Do differences in understory light contribute to tree species turnover along a tropical rainfall gradient?

Background/Question/Methods Understanding how plant communities diverge along environmental gradients will help elucidate the mechanisms underlying niche specialization, the maintenance of diversity and responses to global change. A number of recent studies indicate that species turnover along rainfall gradients in the tropics is strongly influenced by drought resistance. Hence, wet-distribution species are eliminated from dry forests by drought. In contrast, the mechanisms that restrict the abundance of dry-distribution species in wet forests are unknown. In the present study we used common gardens along a rainfall gradient on the Isthmus of Panama to test the hypothesis that understory light availability may influence the distribution of shade-tolerant species. Results/Conclusions We found that understory light is higher and more variable in time and space in the dry relative to the wet forests. We hypothesized that shade-tolerant species, with a distribution in drier, higher light habitats, would have the strongest growth response to increased light. Contrary to our hypothesis, we found that dry-distribution species had a smaller growth response to increased light than did species from wetter, lower-light forests. In fact, in all of our experiments under well-watered conditions, growth was always faster for wet- relative to dry-distribution species. These and other results suggest that the adaptations associated with the drought resistance of dry-distribution species impose constraints on growth under good conditions. When water availability was high, typical of wet forests, mortality went up as light decreased, suggesting that very low light environments are stressful. In contrast, when water availability was low, typical of dry forests, mortality went up as light increased. This response was steeper for the wet-distribution species, suggesting that wet-distribution species may be more constrained by drought plus high light than are dry-distribution species. We conclude that, under conditions that are favorable for plant growth, wet-distribution species may be more competitive than dry-distribution species. Despite the low and less variable light in wet forests, high-light microsites or light gaps may be quite important in growth and recruitment. If so, we hypothesize that one trait that restricts the abundance of shade-tolerant, dry-distribution species in wet forests is that they are less competitive under higher light.