Seedling Density Dependence Promotes Coexistence of Bornean Rain Forest Trees

Density-dependent processes may occur at various spatial scales and may be generated by, and influence, various life stages in a population. When evaluating evidence for density dependence relevant to species coexistence, the nature of forest communities leads logically to a focus on particular scales and life stages. We assessed evidence for the dependence of seedling survival on the abundance of conspecific trees and seedlings over both wide (150-ha) and local (0.16-ha and 1-m2) scales, in a species-rich Bornean rain forest. At the 150-ha scale, we identified a community-level compensatory trend in seedling survival (i.e., more abundant species had higher seedling mortality). There were parallel trends at the population level and on a local scale; seedling survival was inversely related to conspecific seedling density (in 1-m2 quadrats) and tree basal area (in the surrounding 0.16 ha), for five out of 15 abundant species and for all species combined. As expected, seedlings of species with the highest adult abundance in 150 ha experienced the highest local densities of both conspecific seedlings and trees. Local density dependence in single-species analyses confirmed a dynamic interpretation of the community compensatory trend, i.e., that wide-scale seedling survival tends to decrease as a species’ adult abundance increases. Taken together, our findings constitute strong evidence that density-dependent processes contribute to coexistence in this diverse rain forest community.