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.

Twenty-two permanent plots (30 ha in total) were established in seven natural reserves located in different regions of South China, ranging from mid-subtropical to tropical monsoon climate zones. All the forest plots are protected natural mixed forests comprising diverse species and represent a variety of forest types, such as evergreen broad-leaved forests, mid-subtropical evergreen broad-leaved and coniferous forests, mid-subtropical evergreen and deciduous broad-leaved mixed forests, and tropical monsoon rainforests. Data from the fully mapped plots includes all live trees larger than 1 cm, and each individual tree was tagged, numbered, mapped and their species and diameters were recorded. Forest plot data are organized as follows: Number (individual tree number), x [m] and y [m] (tree coordinates), stem diameter at 1.3 m above soil level (dbh in cm) and species. For questions relating to the data please contact the authors.