Leland K. Werden, Justin M. Becknell, Jennifer S. Powers
Tropical forests are complex systems where many factors combine to influence the distribution of species. A rich history of research has focused on how local factors such as soil nutrient availability, climate conditions, and changing environmental conditions at different stages of forest development, or succession, influence the distributions of plant species across tropical landscapes. These factors can influence tree species distributions in both predictable and random ways. Many studies have shown that the occurrence and abundance of certain tree species can be reliably predicted based on a single, or a combination of, these important environmental conditions.
We designed a study in tropical dry forests in Costa Rica, forests that have an annual drought cycle where a majority of trees drop their leaves, to determine how both soil characteristics and changing environmental conditions along succession influence the distributions of tree species. To accomplish this we used data previously collected from 84 plots of different forest ages (from young to mature) distributed over the surrounding landscape. In these plots all trees were counted and identified to species, and the soil characteristics were measured. For each of the 82 tree species that we focused on, we ran a statistical algorithm 1,000 times to robustly determine how the soil and successional characteristics influenced the abundance of tree species in our plots. Our results suggest that both soil characteristics, and environmental conditions at different forest ages, act to influence tree distributions in our study region. What makes our study particularly exciting is the fact that we then used functional traits, or generalized characteristics used to determine plant strategies to use and acquire resources, to understand if these characteristics help to determine why certain tree species preferred to live in specific soil and/or environmental conditions. This approach allowed us to demonstrate that tree species with specific strategies, such as those indicative of how trees deal with drought, also help to determine what soil and successional conditions are ideal for specific tree species.