Community restructuring can maintain diversity across a severity gradient in the absence of foundation species

Phylogenetic community structure can provide insight into the ecological dynamics that drive species co-occurrence and community assembly patterns. Understanding these patterns is important for predicting future community composition in the context of climate change and other anthropogenic disturbances. However, we know little about how these environmental stressors might change interspecific interactions. Here we present an analysis on the role of putative foundation species (cushion-type life forms) in determining the phylogenetic community composition of vascular plants at the local scale. We also investigated how community diversity and structure shifts across an environmental severity gradient. Abundance data for vascular plants was collected within and outside of four focal cushion plant species along an elevation gradient in Canadian alpine tundra. We compared differences in species-specific abundance within plots with foundation plants to the same measure in adjacent open plots. Using a timecalibrated molecular phylogeny, we quantified community structure metrics, including the net relatedness index (NRI), a novel net relatedness to focal index (NRFI), and an index of species co-occurrence across the gradient. We examined the trends in these indices with cushion plant presence/absence and environmental severity using both linear regression and mixed effect models. We found that the loss of species richness at higher elevations was counterbalanced by a decrease in interspecific relatedness, maintaining phylogenetic diversity across the gradient. We found no evidence for community diversity or structure modifications by cushion plants, nor any evidence that species were preferentially associating with these hypothesized foundation species. On the contrary, many species had significantly negative co-occurrence with the cushion plants. Overall, our results suggest that the cushion plant life forms may exhibit interspecific dynamics that are at least as competitive as they are facilitative. The decrease in phylogenetic clustering with elevation suggests that traits that allow persistence in these conditions are convergent at the local