IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY

Abstract

In tropical forests, soil-borne plant pathogens are thought to regulate diversity by driving negative plant-soil feedbacks and promoting density-dependent mortality of seedlings close to parent trees. Pathogens interact with a myriad of other organisms in soil that have the potential to moderate plant disease and in turn its effects on community composition. Studies that consider interactions among microbial groups in the context of plant-soil feedbacks are relatively rare. In order to understand how microbes influence diversity in tropical forests, we need to move beyond the assumption of one-to-one feedbacks between plants and specialized pathogens and consider the potential importance of multiple soil functional groups and environmental heterogeneity in microbe-microbe and plant-microbe interactions. In my dissertation, I investigate antibiotic-producing soil bacteria (genus: Streptomyces) in tropical forest soils. I focus on biotic and abiotic characteristics that influence variation in pathogen suppression across the landscape and among individual trees, and address the potential consequences for plant community composition. In Chapters 1 and 2, I explored Streptomyces communities in tropical dry forests in Costa Rica and asked whether variation was related to soil gradients and/or tree species. There was substantial variation in plant-soil-microbe relationships. In general, variation among tree species was not as great as variation among individual trees. In Chapter 3, I investigated the potential for Streptomyces to influence seedling biomass and plant-soil feedbacks in a tropical moist forest in Panama. Seedling biomass was significantly affected by a three-way interaction between seedling species, soil source species (adult tree species), and densities of pathogen-suppressive Streptomyces in soil. Overall, results suggest that unique relationships among antibiotic-producing Streptomyces, pathogens, and abiotic resources among soils from different adult trees influence seedling biomass and may mediate the strength and specificity of plant-soil feedbacks in tropical forest soils. Future studies which account for multi-dimensional interactions among soils, microbes, and plants will allow us to better understand how aboveground diversity is regulated by diverse microbes in soil.