One important hypothesis to explain tree-species coexistence in tropical forests suggests that increased attack by natural enemies near conspecific trees gives locally rare species a competitive advantage. Host ranges of natural enemies generally encompass several closely related plant taxa suggesting that seedlings should also do poorly around adults of closely related species. We investigated the effects of adult Parashorea malaanonan on seedling survival in a Bornean rain forest. Survival of P. malaanonan seedlings was highest at intermediate distances from parent trees while heterospecific seedlings were unaffected by distance. Leaf herbivores did not drive this relationship. Survival of seedlings was lowest for P. malaanonan, and increased with phylogenetic dissimilarity from this species, suggesting that survival of close relatives of common species is reduced. This study suggests that distance dependence contributes to species coexistence and highlights the need for further investigation into the role of shared plant enemies in community dynamics.
Much of the forest remaining in South East Asia has been selectively logged. The processes promoting species coexistence may be the key to the recovery and maintenance of diversity in these forests. One such process is the Janzen-Connell mechanism, where specialized natural enemies such as seed predators maintain diversity by inhibiting regeneration near conspecifics. In Neotropical forests, anthropogenic disturbance can disrupt the Janzen-Connell mechanism, but similar data are unavailable for South East Asia. We investigated the effects of conspecific density (two spatial scales) and distance from fruiting trees on seed and seedling survival of the canopy tree Parashorea malaanonan in unlogged and logged forests in Sabah, Malaysia. The production of mature seeds was higher in unlogged forest, perhaps because high adult densities facilitate pollination or satiate pre-dispersal predators. In both forest types, post-dispersal survival was reduced by small-scale (1 m(2)) conspecific density, but not by proximity to the nearest fruiting tree. Large-scale conspecific density (seeds per fruiting tree) reduced predation, probably by satiating predators. Higher seed production in unlogged forest, in combination with slightly higher survival, meant that recruitment was almost entirely limited to unlogged forest. Thus, while logging might not affect the Janzen-Connell mechanism at this site, it may influence the recruitment of particular species.