Tropical Trees' Growth Exceeds Seed Dispersal Limits

Tropical trees distance themselves from members of their own species more than they do other species, a new study shows. What's more, trees of the same species exist at distances farther apart than would be expected by chance or the limits of seed dispersal. The results reveal pervasive within-species spatial repulsion in adult trees, providing new insights into the ecological dynamics that stabilize species diversity and enable the exceptionally high diversity of tropical forests. Tropical forests host an unusually high diversity of tree species. For example, some tropical forests contain more than 250 tree species per hectare. However, how hundreds of species coexist on relatively small spatial scales remains a question. It's thought that for species to coexist, performance of a species should decrease as they become more common, preventing any one species from taking over; an individual tree is more likely to survive when surrounded by different tree species with different resource needs, pathogens, and herbivores. As a result, this phenomenon, known as conspecific negative density dependence (CNDD), should produce patterns of within-species spatial "repulsion." While evidence for CNDD has been observed in juvenile trees in forests, the patterns it creates have rarely been demonstrated in adults. Michael Kalyuzhny and colleagues compared the within-species spatial patterns of adult tropical trees (41 species) in a forest plot in Panama with the outcomes of a Dispersal Limitation null model, which estimated species-specific dispersal-distance distributions. Kalyuzhny et al. discovered that many of the adult trees of tropical species show strong spatial repulsion in the forest plot. In some cases, several species were significantly and substantially overdispersed at distances up to ~100 meters. Using a spatially explicit simulation model, the authors demonstrate that such strong spatial repulsion can only arise if species suffer more from competition with their own kind than from other species (heterospecific negative density dependence) and that these observed patterns can result from small-scale CNDD.