Different environmental factors drive tree species diversity along elevation gradients in three climatic zones in Yunnan, southern China.

Elevational patterns of tree diversity are well studied worldwide. However, few studies have examined how seedlings respond to elevational gradients and whether their responses vary across climatic zones. In this study, we established three elevational transects in tropical, subtropical and subalpine mountain forests in Yunnan Province, southern China, to examine the responses of tree species and their seedlings to elevational gradients. Within each transect, we calculated species diversity indices and composition of both adult trees and seedlings at different elevations. For both adult trees and seedlings, we found that species diversity decreased with increasing elevation in both tropical and subalpine transects. Species composition showed significant elevational separation within all three transects. Many species had specific elevational preferences, but abundant tree species that occurred at specific elevations tended to have very limited recruitment in the understory. Our results highlight that the major factors that determine elevational distributions of tree species vary across climatic zones. Specifically, we found that the contribution of air temperature to tree species composition increased from tropical to subalpine transects, whereas the contribution of soil moisture decreased across these transects.