Local species richness is controlled by several interacting processes. Size-asymmetric competition, i.e. when larger plants have a disproportionally larger effect on smaller ones, plays an important role in the dynamics of plant populations, but its importance for community-level properties, such as variation in local species richness, has not been extensively investigated. We constructed a spatially explicit, individual-based Zone-of-Influence model, in which competition occurs within the overlapping areas between individuals. Structural equation modeling (SEM) incorporating environmental variables (topographic and soil resource variables) was implemented to test simulation predictions through three permanent stem-mapped forest plots: Barro Colorado Island (50 ha), Panama, Jianfengling (60 ha) and Heishiding (50 ha), China. Each plot was evenly divided into non-overlapping quadrats with the size of 20 m × 20 m.
Results/Conclusions
Bivariate plots from simulations demonstrated that species richness decreased, increased or had no significant relationships with an increase of the degree of size asymmetry, which depended on the niche breadth of species and the environmental heterogeneity. The direct effect of size-asymmetric competition on local biodiversity was negligible. In contrast, size-asymmetric competition influenced species richness indirectly through its effects on three variables (in order of importance): tree density (the total number of individuals), size variation (the coefficient of variation in diameter), and total stand biomass. Effects of topographic variables on species richness were site-specific, and soil resource variables had little effect on the number of species occurring in quadrats in all three plots. Our study demonstrates the indirect but important effects of size-asymmetric competition for light on local biodiversity in plant communities.