Microbes on the Mountain: Plant-Microbe Associations and Interactions on Mount St. Helens

Abstract

Plant-microbe associations and interactions provide critical context to studies in both community and ecosystem ecology, especially in systems that are relatively new and still undergoing early successional processes. Microbes can colonize the surfaces and interiors of all plant tissues, and these assemblages vary in composition both spatially and temporally, even within the same plant. Endophytes are bacteria or fungi that spend most of their lifecycles living within plant tissues asymptomatically--typically, "endophyte" refers specifically to aboveground tissues such as leaves and stems, and therefore may have direct influences on defenses against herbivory, pathogen or pest tolerance, and even afterlife effects on litter decomposition. Similarly, root-associated symbionts such as mycorrhizae, Frankia, and Rhizobia are especially crucial to host plants' abilities to survive in nutrient-poor environments, such as those found during primary succession. Within these new, harsh systems, advantageous microbial symbioses can be critical for plant establishment and survival, as is the case for pioneer plants with nitrogen-fixing symbionts--e.g. alder, an important native tree species in the Pacific Northwest that is nodulated by Frankia bacteria. Here, we chose red and Sitka alder (Alnus rubra and A. viridis ssp. sinuata, respectively) as plant systems to model multi-scale interactions and associations in the primary successional environment of the Pumice Plain of Mount St. Helens. In Chapter 2, we first surveyed culturable fungal endophytes within red alder to gain a baseline understanding of the structure and composition of endophytes in a closely related species within more established habitats. We were able to use results as a comparison for Chapter 3, where we surveyed culturable fungal endophytes of six woody species on the Pumice Plain of Mount St. Helens. Our results suggest that culturable fungal endophyte communities still seem to be in the early stages of community development on the Pumice Plain, and that pioneer species like Sitka alder are potentially serving as important microbial reservoirs. Since microbial communities within plant tissues can interact with plant genotype to influence ecosystem processes like litter decomposition, we examined population genetics of Sitka alder around Mount St. Helens, in part to determine possible source populations of Pumice Plain colonists (Chapter 4). Finally, to complement our study of Sitka alder genetic structure, we investigated the population structure of Frankia strains in sympatrically-occurring alder species on the Pumice Plain (Chapter 5).