Plant Diversity Patterns and Their Determinants Across a North-Edge Tropical Area in Southwest China

Understanding the diversity patterns within a region is helpful for the implementation of conservation management. Xishuangbanna, located in southwestern China, is notable for its diverse plant species and belongs to a tropical–subtropical transition area. This study investigated the biodiversity patterns for four types of primary forests in Xishuangbanna, namely tropical rainforests, tropical monsoon forests, tropical low-montane evergreen broadleaf forests, and tropical seasonal moist forests. The difference in the forests alongside a set of environments was assessed using non-metric dimensional scaling and partial least-squares discriminant analysis. And, we calculated and compared four diversity metrics for each forest, including species richness, phylogenetic diversity, standardized phylogenetic diversity, and standardized mean phylogenetic distance, and calculated their correlation with 22 environments using multiple stepwise regressions. The results showed that tropical rainforests had the highest biodiversity on account of species richness (with an average of about 40 species) and phylogenetic diversity (with an average of about 3000). Although the values of standardized mean phylogenetic distance were lower than zero for all forests, the tropical seasonal moist forests ranked first. Not only species composition and environments’ differences, especially the temperature seasonality, minimum temperature of the coldest month, latitude, and precipitation of the driest quarter, primarily influenced the forest groupings. The variance in species richness (R2 = 0.57) and phylogenetic diversity (R2 = 0.54) was best explained by a model integrating forest type, soil, climate, and geographic factors. In contrast, the variance in standardized phylogenetic diversity (R2 = 0.48) and standardized mean phylogenetic distance (R2 = 0.39) was primarily influenced by soil and climate factors. We suggest that tropical rainforests and tropical seasonal moist forests should be conservation priorities in conservation management. This study provides insights into community assembly mechanisms and the enhancement of biodiversity conservation management in transitional areas.