Stabilizing and equalizing mechanisms of coexistence in a model of competition for light in forests

Background/Question/Methods   A variety of demographic tradeoffs have been proposed as mechanisms maintaining diversity under competition for light in forests. However, rarely have these tradeoffs been rigorously shown to lead to coexistence through models of competition for light that incorporate heterogeneity in the light environment arising from disturbance dynamics. Even when they have, which tradeoffs actually allow for stable coexistence, in which rare species have an advantage allowing them to invade, and which simply equalize fitness across individuals of different species, has not been determined. In this study we simulated Kohyama's model of competition for light in forests with the aim of determining which tradeoffs allow for stable coexistence of tree species, and which simply equalize fitness. The ultimate goal of the work is to parameterize the model in terms of functional traits of the tree species using observed correlations between functional traits and parameters of demographic performance, and translate the stabilizing and equalizing demographic tradeoffs of the model into expected functional trait patterns. These predicted trait patterns could then be tested to assess the relative roles of stabilizing and equalizing forces of coexistence in specific forests.

Results/Conclusions   We found that an interspecific tradeoff between maximum relative diameter growth rate (which is exhibited in environments with no shading) and sensitivity of survival to shading, as well as an interspecific tradeoff between maximum relative diameter growth rate and sensitivity of growth to shading allowed for stable coexistence. However, a tradeoff between maximum recruitment rate (exhibited in high light environments) and baseline survival in high light environments, as well as tradeoffs in other aspects of demographic performance that are independent of shading, simply equalized fitness across species. Another talk in this session fits multivariate models of the relationship between various parameters of demographic performance and functional traits, allowing the translation of these stabilizing and equalizing tradeoffs into multivariate functional trait tradeoffs. It also compares observed trait patterns with these predicted tradeoffs to begin to assess the relative roles of stabilizing and fitness-equalizing coexistence mechanisms.