Willow canopies and plant community structure along an alpine environmental gradient

Abstract

We examined the impact of Salix lapponum canopies on plantcommunity structure in five sites along a climatic gradient in aglacier foreland in alpine south Norway. Species richness is lowerinside canopies compared to outside in climatically relativelybenign communities, while species richness is not affected bycanopies in the most severe communities closest to the glacier.Differences in species composition inside and outside canopies, asjudged by detrended correspondence analysis, are greater in thebenign communities compared to the severe communities. Variationin the differences in species richness or composition outside andinside canopies within a community is related to differences inthe reduction of photosynthetically active radiation (PAR) bycanopies in only one community. Canopy size does not explaindifferences in species richness or composition between outside andinside canopies in any except one benign community. Our resultssuggest that species responses to canopies are individualistic,and that at the whole-community level negative and positiveimpacts of canopies on species occurrences cancel each other outin severe communities, whereas in benign communities negativeeffects dominate slightly over positive ones in their effects onspecies persistence inside canopies.