Shoreline Vegetation in Axe Lake, Ontario: Effects of Exposure on Zonation Patterns

Abstract

This study tests several hypotheses relating community structure to environmental disturbance, by testing for changes in the zonation patterns of lakeshore vegetation. Twenty—five transects were examined at different positions along an exposure gradient. The range of water depths tolerated by individual species (realized niche width) changes with exposure. Some (Lobelia dortmanna, Utricularia cornuta) reached their maximum on exposed shores, others (Drosera intermedia, Cladium mariscoides) at intermediate exposure, and others (Pontederia cordata, Triadenum fraseri) on sheltered shores. Species richness peaked significantly at intermediate levels of exposure (P < .01). In spite of changes in both species composition and richness, mean niche width did not change with exposure. The distribution of the upper and lower limits of species along the gradient was examined using measures of boundary clustering. The lower and upper boundaries of species were both significantly clustered (P < .001), suggesting discrete communities exist on the shoreline gradient. As exposure increased, upper boundaries became more cluster (P < .002); lower boundaries were unaffected. The distribution of boundaries shifted landward with increasing exposure. This fact is possibly related to an identical landward shift in the lower boundaries of shoreline shrubs, which appear to have a major influence on the distribution of herbaceous shoreline vegetation.