbance. Conventional plant macrofossil analysis is conducted generally on lake sediment or peatland samples where the decomposition process is slow (Warner, 1990 ). Although plant remains are rarely preserved in terrestrial soils (Andersen, 1986 ), conditions at our study sites slowed down the decomposition process (e.g., cold temperatures and short frost-free growing seasons, excessively drained soils and short time lag [about 25 yr] between the onset of caribou disturbance and sampling). As a result, most of the vegetation destroyed by caribou was still present and identifiable in the superficial organic layers.

The main objective of this study was to identify potential shifts in species dominance of ground vegetation of lichen woodlands in the summer habitat of the GRCH in response to caribou grazing and trampling. To do so, we reconstructed the vegetation composition of 14 sites prior to caribou disturbance and evaluated the severity of disturbance based on the amplitude of compositional changes between past and present vegetation.

	MATERIAL AND METHODS

TOP ABSTRACT INTRODUCTION MATERIAL AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
Study area
The study area covers most of the summer habitat of the GRCH. Of the 14 sites sampled, eight sites were located within the calving ground. Four sites were outside the calving ground but still within the range of the summer habitat and the remaining two sites were located in the winter habitat (at the edge of the summer habitat) in the vicinity of Schefferville (Fig. 2). One of these two sites was used as control because no evidence of caribou disturbance was found, a condition rarely met in the study area.

View larger version (60K): [in this window] [in a new window]   Fig. 2. Location of the study area, including calving ground and summer habitat of the George River caribou herd (GRCH), northeastern Québec-Labrador. Eight sites were sampled in 1999 (circle) and six in 2000 (diamond)

Linear surveys of present vegetation were performed in each trail along the five transects. Plants were tallied along each 2-m transect using 20 10-cm long sections. The linear cover (cover classes: <1%, 1.1–10.0%, 10.1–20.0%, etc.) of living plants (lichens, mosses, low shrubs, etc), litter, and mineral soil crossing the transect was determined for each 10-cm section. Thus, the survey in each trail was based on 100 measurements used for the calculation of species relative cover. Based on each of the 20 sections/transect, the mean cover of living plants, litter, and mineral soil along each transect was calculated, using the central value of the cover class rather than the cover class itself (e.g., cover class 10.1–20.0% replaced by 15%).

Plant remains were recovered in each trail for transects #1, #3, and #5 thought to be representative of the plant macrofossil assemblage at each site. Plant remains correspond to dead plant parts directly attached to living plants (as in the case of the decaying basal part of Cladina podetia) or buried in the uppermost 1 cm of the organic horizon (F horizon). The position and good preservation of plant remains in the uppermost part of the organic horizon indicate their young age and recent burial relative to the long-term development of the vegetation cover at each site. Along each transect, contiguous slices (10-cm long, 5-cm wide) of living plants and the underlying organic horizons were extracted. The samples were then carefully packed to preserve the original stratigraphy. In the laboratory, samples were dried at room temperature and plant remains were exposed by removing the living plants.

Plant remains were then surveyed along the 20 10-cm lines of the 2-m transect. Each 10-cm line was divided in 1-cm sections, and plant cover was determined for each millimeter of 1-cm section under a dissecting microscope (40x, Fig. 3). Plant remains were identified at the genus level. Special attention was given to lichen and moss species because ground vegetation of lichen woodlands is largely dominated by different lichen and moss assemblages rather than by vascular plants. However, shrubs and herbs were also identified and surveyed when present. Plant cover prior to caribou disturbance was then reconstructed using plant remains from the six surveyed 2-m transects.

View larger version (130K): [in this window] [in a new window]    Fig. 3. Plant remains from organic horizons of lichen woodlands in the summer habitat of the George River caribou herd (GRCH), northeastern Québec-Labrador. Clockwise from top left: Cladina, Stereocaulon, Pleurozium, liverworts, and Vaccinium

Data analysis
Principal component analysis (PCA) on plant remains and present vegetation data was conducted to evaluate changes in the relative abundance of the different plants as a result of caribou disturbance over the last 25 yr (mixed effects of grazing and trampling). Relative abundance of species in the present ground vegetation was calculated using living plant cover only. Litter and exposed mineral soil were excluded from the current ground cover data in order to homogenize past and present vegetation data sets. Because we used standardized data, the PCA was performed on the covariance matrix. Scores were then calculated using the centered variables instead of the standardized ones (SAS 6.12, Cary, North Carolina, USA).

