Biotic and abiotic neighborhood effects on seedling survival in a tropical forest

Understanding the mechanisms maintaining diversity in species rich communities remains a major challenge in ecology.  In tropical forests, many of the theories proposed to explain species coexistence hinge on mechanisms that shape spatial patterns of seedling survival.  Such patterns have been hypothesized to depend on both the local biotic neighborhood, through increased seedling mortality in areas of high conspecific density, and on the abiotic environment, as species with differing regeneration requirements will survive better in different habitats. Using data from a large-scale, community-wide study of established seedlings in a 50-ha Forest Dynamics Plot (FDP) located in central Panama, we tested for the significant effects of the local biotic and abiotic neighborhood on seedling survival. Specifically, we used maximum likelihood methods to parameterize and evaluate spatially explicit models of survival that included effects of conspecific seedling and adult neighbor density, edaphic habitat type, and interactions between density and habitat.  We found that seedling survival declined with increasing local density of conspecific neighbors and that the effect of conspecific neighbors was significantly different from that of heterospecific neighbors.  There was a significant interaction between edaphic habitat type and density, indicating that habitat not only influences the probability of survival directly, but also drives spatial variation in the strength of density dependence.  Our results suggest both abiotic and biotic neighborhood variables, as well as their interaction, shape spatial patterns of seedling survival and potentially promote species coexistence in diverse tropical tree communities.