Local species composition is shaped by plant responses to abiotic and biotic factors. Tropical forests species composition changes substantially along rainfall gradients and with temporal trends in precipitation within a site as a consequence of interspecific differences in responses to water availability. Patterns of plant species abundance are also influenced by reduced individual performance in areas of high conspecific density. Such conspecific negative density-dependence (CNDD) is central to stable species coexistence. Studies have shown that natural enemies, and specifically fungal pathogens in the soil, are the most important factor contributing to CNDD among tropical forest plants. Because fungal pathogens and other microbes respond strongly to water availability, changes in water availability are expected to alter plant-pathogen interactions, and thereby plant performance and CNDD. Research into the impacts of temporal variation in water availability on tropical plants has focused on direct effects of drought on plants. In contrast, responses of plant-pathogen interactions to temporal variation in rainfall and the consequences for plants are virtually unknown. We analyzed 16 years of seedling census data for Barro Colorado Island (BCI), Panama, to investigate how seedling survival responded jointly to CNDD and interannual variation in soil moisture as an indicator of water availability.
Results/Conclusions
We found that on average, CNDD is stronger in years with higher soil moisture. Thus, seedling survival in areas of high conspecific density was lower in wetter years than in drier years. This result supports the hypothesis that increased rainfall could increase plant-pathogen interactions in tropical forests because wetter conditions favor pathogens and promote the incidence of infection. Our result is inconsistent with the hypothesis that drought could increase CNDD by increasing the susceptibility to attack of drought-stressed hosts. We also found, however, that seedling survival in areas of low conspecific density was higher in wetter years, than in drier years. This result supports the hypothesis that higher soil moisture reduces seedling mortality by alleviating drought-stress, which is known to kill many seedlings in tropical forests. Our long-term analysis of seedling survival reveals that the effects of a key abiotic factor, water availability, interact with those of a key biotic factor, conspecific density, to create a complex spatio-temporal mosaic of conditions with important consequences for seedling recruitment.