Tree diversity, environmental heterogeneity, and productivity in a Mexican tropical dry forest

Species diversity-environmental heterogeneity (D-EH) and species diversity-productivity (D-P) relationships have seldom been analyzed simultaneously even though such analyses could help to understand the processes underlying contrasts in species diversity among sites. Here we analyzed both relationships at a local scale for a highly diverse tropical dry forest of Mexico. We posed the following questions: (1) are environmental heterogeneity and productivity related ?; (2) what are the shapes of D-EH and D-P relationships ?; (3) what are individual, and interactive, contributions of these two variables to the observed variance in species diversity ?; and (4) are patterns affected by sample size, or by partitioning into average local diversity and spatial species turnover ? All trees (diameter at breast height >= 5 cm) within twenty-six 0.2-ha transects were censused; four environmental variables associated with water availability were combined into an environmental heterogeneity index; aboveground standing biomass was used as a productivity estimator. Simple and multiple linear and nonlinear regression models were run. Environmental heterogeneity and productivity were not correlated. We found consistently positive log-linear D-EH and D-P relationships. Productivity explained a larger fraction of among-transect variance in species diversity than did environmental heterogeneity. No effects of sample size were found. Different components of diversity varied in sensitivity to environmental heterogeneity and productivity. Our results suggest that species' differentiation along water availability gradients and species exclusion at the lowest productivity (driest) sites occur simultaneously, independently, and in a scale-dependent fashion on the tree community of this forest.