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.
