Declining Small-Stem Abundance and Contrasting Intraspecific Spatial Dynamics in a Tropical Montane Rainforest on Hainan Island, China

Dispersal limitation, habitat heterogeneity, and density-dependent processes shape tree distributions, but their spatial signatures can overlap with those of vegetative reproduction. We examined diameter structure and intraspecific spatial patterns of eight dominant species using censuses from 2016, 2020, and 2025 in a 6 ha tropical montane rainforest plot in Bawangling, Hainan Island, China. Topography-adjusted pair-correlation functions described aggregation within and associations between diameter classes. Random-thinning simulations retained subsets of the 2016 locations at the observed 2025 abundance to assess whether fewer stems alone could account for changes in small-stem aggregation. In the monitored populations, small-stem abundance declined by 15.77%, large-stem abundance increased by 4.97%, and total basal area increased by 6.29%. Spatial responses differed: Symplocos adenophylla showed weaker small-stem aggregation and small–large association, whereas Hancea hookeriana became more aggregated despite a weaker association with large conspecifics. Observed aggregation changes in six species, including the largest decrease, fell within the random-thinning intervals; increases in H. hookeriana and Polyalthia laui exceeded the upper bounds. Declining abundance therefore did not consistently weaken aggregation. These contrasts are compatible with different combinations of stem entry, persistence, and growth, but do not distinguish seed-mediated establishment from undetected vegetative spread or demonstrate a common increase in density-dependent losses.