	RESULTS

TOP ABSTRACT INTRODUCTION MATERIAL AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
Ground vegetation
Living ground vegetation covered between 30 and 96%, from the most disturbed site to the control site, the remaining surface being litter and mineral soil (Table 2). However, for most sites (nine out of 14), ground cover varied between 40 and 65%. Lichens were the major component of the vegetation (55–93% cover). Cladina, Cladonia, and Stereocaulon were dominant plants, while other lichens (Alectoria, Cetraria, Nephroma, Peltigera, and several crustose lichens) were rare, even though their relative abundance reached 10% at some sites (e.g., Cetraria in site 50). Companion plants were mosses and liverworts (0–27%), shrubs (5–24%), and herbs (0–4%). Dicranum was the most abundant of the moss and liverwort group, and Vaccinium and Empetrum were the most abundant of the shrub group.

PCA analysis
Principal component analysis was used to ordinate the study sites according to composition of ground vegetation before and after caribou disturbance (Fig. 4). The first two axes of PCA explained 91.4% of the variance. Axis 1 (79.4% of the variance) was positively correlated with Cladina (score of 0.92) and negatively correlated with Cladonia (–0.29) cover. Axis 2 explained an additional 12% of the variance and was positively correlated with Cladonia (0.57) and negatively correlated with Stereocaulon (–0.79) covers.

View larger version (13K): [in this window] [in a new window]   Fig. 4. Principal component analysis using plant remain and present vegetation data of lichen woodlands of the summer habitat of the George River caribou herd (GRCH), northeastern Québec-Labrador. Past and present vegetation composition of each site are represented by a circle and an arrowhead, respectively. (Broken line: group based on past vegetation)

Position of the sites in the PCA analysis, based on present vegetation, was directly linked to the relative abundance of the different lichens, in relation to the severity and type (trampling or grazing) of caribou disturbance. Four major vegetation types were identified: Cladina-dominated ground vegetation; reduced Cladina and increased Cladonia cover; reduced Cladina cover with no increase of Cladonia; and finally, reduced Cladina and Stereocaulon cover combined with an increase of Cladonia.

	DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIAL AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
In this study, we used a new approach to evaluate caribou-induced changes in ground species dominance over a short period based on macrofossil analysis. The use of this paleoecological method was possible because plants prior to caribou disturbance were still identifiable in the organic topsoil. Our sampling scheme was designed to evaluate the most important botanical changes relative to present composition of ground vegetation as it was based on the analysis of the extensive trail networks crossing the studied lichen woodlands. Although relative species abundance is thought to be representative of the ground vegetation of the lichen-spruce woodlands, we need to point out that no consideration was given to trail densities.

Past and present vegetation data
Prior to caribou disturbance, the ground vegetation of most lichen woodlands was largely dominated by lichens. Most of the sites (10/14) were dominated by Cladina species typical of late-successional stages (Hustich, 1951 ; Ahti, 1959 ; Bergerud, 1971 ; Auclair, 1983 ; Morneau and Payette, 1989 ). Based on the Cladina dominance, sustained growth conditions in absence of ground disturbance was likely during the decades preceding growth of the GRCH. However, Stereocaulon-dominated sites were probably associated with light caribou disturbance. Stereocaulon typically increases to the detriment of Cladina in response to light disturbance (Ritchie, 1960 ; Scotter, 1964 ; Ahti and Hepburn, 1967 ; Haapasaari, 1988 ). Some of the factors likely to explain Stereocaulon abundance on disturbed sites are better trampling resistance (Haapasaari, 1988 ), thalli less thoroughly eaten by caribou (Haapasaari, 1988 ), and faster growth rate (Hustich, 1951 ). More recent quantitative work in northern Finland (Kärenlampi, 1971 ; Crittenden et al., 1994 ; Kytöviita and Crittenden, 2002 ), however, indicates that several Cladina species are growing faster than Stereocaulon.

Aerial photo interpretation of the sites prior to 1975 corroborates our plant-remain data, which suggested the dominance of a continuous lichen (Cladina) carpet before the period of caribou disturbance. Although the identification of the dominant species from aerial photos cannot be validated directly, continuous lichen carpets in northeastern Canada have always been reported to be Cladina-dominated in field surveys (Hustich, 1951 ; Bergerud, 1971 ; Morneau and Payette, 1989 ; Riverin and Gagnon, 1996 ). Also, according to aerial photos, no caribou trails were present in the studied sites prior to 1975. Although seldom-used trails could have been overlooked, only a few well-developed trails, particularly in fen peatlands, were present at the time the aerial photos were taken. Obviously, trail networks at that time were ill-developed in comparison to now, which strongly suggests the absence of significant caribou disturbance at that time.

