Relative effects of dispersal limitation, environmental filtering and limiting similarity on the assembly of south subtropical forest communities in Guangdong, China

Understanding the relative roles of dispersal limitation, environmental filtering, and limiting similarity in driving forest community assembly across spatial scales and ontogenetic stages is fundamental to revealing the mechanisms governing forest dynamics. In this study, we used species composition and functional trait data from permanent plots at multiple spatial scales in a subtropical evergreen broad-leaved forest in Dinghushan, Guangdong, China. We applied the STEPCAM (Structured Ecologically-Probabilistic Community Assembly Model) to quantify the relative effects of the three assembly processes. Our results showed that: (1) Across all scales and stages, dispersal limitation dominated community assembly, with a mean relative effect of 76.7%, followed by environmental filtering (15.4%) and limiting similarity (7.9%), and the differences among the three processes were highly significant (P < 0.01). (2) As spatial scale increased, the relative effect of dispersal limitation remained stable (P > 0.05), whereas the effect of environmental filtering decreased significantly and that of limiting similarity increased. (3) The relative effects of the three processes varied markedly across tree developmental stages. With increasing tree size (from saplings to adults), the relative effect of dispersal limitation first increased and then decreased, while the niche-based processes—environmental filtering and limiting similarity—showed a gradually increasing trend. Collectively, our findings indicate that dispersal limitation is the predominant assembly mechanism in this subtropical forest, with environmental filtering and limiting similarity playing secondary but scale- and stage-dependent roles. Therefore, conservation and management strategies for this forest type should target species across different spatial scales and ontogenetic stages to maintain community assembly processes.