Spatial dispersion patterns of trees in a tropical rainforest in Xishuangbanna, southwest China

Spatial dispersion patterns of trees at different life stages are an important aspect to investigate in understanding the mechanisms that facilitate species coexistence. In this paper, Ripley's univariate L(r) and bivariate L-12(r) functions were used to analyze spatial distribution patterns and spatial associations across different life stages of 131 tree species in a 20-ha plot of a tropical rainforest in Xishuangbanna, southwest China. Our results show that: (1) Saplings of 109 (83.2%) species have a significant clumped distribution, which confirms the ubiquity of clumped spatial distributions among tropical tree species. (2) Adults of 126 (96.2%) species have a random distribution suggesting that density-dependent mortality can make the spatial pattern of tropical trees more regular with time. (3) At small scales (0-10 m), 95 (72.5%) species have a neutral or negative sapling-adult association, implying that there is recruitment limitation within the vicinity of their conspecific adults. The reduction in spatial clumping in going from younger to older life stages and the neutral or negative sapling-adult association imply density-dependent mortality in the vicinity of adult trees. In accordance with the Janzen-Connell hypothesis, such density-dependent mortality can free up space for other species to colonize, contributing to the maintenance of species diversity.