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Plant succession after active layer detachment slides, in high Arctic tundra, Fosheim Peninsula, Ellesmere Island, Canada Desforges, Manon

Abstract

Vegetation succession patterns and processes were studied after active layer detachment slides. These natural disturbances were grouped into four age categories in the valleys Hot Weather Creek and Big Slide Creek on the Fosheim Peninsula, Ellesmere Island (80°N). A time-space substitution was used to build a surrogate revegetation sequence in order to evaluate primary succession in the scar and secondary succession in the toes of the slides. Vegetation cover and environmental characteristics were measured using haphazard sampling along transects laid across the slide, during the summer of 1994. TWLNSPAN and detrended canonical correspondence analysis were used together to examine vegetation patterns in relation to environmental variables. Sexual reproduction effort was measured from harvested seed and seed bank samples to assess it role as one mechanism driving plant succession. The change in viable seed production for species of two life history groups (ruderal and late-sere grasses and forbs) and the change in seed bank density and composition were examined in relation to the above-ground vegetation and terrain age. Variation in species composition was accounted for by a combination of terrain age and environmental factors. In the scar, where environmental conditions improved over time, primary succession was directional with eventual replacement only in the oldest terrain. The succession followed four main stages of dominance: ruderal grass and forb -> late sere grasses and forb -> shrubs, late sere forbs and grasses -> shrubs and cushion plants. In the toe, which experienced and reduction in soil moisture over time, a retrogressive succession was observed with reduced density and diversity of species, and shift to species better adapted to surviving drought. Large seed production by ruderal species in all terrain ages of the scars suggest that the absence of these species in the oldest terrain does not result from a reduction of resource allocation to reproductive efforts due to possible inter-species interaction. However, the slight reduction of seed production in the more severe environment of the aging toes indicates the important role of physical environmental factors. Total seed bank densities were large and more comparable to temperate environments or disturbed habitats, confirming the importance of sexual reproduction efforts in these ecosystems. Dissimilarity between the germinable seed bank and the extant vegetation composition throughout the successional sequence was expressed by the large number of seeds of late sere grasses in the young scars and the large number of seeds of ruderal species in the old scars. This suggests that seed dispersal occurs at a scale beyond the local vegetation cover, most likely as a result of winter seed rain. This study agrees with succession models that incorporate levels of environmental severity. In habitats with low environmental stress, patterns of community change, with eventual species replacement, were possibly governed by the same sort of processes described in more temperate environments and classical models of succession, but with a much slower rate of change. However, with increases in environmental stress, succession shifted from species replacement to species establishment and survival.

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