Mechanisms of local regeneration and community assembly under broad-leaved Korean pine ( Pinus koraiensis ) forests in northeastern China

Aims Seedling and sapling community assembly is a key process affecting forest regeneration and succession, yet the spatial aggregation patterns and assembly mechanisms of locally regenerating communities in broad-leaved Korean pine ( Pinus koraiensis ) forests remain poorly understood. To address this gap, we identified aggregation scales and patch sizes and elucidated the assembly mechanisms of local seedling and sapling communities. Methods We conducted stand and regeneration surveys in four broad-leaved Korean pine forest plots. For each regenerating individual, we recorded its species identity, spatial position, and height. Leaf traits were measured at the species level, and local canopy environmental variables were quantified based on stand survey data. Ripley’s g(r) , fitted using a Thomas process with a two-nested-scale function, was used to identify aggregation scales, and hierarchical clustering was used to delineate local communities. Abundance-based and presence-based standardized effect sizes of mean pairwise trait distance (SES.MPD), standardized effect sizes of Pielou’s evenness (SES.E), and the degree of distribution tendency ( D ) were then used to characterize community structure, and redundancy analysis (RDA) was used to assess trait–environment relationships. Results Both local seedling and sapling communities exhibited two characteristic aggregation scales, at approximately 1 and 15 m. Compared with sapling communities, seedling communities showed stronger aggregation at both the small and large scales, had more patches and more individuals per patch. Across 1–15 m scales, abundance-weighted SES.MPD , presence-based SES.MPD , SES.E , and their corresponding D values indicated non-random functional structure in both communities, with stronger abundance-based patterns in seedlings but more consistent presence-based patterns in saplings, and stronger signals of competition at smaller scales in saplings. The RDA showed that environmental explanatory power was relatively weak overall, but increased with spatial scale and developmental stage. Compared with the seedling communities, the sapling communities demonstrated weaker or even opposite correlations in community-weighted mean leaf area (CWM.LA), specific leaf area (SLA), and leaf width (LW) with vertical distribution tendency degrees and local coniferous leaf area index. Conclusions Overall, stochastic processes played a major role in shaping local regeneration communities. Limited dispersal caused regenerating individuals with similar traits to cluster across spatial scales. As seedlings developed into saplings, the influence of canopy conditions progressively accumulated, and local competition apparently became stronger, especially at small spatial scales. These findings suggest that the mechanisms shaping regeneration communities change during development, and that seedlings and saplings respond differently to canopy structure.