The relative role of regional species pools in determining tree species richness and rarity in Chinese subtropical and tropical forests.

The species pool hypothesis argues that local species diversity mainly depends on regional diversity, which is influenced by dispersal, historical and current environmental conditions. We hypothesize that regional factors, such as the size of the regional species pool, current climate, topographical variability, and historical climate stability, also impact local species-abundance patterns, like the rarity of local species, though their specific effects are not yet well understood. Analyzing data from 3307 species across 3923 forest plots in Chinese subtropical and tropical regions, we employed boosted regression trees and structural equation modeling to assess the roles of regional species pool size along with climatic seasonality, topography, and soil factors, in shaping local richness and rarity. We found that local tree species richness declined with increasing latitude, while species rarity decreased from west to east. The factors such as current regional environment, paleoclimate stability, and human disturbance significantly affected local richness and rarity, primarily through their effects on regional species pool size. The impacts of regional mean temperature and elevational range on local richness surpassed local influences. Conversely, local climatic seasonality exerted the strongest influence on species rarity, followed by human activity. Overall, the findings indicate that regions with large regional species pools tend to support diverse communities with high proportions of rare species.