All leaves harbor diverse communities of fungi, both on their surface and within their tissues. The startling diversity of this group of microorganisms has just begun to be explored, and very little is yet known about their function in ecosystems. This project seeks to understand one possible role of these fungi: how they influence the process of decomposition. The general model of decomposition is that leaf litter is colonized and broken down by soil organisms after it reaches the ground. If some of the fungi inhabiting healthy, living leaves are latent decomposers, then they would have a head-start in breaking down litter compared to soil organisms, and their presence as the leaves senesce could even influence how decomposition proceeds. This project will test this hypothesis by using next-generation high throughput sequencing technologies, paired with biochemical and nutrient assays, to identify the fungi in a single, widely dispersed tree species (Metrosideros polymorpha), and trace these fungi through the process of decomposition in manipulative lab and field based experiments across an environmental matrix in Hawaii.
The broader impacts of this study include the participation of high school students and undergraduates from both California and Hawaii. The results of this research have important implications for understanding nutrient cycling and microbial biodiversity in tropical ecosystems, and will be communicated through journal articles, conference presentations, and via ongoing interactions with conservation biologists and land managers in Hawaii.
Intellectual Merit The goal of this research was to better understand the distribution of foliar fungal endophytes across a tropical landscape. Foliar fungal endophytes are microfungi that live within healthy leaves of all plants yet surveyed; these fungi could have important effects on their host's ability to handle environmental stress, resist herbivory, and mediate the effects of pathogens. However, we know very little about the factors that control how these species are distributed across landscapes, and even less so in the tropics. Understanding those distributions could potentially facilitate better modeling of pathogen outbreaks and inform conservation decision making. This research, which was conducted as part of a PhD dissertation, found that environmental factors such as elevation and rainfall strongly structure the membership of these microbial communities, and that these communities show very high between-site diversity (that is, even within the same host plant, within 10s of km from each other, the communities of microfungi are very different from one another and that those differences are predictably structured). The results of this research also show a very high level of fungal species richness (the number of species in a given area, determined in this case via high-throughput DNA sequencing methods)—suggesting that more than 4000 fungal species could be living associated with the leaves of a single host plant. This richness is higher than the number of all fungal species yet documented in Hawai'i, from all habitats. More broadly, this research has contributed to our understanding of the extent to which environmental controls (temperature, rainfall, elevation, soil nutrients, geographic distance) influence biogeographic patterns of eukaryotic microbes in the environment. It has highlighted the remarkable fungal diversity that can exist in association with a single host tree species over landscape scales. Broader Impacts Through this project, Co-PI Zimmerman has mentored three undergrads from Stanford and has presented on the results at a public meeting in Hilo, Hawaii on Hawaiian Ecology, at the Hawaii Conservation Conference in Honolulu, HI, as well as at several national and international ecology conferences. The training Co-PI Zimmerman received during the course of this grant strengthened his ability to conduct research in environmental microbial ecology and to use the most recently-developed methodological approaches to do so. Finally, the results of this research may have implications for understanding how native species will be impacted by increasing exchange of foliar pathogens worldwide.
