Phylogenetic Turnover in Biomes is Determined by Environmental Harshness, Geographic Distance, and Climate Stability

Biomes are climatically and spatially structured geographic regions with distinct floristic communities. This discretization makes biomes natural geographic delineations for identifying environmental variables that constrain the evolutionary diversification and environmental filtering of vascular plant diversity. Using over 220 million occurrence records, we compile a dataset of 68,515 vascular plant species to measure alpha and beta diversities between and within biomes using two-degree grid cell community delineations. We test the hypothesis that patterns of biodiversity among biomes reflect their distinct environmental conditions, and that these conditions contribute to community assembly processes within them. Our results suggest that: 1) tropical biomes are characterized by an overabundance of long phylogenetic branches, while temperate biomes have an overabundance of short branches; 2) phylogenetic dissimilarity within a biome type is predicted by overall connectivity and harshness of the biome type (e.g., extreme temperatures and precipitation, seasonality, stability through time); 3) geographic distance is the largest predictor of both taxonomic and phylogenetic beta diversities, however taxonomic beta diversity is significantly more driven by geographic distance than is phylogenetic; 4) specific bioclimatic predictors of phylogenetic turnover among grid cells within biomes vary by biome type, and geographic distance exhibits more importance in environmentally harsh biomes like tundra, reflecting the environmental filtering of regional species pools coupled with local in situ diversification. In the face of rapid global change, biomes with low connectivity and ones that occupy climatic extremes may be at heightened risk of biodiversity loss.