A multi-scale approach reveals random phylogenetic patterns at the edge of vascular plant life

Mountain plant diversity results from a myriad of factors, including evolutionary history, species pools, abiotic and biotic constraints. For instance, increasing stress (e.g., elevation) often selects communities with species originating from fewer, and more closely-related clades. We assessed phylogenetic diversity and turnover of plant communities by considering multiple drivers simultaneously: 1) the species pools; 2) the regional context, i.e., two regions of the arid Trans-Himalaya (the Karakoram Range and Little Tibet) with distinct history, climate and species richness (regional scale); 3) the abiotic constraint with communities spread out along an elevation gradient in each region (community scale); 4) the biotic constraint, i.e., species co-existing inside a competitive dominant species (cushion plant) vs. its adjacent open area in each community (neighbourhood scale); and 5) two phylogenetic scales (overall vs. recent events in the assembled phylogeny). We found random phylogenetic patterns for all spatial and phylogenetic scales, irrespective of the regional context, and the abiotic and biotic conditions under consideration. Contrary to expectations, we observed a tendency for phylogenetic evenness in Little Tibet and in the Karakoram Range with increasing elevation. The observed phylogenetic patterns were mostly explained by region, elevation and interactions among the multiple drivers under considerations. Phylogenetic scale, species pools and cushion plants explained patterns to a lower but significant extent. The studied phylogenetic patterns emerge from the complex interplay between multiple drivers, and challenge the common view that increasing spatial and phylogenetic scales, as well as increasing biotic and abiotic constraints, select communities with species originating from fewer, and more closely-related clades.