Impact of experimental soil moisture manipulation on tropical tree seedling demographic fates and functional traits

Sammanfattning: In tropical regions, climate change is predicted to lead to increased drought frequency and intensity. The extent to which this will shift the functional diversity of tropical tree communities is unknown due to a limited understanding about the response of seedlings to variable soil moisture. We addressed this issue using an experimental approach in the Luquillo Experimental Forest, Puerto Rico. We exposed seedlings of eight tree species, representing different successional stages, to an experimental soil moisture gradient. We evaluated (1) How species mean trait values relate to species-specific demographic responses to drought, (2) How intraspecific variation in functional traits relates to a soil moisture gradient, (3) The extent to which demographic response to short-term experimental drought mirrored long-term demographic response of seedlings to natural variation in soil moisture. Growth and survival of species with more `conservative` functional strategies tended to be more sensitive to a change in soil moisture and more tolerant to drought compared to species with more `acquisitive` strategies. In addition, traits of individual seedlings within species varied with respect to soil moisture, suggesting a potential role for phenotypic plasticity in response to drought. Specifically, this response was significant for three species (Inga laurina, Guarea guidonia, Schefflera morototoni) and was primarily associated with relative carbon investment in leaves and roots. Species demographic responses to soil moisture in experimental and long-term studies were weakly positively correlated but more variables are at play under natural conditions, which partly decouples these responses. Overall, our results suggest that tree species with `conservative` functional strategies are likely to become more common under increasing drought frequency and intensity in the Luquillo Mountains of Puerto Rico. However, understanding the broader implications of our findings will require considering the effects of other disturbances, including hurricanes, which may have contrasting effects. Furthermore, intraspecific variation in functional traits is likely to influence how the seedlings of tropical tree species cope with drought.