Effects of Climate Change and Related Processes on Communities of Symbiotrophic Fungi Associated with Ponderosa Pine (Pinus Ponderosa)

Through hundreds of millions of years of association, fungi that live in symbiosis with plants have diversified in form and function. Today symbioses are increasingly recognized as a functional extension of each partner that can modify the host phenotype and impact host health and resilience to stress. Plant-mutualistic fungi such as ectomycorrhizal fungi on roots and endophytes in leaves contribute meaningfully to plant and ecosystem processes, but often are studied separately. As sweeping changes in climate alter temperature, the timing and quantity of precipitation, the length of growing seasons, and the occurrence and intensity of disturbances, there is a need not only to characterize existing diversity of plant-associated fungi, but to understand how these symbionts respond individually and together to environmental stress. In this dissertation, I examine how climate change and related processes impact ectomycorrhizal and endophytic fungi associated with an important tree species of western North America (Pinus ponderosa sensu lato). I first examine the extent to which these fungal communities are structured by climate and related factors along an elevation gradient in the Santa Catalina Mountains, an iconic range in the Madrean Sky Island Archipelago of southern Arizona. I show that trees at cooler, wetter, high-elevation sites host a distinct ectomycorrhizal community compared to those at warmer and drier sites at lower elevations. In contrast, foliar endophyte communities show no structure across the elevation gradient. I then widen my focus to a regional scale, examining the importance of a climate-relevant disturbance – wildfire – on ectomycorrhizal communities. My study in two ranges of the Sky Islands shows that wildfires, which are increasing in intensity and frequency as our region warms and dries, strongly impact ectomycorrhizal communities, with recovery periods on the scale of decades. My colleagues and I demonstrate that ectomycorrhizal communities in geographically distinct forests have similar responses to fire in terms of shifts of richness and diversity, but not in terms of species composition. Finally, I expand my focus to encompass the range of Ponderosa pine in Arizona, evaluating the distribution and diversity of ectomycorrhizal and endophytic fungi in anciently disjunct forests spanning >500 km that are isolated by desert, grassland, chaparral, and woodland. I show that the composition of these fungal communities decreases in similarity with increased distance between sites, with assemblages of ectomycorrhizal fungi shaped by dispersal limitation and assemblages of endophytic fungi influenced by environmental dissimilarity. Together these studies represent the first major characterization of ectomycorrhizal and endophytic fungi associated with Pinus ponderosa in the southwestern USA, highlighting the sensitivity of two vital portions of the mycobiome of this important coniferous species to diverse pressures associated with climate change