Abstract: Severe anthropogenic disturbances have caused significant forest degradation, making the exploration of community assembly mechanisms crucial for developing effective restoration strategies. These mechanisms may vary across size classes due to ontogenetic niche shifts in plants; however, this process remains insufficiently explored. In this study, we surveyed woody plants across five size classes within degraded forests and measured habitat conditions and neighborhood effects, aiming to investigate the size-class dependence of environmental filtering and biotic competition. The results showed that: (1) Due to ontogenetic niche shifts, the assembly mechanisms for seedlings and large trees differed, shifting from varying primarily by environmental filtering to a combination of filtering and competition. (2) These mechanistic differences resulted in the β-diversity among smaller size classes being primarily driven by species turnover and balanced variation in abundance, whereas nestedness and abundance gradients became the main drivers of structural dissimilarity among larger size classes. (3) At the family level, dominant families achieved intraspecific and interspecific coexistence through ontogenetic niche shifts and differentiated size structures. Overall, the relative importance of environmental filtering and biotic competition shifts significantly across size classes. These findings emphasize that forest restoration practices must account for the differentiated requirements of plants at different developmental stages. Based on these findings, we propose targeted forest restoration strategies to provide guidance for forest recovery efforts.

Key words: Habitat filtering, Competition, Species diversity, β-diversity, Ontogenetic niche shift