Host and symbiont diversity are inextricably linked across partnerships and ecosystems, with degree of partner reliance governing the strength of this correlation. In many forest soils, symbiotic ectomycorrhizal fungi coexist and compete with free-living saprotrophic fungi, with the outcomes of these interactions shaping resource availability and competitive outcomes for the trees aboveground. Traditional approaches to characterizing these communities rely on DNA sequencing of a ribosomal precursor RNA gene (the internal transcribed spacer region), but directly sequencing the precursor rRNA may provide a more functionally relevant perspective on the potentially active fungal communities. Here, we map ectomycorrhizal and saprotrophic soil fungal communities through a mixed hardwood-conifer forest to assess how above- and belowground diversity linkages compare across these differently adapted guilds. Using highly spatially resolved transects (sampled every 2 m) and well-mapped stands of varying host tree diversity, we sought to understand the relative influence of symbiosis versus environment in predicting fungal diversity measures. Canopy species in this forest included two oaks (Quercus agrifolia and Quercus douglasii) and one pine (Pinus sabiniana). At the scale of our study, spatial turnover in rRNA-based communities was much more predictable from measurable environmental attributes than DNAbased communities. And while turnover of ectomycorrhizal fungi and saprotrophs were predictable by the presence and abundance of different canopy species, they both responded strongly to soil nutrient characteristics, namely pH and nitrogen availability, highlighting the niche overlap of these coexisting guilds and the strong influence of aboveground plants on belowground fungal communities.

Forest outcomes are contingent upon the persistence of crucial symbioses between plants and soil fungi – mycorrhizae. To understand the vulnerability or resilience of these ecosystems, which are the dominant land cover type and house the bulk of terrestrial carbon, we must account for ecology of mycorrhizal symbioses. Temperate and boreal forests are primarily reliant on the guild known as ectomycorrhizal fungi which facilitate nutrient cycling and plant nutrient uptake under the canopies of more than 60% of the world’s trees. In this symbiosis, both the host trees and mycorrhizal fungi often have the capacity to interact with multiple partner species, even at the same time. This interaction network creates a complex set of both direct and indirect interactions that can determine the dynamics of forest communities.

By combining field studies, mesocosm experiments, and theoretical modeling, my research interrogates how ectomycorrhizal fungi and their host trees interact to shape community structure and drive system-level ecological outcomes. In this dissertation, I share three research efforts conducted over the course of my graduate studies each of which use different techniques to scale from idealized ecological systems to the full complexity of the natural world.

In Chapter 1, I share a multi-year restoration effort working to better understand the ecology of an endemic Southern California conifer, the bigcone Douglas-fir (Pseudotsuga macrocarpa). In this project, we planted 1,728 seedlings in into backcountry sites in the scar of the 2007 Zaca Fire. We combine fire severity, slope position, habitat type, soil inoculation, and summer watering in a mixed experimental design to inform USFS restoration efforts in the future, following the seedlings for more than two years to observe long-term outcomes. Using Bayesian mixed models, we assessed the direction and strength of each contributing factor, leading to insights into improved planting locations and conditions for bigcone Douglas-fir.

I build upon this field-based work in Chapter 2 where I report on a greenhouse experiment looking at the interactions between host species and soil microbial inoculum. We planted seedlings of two species, the bigcone Douglas-fir (Pseudotsuga macrocarpa) and the canyon live oak (Quercus chrysolepis), into two live field soils and one sterile soil for a year. We then uprooted seedlings, washed them, and replanted them with a neighbor of variable backgrounds, allowing us to observe the influence of soil and neighbors together. We then incorporated a simulated drought to provide greater insights into ectomycorrhizal dynamics under current ecological stressors. We find strong, lasting effects of soil on seedling outcomes with negative plant-soil feedbacks under drought conditions. These results point to the importance of understanding soil microbial partner availability and the context dependence of stressors for mixed forest dynamics under climate change.

Finally, in Chapter 3 I develop a mathematical model depicting host trees and ectomycorrhizal fungi in a shared network. In this model, we allow for identity-based and quality-based partner rewards in the mutualism network, integrating two common mechanisms for partner control in empirical studies. By specifically outlining and interrogating the combined effects of these pathways, we help build an understanding of the impacts of different assumptions about the nature of the symbiosis. We then use this model to assess the role of ectomycorrhizal network connectivity in forest community stress responses, finding that agnostic partner allocation (without strong partner control mechanisms) leads to the most stable forest communities through stress. This result points to important questions for empiricists working to understand the evolutionary benefits of partner sanctions in the ectomycorrhizal symbiosis.

A large part of ecosystem function in woodland systems depends on soil fungal communities. However, global climate change has the potential to fundamentally alter these communities as fungal species are filtered with changing environmental conditions. In this study, we examined the potential effects of climate on host-associated (i.e. tree-associated) soil fungal communities at climatically distinct sites in the Tehachapi Mountains in California, where more arid conditions represent likely regional climate futures. We found that soil fungal community composition changes strongly across sites, with species richness and diversity being highest at the most arid site. However, host association may buffer the effects of climate on community composition, as host-associated fungal communities are more similar to each other along the climatic gradient than the whole fungal community. Lastly, an examination of functional traits for ectomycorrhizae, a well-studied guild of fungal mutualist species, showed that stress-tolerant traits were more abundant at arid sites than mesic sites, providing a mechanistic understanding of these community patterns. Taken together, our results indicate that fungal community composition will likely shift with future climate change but that host association may buffer these effects, with shifts in functional traits having implications for future ecosystem function.