Effects of experimental warming, disturbances, and species interactions on tropical tree community demography

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Tropical forests are the most diverse ecosystems in the world with some forests having hundreds of tree species per hectare. The maintenance of this diversity is believed to depend upon complex interactions among species. However, the relationships and interactions among species can be altered by climate change and disturbances such as hurricanes that disrupt resource conditions that govern seedling and sapling interactions in the forest understory. This project will combine a shade house experiment with novel molecular approaches to study how adult trees influence the interactions among seedlings of species on the forest floor beneath their canopies. In addition to disentangling how disturbances and warming may shift the strength and direction of relationships among species, this project thus investigates how plants can control plant-plant interactions by secreting specific molecules from their root tips. These secretions influence which micro-organisms become common and these micro-organisms in turn influence seedling success (either positively by promoting beneficial microbes, or negatively by promoting pathogenic microbes). Specifically, seedlings of canopy trees will be grown in pots at different densities in a shaded greenhouse under a range of growing conditions to simulate global warming and changes in canopy cover (light) that occur with hurricane removal of the adult tree canopy. They will be grown in different combinations of density and with different combinations of the field based micro-organisms that typically associate with the roots of different species of adult trees. The outcome of this work will help us to better predict whether tropical forests will maintain their high level of diversity as the world’s climate continues to change and how changes in light associated with canopy removal during hurricanes or cyclones can change species relationships. In addition to the scientific benefits, this project will offer educational and research opportunities to under-represented students. The study site has an established successful internship program, where recently graduated university students, primarily from Puerto Rico, are provided the opportunity to participate in research. They will be offered the opportunity for internships with the project and gain research experience from it. The team will also collaborate with a non-profit organization in Puerto Rico to coordinate field trips for elementary and middle school students to visit the research site and learn about the project. Overall, this work will contribute to the scientific and broader community understanding of what tropical forests might look like in the future. The maintenance of tropical forest diversity is hypothesized to rely on complex species interactions such as mutualism, predation/pathogen attack, and competition, that regulate seedling dynamics and tree species coexistence. These diversity maintaining processes could be fundamentally altered under projected changes in climate and disturbance regime, yet we have little understanding of how these factors will act independently or in concert to affect biological diversity in tropical forests. This project builds and expands upon an existing infrared warming experiment (Tropical Response to Altered Climate Experiment, TRACE). Based upon preliminary data, the team hypothesizes that the strength and direction of density dependence changes with warming and disturbances because of modifications in species interactions such as associations with mycorrhizal fungi (H1), and that plants can in part control these changes through root exudates (H2). This 2-year project combines a shade house experiment with -omic approaches to evaluate how changes in light, density, microclimate and microbes influence density dependence in different forest species. Specifically, seedlings from tree species will be grown in pots subjected to two soil source treatments (e.g. soil from warming versus control plots at the tRACE site), four microbial treatments (no mycorrhizal fungi, no pathogens, no microbes, control), three densities (low, medium, high), and two light treatments (low and high). Broader impacts of the study will fall into three main foci 1) provide research and educational opportunities in science, technology, engineering and math (STEM) with an emphasis on diversity and inclusion, 2) community outreach and education, and 3) generate research that will provide insight into the future of tropical forested ecosystems in a changing world.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.