Degradation process
Ground vegetation faced severe trampling and grazing pressures as caribou density increased in the summer habitat following the rapid growth of the GRCH. The most striking impact of caribou disturbance was the destruction of the lichen carpet, as reported elsewhere (Manseau et al., 1996 ; Morneau and Payette, 1998 ), resulting in a directional shift in species dominance, from Cladina-dominated vegetation to more complex and diverse lichen assemblages accompanied by mosses, shrubs, and herbs.

The dominant ground species in stands with light or moderate disturbances had no major change, because destruction of the lichen cover was restricted to caribou trails. Although light and moderate trampling can favor short-distance lichen dissemination (Heinken, 1999 ), severe trampling can have deleterious effects on lichens. At the landscape scale, degradation of ground vegetation in lichen woodlands of the summer habitat is closely associated with severe trampling. For lichen species other than those in the genera Cladina and Stereocaulon, the impact of caribou disturbance on ground vegetation can be compared to that of forest fire. By destroying the lichen cover, caribou trampling causes mineral soil to be exposed, thereby creating favorable conditions for the establishment of crustose lichens and several early successional Cladonia species. The relative abundance of mosses and dwarf shrubs generally increases with the destruction of the lichen carpet, although to a lesser extent.

With the destruction of the Cladina stellaris carpet, the respective influences of trampling and grazing can be evaluated by looking at the sequence of lichen recovery. Present vegetation with small Cladina stellaris podetia (originating from sprouting of surviving lichen thalli) is possibly the result of the combined effects of heavy grazing (most living thalli removed) and light/moderate trampling (dead residual lichen carpet still present). However, the sequence of vegetation recovery with Cladonia and crustose lichens is likely initiated when heavy trampling that exposes the mineral soil ceases. Grazing severity is difficult to evaluate in this particular case because heavy trampling destroyed any former grazing evidence.

Lichen succession
All sampled sites were old-growth lichen woodlands (>200 yr old; Morneau, 1999 ), in an area with a long fire-return interval and rare recently burned stands. In absence of disturbance (fire and caribou), old-growth lichen woodlands are self-maintained during several centuries in northern Québec (Payette and Morneau, 1993 ).

Successional processes are still at an early stage at the caribou-disturbed sites as the activity of the GRCH only decreased in the early 1990s (Boudreau et al., 2003 ). Although comparisons can be made, successional trajectories for this type of disturbance are, for many reasons, more complex than those associated with fire disturbance: late successional lichen species are still present at every site, mineral soils are not always exposed and, more importantly, caribou can come back to a site and change the successional pathways anytime. Furthermore, plant composition in disturbed stands includes early (crustose lichens and Cladonia) and late (Cladina stellaris thalli and fragments dispersed in the stands) successional species. Such plant assemblages produce successional stands with high lichen diversity. In the absence of caribou activity, lichen succession will progress, with Cladina species becoming progressively dominant over the next 100 years until a thick Cladina stellaris carpet covers most of the ground surface (Morneau and Payette, 1989 ).

In conclusion, our results show that caribou disturbance plays an important role determining lichen abundance and diversity. Destruction of the Cladina-species-dominated lichen carpet allows other lichens, particularly species of early successional stages, to increase in abundance. However, the impact of caribou disturbance on mosses, shrubs, and herbs are less apparent, although small increases in relative cover were observed. Further research should emphasize the need to partition the respective influence of grazing and trampling in the degradation process.

	FOOTNOTES

 
¹ The authors thank Hydro-Québec, the Natural Sciences and Engineering Research Council of Canada (NSERC), and the Department of Indian and Northern Affairs for financial support. S. Boudreau received scholarships from the Fonds pour la Formation de Chercheurs et l'Aide à la Recherche (Fonds FCAR) and the Association of the Canadian Universities for Northern Studies (ACUNS). The authors acknowledge the assistance of C. Morneau, S. Desaulniers, C. Lambert, M. Longchamps, P. Morin, and J. Théau in the field and the laboratory, and the statistical advice of G. Houle and H. Crépeau. Thoughtful comments on an earlier draft by G. Houle, D. Coxson, P. Crittenden and an anonymous referee were greatly appreciated.

² serge.payette{at}bio.ulaval.ca .

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