Recruitment asynchrony and stable coexistence in trees: Using theory and long-term data to infer community dynamics of long-lived species

Explaining the diversity of natural communities is a long-standing challenge to ecologists. Competition for common resources is expected to limit the number of species in any community, but various coexistence mechanisms have been identified that can promote diversity. Tropical forests are among the most diverse communities on Earth, but the extent to which known coexistence mechanisms can explain this diversity is highly debated. This thesis uses mathematical models and long-term demographic data to addresses the ability of a particular mechanism---the storage effect---to account for tropical tree diversity. A forest dynamics model was developed that captures species-specific recruitment through seed, sapling, and adult stages, and parameterized using records of seed production and seedling recruitment. It is first shown for 108 species at Barro Colorado Island (BCI), Panama, that the storage effect can reduce competition between species by almost 70% on average. Applying this approach across 8 different forests with substantial long-term records (including 5 tropical, 2 temperate, and 1 boreal forest) it is shown that the strength of this mechanism increases with decreasing latitude. This reduction in competition creates a gradient in species interactions that contributes to the maintenance of the lower extinction rates and higher diversity of tropical forests. As a final caveat to these results, I use a spatially explicit mathematical model to demonstrate that dispersal limitation can have important consequences on coexistence by the storage effect. These results suggest that a more accurate assessment of coexistence between tropical trees requires more detailed knowledge of dispersal; unfortunately, the appropriate data to do more than a rudimentary assessment may not exist for BCI or any other forest. Overall, observed patterns of reproduction in several forests indicate that the storage effect can play a major role in promoting coexistence in even highly diverse plant communities, although its strength does increase with decreasing latitude. The latitudinal gradient corresponds to differences in seasonality, suggesting that the synchronization imposed by seasonality ultimately decreases the potential for the storage effect. These results must be considered preliminary, however, because dispersal limitation and other sources of spatial heterogeneity can impact the strength of coexistence.