RAPID: Microbial signatures of tropical trees across human land use gradients before and after hurricane disturbance: implications for the disruption of coexistence mechanisms

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This is a Rapid Response Research (RAPID) project to address the impacts of Hurricanes Maria and Irma in Puerto Rico. The overall goal is to understand how natural disturbances such as hurricanes impact plant biodiversity in tropical forests on the Island. Experiments and surveys before and after the hurricanes will be used to determine whether or not the hurricanes disrupted the process thought to be responsible for maintaining the incredible diversity of plant life there. Tropical rain forests are one of the most diverse ecosystems on Earth, and provide habitats for more than half of all plant and animal species, but we still do not fully understand how such immense diversity is maintained or how human and natural disturbances affect it. One well-known hypothesis predicts that species-specific natural enemies (disease-causing microbes) reduce the competitive ability of some trees, thereby allowing other trees to grow and survive, but detailed tests of this hypothesis are few. This project will test the hypothesis in various ways, by examining whether tree and soil microorganism distribution patterns are in sync with one another, and whether such interactions were altered by massive inputs and mixtures of leaf litter from recent hurricanes Irma and Maria. The studies will take place through the auspices of the Luquillo Long-term Ecological Research (LTER) site in El Yunque. The intellectual and technical capacity building for students and researchers in Puerto Rico and Oregon will directly benefit goals of improving the competitiveness of the United States in research, as well a broadening participation in science.The Janzen-Connell hypothesis is one of the main hypotheses evoked to explain how tree diversity in tropical forests is maintained. To explicitly test the underlying assumption that trees have unique microbial signatures in the soils surrounding them, the DNA of fungi and bacteria will be sequenced from soils sampled at 0-10m within the bases of nine target trees before and after the hurricane in the 16-ha Luquillo Forest Dynamics Plot, which has a gradient of human land use. To determine whether or not the strength of Janzen-Connell effects was altered, and how land use legacies interact with these processes, seedling transplant experiments will be conducted, and pathogens and mycorrhizal fungi will be isolated and used for further experiments. In combination with this team's extensive pre-hurricane data, the following hypotheses will be tested: H1: Sudden inputs of mixed leaf litter and debris from hurricane-force winds attenuate microbial signatures of canopy trees, which weakens Janzen-Connell effects due to diluted species-specific pathogen loads near canopy trees; H2: Mycorrhizal fungal colonization, richness, and diversity decrease following hurricane disturbance due to a lack of photosynthetic input from defoliated trees, making seedlings more vulnerable to generalized pathogen attack and nutrient limitation; H3: Recent human land use interacts with hurricane disturbance to further impede microbially-mediated coexistence processes. Since human disturbance is pervasive in all tropical forests, and hurricane disturbance is increasing in intensity across a wide range of tropical areas from the Caribbean to the Asian tropics, results of this study will have generalizable implications for predicting tropical forest dynamics, and may provide insight into some of the unexplained variation observed in studies of forest recovery.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.