Spatial patterns of plant species richness along a forest edge: What are their determinants?

The impact of forest edges on plant species richness is poorly understood. In a sugar maple (Acer saccharum) stand in Southern Québec, variations in species richness and spatial pattern were observed as a function of distance to the edge and were associated with different environmental variables. Unexpectedly, light was not higher and soil moisture and organic matter were not lower close to the edge than in the forest interior: edge structure was most likely responsible for these results. At a 2 m2 scale, richness tended to decrease with distance to the edge, as did fine-scale species turn-over. Erythronium americanum patch size increased with distance to the edge, most likely because of differences in clonal growth along the gradient. Cover of newly emerged seedlings of sugar maple decreased with distance to the edge, in association with soil moisture and pH. Path analyses revealed significant links between environmental (micro-elevation, irradiance, soil organic matter, moisture, and pH) and biotic variables (richness, E. americanum cover, and A. saccharum cover), but only a low proportion of the variance of the biotic variables was explained. At the scale studied, stochastic processes may be more significant than deterministic processes for species richness.