Rainfall variability and deciduous-evergreen coexistence in tropical forests

In tropical forests, deciduous and evergreen leaf habits represent contrasting tree adaptations to precipitation seasonality. Both rainfall seasonality and interannual variation in rainfall are determinants of forest deciduousness, but their relative influence is not well understood. In this study, we evaluate the extent of deciduous-evergreen coexistence in tropical forests and develop a simple model of competition for water between leaf habits. Using this model, we formalize two mechanisms representing rainfall variability across time scales that may explain their stable coexistence: the temporal storage effect via interannual variability in rainfall vs. rainfall partitioning via evergreen access to dry-season rainfall. In our model, both mechanisms resulted in coexistence, but coexistence was more robust via resource partitioning. Empirically, remotely sensed deciduousness increased with precipitation seasonality, but effects of interannual rainfall variability on deciduousness were minor. We hypothesize that dry-season rainfall may prove a stronger influence on coexistence between leaf habits, and that changes in rainfall seasonality will have a greater impact on forest deciduousness than changes in the interannual variability of rainfall.