Numerous field studies have measured mycorrhizal dynamics under additions of nitrogen (N), phosphorus (P), or atmospheric CO2 to test the hypothesis that plants should invest in mycorrhizal fungi when soil nutrients are limiting. Here meta-analyses were used to integrate nutrient responses across independent field-based studies. Responses were compared between ecto- and arbuscular mycorrhizal fungi, and among fertilizer types, methods of measurement, biomes, and lead investigators. Relationships between degree of response and study length, fertilization rates, total amounts of nutrients applied, and numbers of replicates were also tested. Across studies, mycorrhizal abundance decreased 15% under N fertilization and 32% under P fertilization. Elevated CO2 elicited a 47% increase. Nitrogen effects varied significantly among studies, and P effects varied significantly among lead investigators. Most other factors did not affect mycorrhizal responses. These results support the plant investment hypothesis, and suggest that global standing stocks of mycorrhizal fungi may increase substantially under elevated CO2 but decline moderately under P additions. Effects of N deposition may be difficult to predict for individual ecosystems, with a slightly negative influence overall.

Tropical montane cloud forests (TMCF) are defined by their characteristic cloud cover. This cloud cover endows these forests with unique structural and functional characteristics. However, climate change is reducing cloud cover, and causing warmer temperatures and longer dry periods in TMCF. Studies show that there will be devastating effects on plant and animal species aboveground. But, few studies have investigated soil communities and associated processes in TMCF. The goal of this dissertation was to characterize soil fungal communities and associated properties to determine how they respond to elevation. And in turn, assess how soil fungi and decomposition may respond to projected climate change.

In Chapter 1, I characterized soil fungal communities and associated properties along an elevation gradient in a TMCF in Monteverde, Costa Rica. I found that soil properties, fungal communities, and microbial processes varied with elevation and across seasons. In Chapter 2, I simulated the effects of climate change by performing a soil translocation experiment across this same gradient. I found that fungal decomposers and pathogens may increase under warmer, drier conditions. I also found that decomposition increased under these warmer, drier conditions. The results suggest that decomposition may increase, and plants and animals may be exposed to more pathogens. In Chapter 3, I compared canopy and ground soils along two elevation gradients. I also assessed if these two soil types differed in how their properties, fungal communities, and extracellular enzyme activity (EEA) varied with elevation. I found that fungal communities differed between canopy and ground soils. Moreover, canopy soils had greater EEA than ground soils, suggesting that canopy soils have a greater decomposition potential and may contribute greatly to C cycling. Carbon dynamics in canopy soil may also be particularly responsive to climate change even more than we might predict based on observations from ground soil. Collectively, the results of my dissertation provide insights into how the structure and function of TMCF may be altered through changes in the fungal community.

While 80% of plant species are associated with mycorrhizal fungi, the impacts of ecological restoration on mycorrhizal fungi are rarely explored in scientific literature. Ecological restoration is an interdisciplinary endeavor, requiring partnerships with a diverse array of stakeholders. Increasing partnerships with restoration practitioners and mycologists will fill important knowledge gaps in our understanding of restoration efforts. Since mycorrhizal fungi provide a myriad of services to plant hosts, the efficacy of restoration efforts can improve with such partnerships. For my first chapter, I collaborated with the Newport Bay Conservancy to explore the effects of invasive species removal on mycorrhizal fungi. For my second and third chapters, I partnered with Save the Redwoods League to examine the response of mycorrhizal fungi to restorative thinning and road removal. Taken together, my dissertation chapters provide evidence that fungal communities play an important role in the success of restoration efforts and can inform management decisions.

In Chapter 1, I assessed the effect of full and selective removal of the invasive Brazilian pepper tree (Schinus terebinthifolius) on mycorrhizal fungi. To provide a trajectory for the restoration of the mycorrhizal community, I examined the effect of selective removal on the native Arroyo willow (Salix lasiolepis). An important distinction between the selective and full removal was that the full approach deployed a soil amendment. The results showed a disparity between full and selective removal, where the diversity of ectomycorrhizal fungi (EMF) declined significantly after selective removal. However, full removal led to a significant increase in the diversity of EMF. The results of our study point to the soil amendment as a significant factor that led to the increase of EMF.

For Chapter 2, I assessed the effects of logging and restorative thinning in second-growth redwood (Sequoia sempervirens) forests. Mycorrhizal fungi are important in these ecosystems, where redwoods use arbuscular mycorrhizal fungi (AMF) and EMF. However, following timber harvests, pine trees (Pinaceae) were aerially seeded. Since pine trees use EMF exclusively, the encroachment of pine woodland can restrict the use AMF by redwood forests. Our predictions were supported with the significantly lower abundance and diversity of AMF in second-growth forests. This trend was found in soils and a specialized root structure for AMF known as the rhizonode. When pine trees were removed, we expected the presence of AMF would change. However, the presence of AMF was still lower compared to old-growth. The results of our study provide a new trajectory for the restoration of second-growth forests, where restoration efforts should aim to establish a mycorrhizal community indicative of old-growth.

Chapter 3 explored the effect of road removal on mycorrhizal fungi and the broader fungal community. In this chapter, I continued the examination of restoration efforts in second-growth redwood forests. Since 2004, road removal has been an integral part of ecosystem restoration. I expected road removal efforts would lessen the diversity and abundance of mycorrhizal fungi. However, we found a recent road removal led to an increase in the abundance of EMF. In accordance with our predictions, pathogenic fungi that are detrimental to redwood health were lowest in old-growth forests and the oldest removal site. Moreover, saprotrophic fungi involved in decomposition were lowest in the road-center compared to the roadside. The results of this study show that restoration efforts should focus on restoring saprotrophic fungi in the road-center, which facilitate nutrient cycling and the recovery of redwood forests.

Understanding the response of soil microbial communities and decomposition to global environmental changes is central to our ability to accurately forecast future terrestrial carbon (C) storage and atmospheric CO₂ levels. Increases in the frequency and severity of disturbance events are one element of global change in terrestrial ecosystems. The goal of this dissertation was to measure the response of soil microbial communities and decomposition to disturbance events and to examine the mechanisms underlying post-disturbance changes in decomposition. In the first part of my dissertation work I explored these questions within the context of wildfires in boreal forests. Chapter 1 characterized soil microbial communities and the rate of decomposition across a fire chronosequence in interior Alaska. I found that boreal forest fires reduced soil microbial abundance, altered fungal community composition, and suppressed litter decomposition. Chapter 2 investigated whether soil microbial responses to boreal forest fires differ as a function of fire severity. I demonstrated that higher severity fires elicited greater reductions in soil microbial biomass and larger shifts in fungal community composition than lower severity fires. Chapter 3 tested the mechanisms through which boreal forest fires alter decomposition processes. I discovered that decomposition rates were slower in recently burned forests because of post-fire reductions in soil moisture and C substrate quality.

In the second part of my dissertation I expanded my findings to other types of disturbance events using meta-analysis. Chapter 4 reviewed the response of soil microbial biomass to fires. I found that soil microbial biomass was significantly lower in recently burned ecosystems, but the response of microbial biomass to fire differed by fire type and biome. Chapter 5 examined soil microbial responses to abiotic (fire, harvesting, storms) and biotic (insect infestation, pathogen outbreaks) disturbances in forests. I observed that abiotic disturbances significantly reduced soil microbial biomass, while changes in microbial biomass were non-significant following biotic disturbance events. Collectively, these findings suggest that reductions in soil microbial biomass and decomposition rates following abiotic disturbances are likely to slow the transfer of C from soils to the atmosphere and provide a negative feedback to rising atmospheric CO₂ concentrations and global change.