Strategy Space and the Disturbance Spectrum: A Life-History Model for Tree Species Coexistence

The disturbance spectrum consists of disturbance patterns differing in type, size, intensity, and frequency. It is proposed that tree life-history traits are adaptations to particular disturbance regimes. Four independent axes are proposed to define the dominant dimensions of tree strategy space: shade tolerance, tree height, capacity for vegetative reproduction, and seed dispersal distance. A fitness model was developed to elucidate interactions between the proposed life-history traits. The model shows how alternate life-history sets can coexist when disturbance patterns fluctuate in space and time. Variable disturbance regimes were shown, based on data and simulation results, to enhance species coexistence, as predicted. The strategy space model accurately predicts the number of common tree species for the eastern United States, boreal Canada, and southwestern piñon-juniper woodlands. The model also provides an explanation for latitudinal gradients in tree species richness in North America and Europe. The proposed model predicts a relationship between disturbance characteristics and the species composition of a forest that allows for the coexistence of large numbers of species. The life-history traits of size, growth rate, life span, shade tolerance, age of reproduction, seed dispersal distance, and vegetative reproduction are all incorporated into the model.