Localized neighborhood species mingling is correlated with individual tree size inequality in natural forests in South China

• Key message We identified the significant spatial relationships between tree species mingling and size inequality in various species-rich natural forests in South China. Forest communities with low species mingling, or segregation of heterospecific trees, exhibited increased segregation of trees with dissimilar sizes. Large trees were more likely to be associated with high species mingling, and variation in neighboring tree sizes would also increase neighborhood species diversity. • Context Numerous hypotheses such as Janzen–Connell and herd immunity effects have been proposed to explain the coexistence of diverse species in plant communities. However, these mechanisms for maintaining diversity have been studied primarily through the lens of species diversity, whereas tree size diversity or relationships between size and species diversity resulting from local interactions have rarely been investigated. • Aims We aimed to identify relationships between spatial species mingling and tree size inequality using extensive plot data from species-rich forest ecosystems. • Methods We applied neighborhood-based methods and explored relationships between spatial species mingling and size inequality indices in 22 large forest plots (30 ha in total) in subtropical and tropical China. • Results Forest communities with low species mingling, or segregation of heterospecific trees, exhibited increased segregation of trees with dissimilar sizes. In most plots, large trees (e.g., dbh > 30 cm) were extensively associated with high species mingling, and individual-tree neighborhood species and size diversity were closely correlated. • Conclusion Our findings suggest that both the spatial patterns of conspecific dispersal and neighborhood interactions play an important role in determining the spatial patterns of tree size and species diversity and eventually in shaping the structure of forest communities.