Willow Canopies and Plant Community Structure along an Alpine Environmental Gradient

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We examined the impact of Salix lapponum canopies on plant community structure in five sites along a climatic gradient in a glacier foreland in alpine south Norway. Species richness is lower inside canopies compared to outside in climatically relatively benign communities, while species richness is not affected by canopies in the most severe communities closest to the glacier. Differences in species composition inside and outside canopies, as judged by detrended correspondence analysis, are greater in the benign communities compared to the severe communities. Variation in the differences in species richness or composition outside and inside canopies within a community is related to differences in the reduction of photosynthetically active radiation (PAR) by canopies in only one community. Canopy size does not explain differences in species richness or composition between outside and inside canopies in any except one benign community. Our results suggest that species responses to canopies are individualistic, and that at the whole-community level negative and positive impacts of canopies on species occurrences cancel each other out in severe communities, whereas in benign communities negative effects dominate slightly over positive ones in their effects on species persistence inside canopies.