Biogeographic affinity partly shapes woody plant diversity along an elevational gradient in subtropical forests.

The ecological and evolutionary mechanisms underlying montane biodiversity patterns remain unresolved. To understand which factors determined community assembly rules in mountains, biogeographic affinity that represents the biogeographic and evolutionary history of species should incorporate with current environments. We aim to address two following questions: 1) How does plant taxonomic and phylogenetic diversity with disparate biogeographic affinities vary along the subtropical elevational gradient? 2) How do biogeographic affinity and environmental drivers regulate the community assembly? We collected woody plant survey data of 32 forest plots in a subtropical mountain of Mt. Guanshan with typical transitional characteristics, including 250 woody plant species belonging to 56 families and 118 genera. We estimated the effects of biogeographic affinity, climate and soil properties on taxonomic and phylogenetic diversity of plant communities employing linear regression and structural equation models. We found that the richness of temperate-affiliated species increased with elevations, but the evenness decreased, while tropical-affiliated species had no significant patterns. Winter temperature directly or indirectly via biogeographic affinity shaped the assemblage of woody plant communities along elevations. Biogeographic affinity affected what kind of species could colonize higher elevations while local environment determined their fitness to adapt. These results suggest that biogeographic affinity and local environment jointly lead to the dominance of temperate-affiliated species at higher elevations and shape the diversity of woody plant communities along elevational gradients. Our findings highlight the legacy effect of biogeographic affinity on the composition and structure of subtropical montane forests.