Plant communities and landscape diversity along a Canadian arctic river

We analysed the structure and diversity of the vegetation along an Arctic river to determine the relationship between species richness and plant community structure. We examined whether variation in species richness along the corridor is structured as (1) an increase in the number of communities due to increasing landscape heterogeneity, (2) an increase in the floristic distinctiveness (beta-diversity) of communities, or (3) an increase in within-community richness (alpha-diversity) as species-poor communities are replaced by species-rich communities. We described 24 community types and analysed the relationship between site vascular species richness (gamma-diversity) and beta-diversity, alpha-diversity, site environmental heterogeneity, and the number of distinct plant communities. We also measured diversity patterns of vascular, bryophyte, and lichen species within communities and examined their relationship to community-level estimates of environmental factors. We found that an increase in site species richness correlated with an increase in the number of communities (r(2) = 0.323, P = 0.0173) and beta-diversity (r(2) = 0.388, P = 0.0075), rather than an increase in the a-diversity of individual communities. Moisture and pH controlled most of the differences in composition between communities. Measures of species richness and correlations with moisture and pH within communities differed among vascular, bryophyte, and lichen species. Bryophyte richness was positively correlated with moisture (r(2) = 0.862, P = 0.0010) and lichen richness was negatively correlated with moisture (r(2) = 0.809, P = 0.0031). Vascular plants had a peak in richness at pH 6.5 (r(2) = 0.214, P < 0.0001). We conclude that site variation in vascular richness in this region is controlled by landscape heterogeneity, and structured as variation in the number and distinctiveness of recognizable plant communities.