Tropical forests harbor high diversity. Even more striking is the extremely high numbers of species that can coexist at a single site (> 650/hectare). Recent research has focused on identifying which interactions with neighbors are those that actually promote coexistence and lead to negative density-dependent growth and survival. Two prominent hypotheses to explain negative density dependence are resource competition, where conspecifics compete for the same resources, and pest pressure, where attack by species-specific pests increases with conspecific density. Using data from the 50-hectare forest dynamics plot on Barro Colorado Island in Panama, I compare the influence of resource acquisition traits and defense traits on tree growth, survivorship, and recruitment for focal trees of the tropical tree genus Inga (Fabaceae). While studies have used resource acquisition traits in their analysis of density dependence, no studies have simultaneously integrated defense traits. Here I use generalized linear models to predict the effect of neighborhood similarity based on the following traits: 1) Defense - phytochemistry, phenology, development, ants, hairs 2) Resource Acquisition - specific wood density, maximum tree height, specific leaf area and leaf elemental composition. These models provide insight as to the relative importance of these two proposed mechanisms for explaining negative density dependence.
Preliminary results show that similarity in defense traits leads to decreased survival, while similarity in resource traits leads to an increased survival in sapling and juvenile trees. Patterns for the relationship between growth and trait based neighborhood similarity varied among individual traits measured, however, chemical similarity showed a clear negative relationship with growth. In contrast, the defensive trait related to interactions between plants and ants, showed a positive association with tree growth. The latter finding supports previous work that ant associated plants tend to cluster. Lastly, resource competition traits show little association with sapling tree growth with the exception of wood density (slightly positive) and maximum tree height (slightly negative). Overall these findings support the hypothesis that pest pressure is the dominant driver of negative density dependence in tropical forest communities, especially for understory saplings. This study builds off of a large body of research supporting that negative density dependence during early life stages facilitates coexistence by integrating both defense and resource acquisition traits rather than using phylogeny as a proxy for trait similarity. I will continue this work by testing the relationship between similarity in the herbivore community between two species and tree survival and growth.