Collaborative Research: Testing a microbial-association-distribution hypothesis to explain spatial distributions and species co-existence in a community of epiphytic plants

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Nearly all wild plants depend upon beneficial fungi, called mycorrhizal fungi, to obtain adequate nutrition. This relationship between plants and fungi may help solve a key puzzle in the study of biodiversity, how so many species of plants can compete for the same resources of water, light, and soil nutrients and yet co-exist in the same natural community. This project will conduct experiments in a highly diverse tropical forest in Costa Rica to test if: 1) different plant species associate with different fungi; 2) species only occur where the appropriate beneficial fungi for that species are present; and 3) association with different fungi reduces competition between plant species and promotes co-existence. The research will focus on the rich array of orchids that grow on the trunks and branches in the forest, since it is already known that these species form close associations with mycorrhizal fungi. Results will advance understanding of the mechanisms that maintain natural diversity, and the project will disseminate findings to the public as well as to the scientific community to promote conservation of the diversity in tropical forests. The project will also train undergraduate and graduate students and postdoctoral researchers, including members of groups under-represented in science, and promote international scientific collaboration. The proposed conceptual model for how plant-fungal associations could shape orchid communities and their niche structures is that fungi are themselves resources exploited by the plants, as well as the means by which plants acquire resources such as water and nutrients, and thus determine plant spatial distributions and species coexistence. This hypothesis will be addressed through two integrated studies that will (1) conduct the first such spatial sampling of orchids, fungi, and environmental variables in a tropical forest and (2) experimentally determine whether orchid seed establishment is limited by the distribution of fungi by placing seed packets in locations with and without suitable fungi. Both studies will employ next-generation sequencing methods to identify fungi in the environment and will include novel statistical analyses of the niches and interaction networks of orchids and fungi. Findings will show whether (1) fungi are patchily distributed, (2) orchids differ in their fungal associations, (3) orchid seedling establishment and adult distributions are limited by fungal distributions, and (4) co-occurring orchid species show niche partitioning in their use of fungi and in the substrates from which their associated fungi draw resources.