Simulating Stationary Size Distribution of Trees in Rain Forests

A simple dynamic model of the distribution of tree size (trunk diameter) in natural rain forests is presented. Based on data of permanent plot measurements in a tropical rain forest and a warm-temperate rain forest, the cumulative basal area density of trees larger than a given tree, at any particular time, is used to express the effect of suppression, or one-sided competition, on the growth rate of that tree. It also shows that increasing the basal area density of all trees in the stand depresses the rate of recruitment from the pool of seedlings. Mortality is treated as independent of the cumulative basal area. Simulation with the model, applying the one-dimensional drift-diffusion equation, reproduces the observed course of reforestation after clear-felling and leads to convergence to a unique stationary size distribution by 200 years. This concuts with the size distribution observed in primary forest stands. The present model represents an extension of density-dependent population growth models to size-structured tree populations.