Understanding the mechanisms shaping the distribution of organisms sheds light on local and regional biodiversity and the susceptibility of communities to environmental change. Tropical forests are characterized by very high species diversity. A large fraction of this diversity can be explained by species turnover along environmental gradients. Within the tropics, rainfall can vary by a factor of 10 between dry and wet forest, and many biotic and abiotic factors correlate with this gradient. One important change along this gradient is seasonality in the precipitation, with drier forests suffering longer and stronger annual droughts. Currently, mounting evidence suggest that dry forest composition is linked to plant adaptations to cope with drought. Other important stressors that covary with the rainfall gradient include natural enemies, soil fertility and understory light. While drought may determine forest composition in drier forests, these other variables may determine forest composition in wetter sites. We established common garden experiments along a rainfall gradient in the Isthmus of Panama in order to experimentally evaluate the relative impacts of these variables as determinants of plant distributions.
Results/Conclusions

Seasonal drought significantly increased the mortality of wet-distribution species in the drier site, and its effect was reversed with water supplementation. Wetter forests had significantly higher herbivore and pathogen attack, and lower understory light levels and soil fertility. However, these variables impacted equally the leaf damage, growth and mortality of both dry-and wet-distribution species, suggesting that they do not preferentially exclude dry-distribution species. Instead, dry-distribution species had significantly slower growth rates than wet-distribution species, may be due to trade-offs imposed by their drought adaptations. This difference in growth rate combined with the poorer growing conditions in the wetter forest to cause dry-distribution species to have slightly negative growth rates. We conclude that the lack of adaptations to seasonal drought is the most important limitation to the colonization of wet-distribution species in drier forests, while strong competition in poor growing conditions is the most important limitation for slow growing, dry-distribution species in the absence of drought stress.