Character convergence, diversity, and disturbance in tropical rain forest in Guyana

The level of tree diversity varies greatly between sites in Guyana and decreases along a gradient from south to north. We conducted a study to understand what controls this gradient of diversity using data from country-wide forest inventories. Analysis of tree diversity on the basis of soil and rainfall effects in an area of 15 x 10(6) ha showed that soil exerts a strong local effect on tree diversity, whereas rainfall, potential evapotranspiration, and length of the dry season do not. Community averages of disturbance-sensitive attributes of trees, such as seed mass or wood density, were not correlated with variation in either rainfall or soil, but were strongly correlated with tree diversity. As such, the forests in central Guyana were characterized by generally a high average wood density, large seeds, and rodent or unassisted dispersal. These forests also displayed the highest single dominance, thus lowest diversity and low percentages of pioneers in the community. The more highly diverse forests in south Guyana showed much lower average community wood density and seed mass, and bird and primate dispersal were more common than in central Guyana. Large-scale spatial variation in diversity and community averages of life history characteristics are here considered the product of local convergence of species with similar characteristics. Characteristics bestowing competitive superiority in environments with little disturbance ultimately lead to a decline in diversity through competitive exclusion, much as that predicted by the intermediate disturbance hypothesis and dynamic equilibrium theory. The more diverse communities are associated with characteristics of superior colonizers: low wood density, small seeds, and good dispersal. Variation in diversity at smaller spatial scales is strongly affected by soils.