Impacts of ponderosa pine forest restoration treatments on the ectomycorrhizal fungal community and fine root biomass in the Blue Mountains of Oregon

Before the arrival of Euro-Americans, the inland Pacific Northwest was settled by native people whose frequent intentional burning of the landscape promoted open stands dominated by large fire-resistant ponderosa pine. Fire suppression for nearly a century, livestock grazing, and logging of the largest trees has resulted in forests characterized by increased densities of small trees with closed canopies and unusually high fuel loads. Such structural changes to the forest ecosystem are particularly evident in the Blue Mountains of Oregon and Washington where forest managers are applying prescribed fire and thinning to reduce the risk of stand-replacing wildfire and reestablish stand structure characteristics seen prior to Euro-American settlement. Aboveground ecosystem recovery after disturbance is directly linked to the survival of ectomycorrhizal fungi (EMF) that form an obligate symbiosis with roots of tree species in the Pinaceae. EMF are critical for the uptake of nutrients by the tree host. The research comprising this dissertation explores the response of the EMF community structure and composition, live fine root biomass, and duff (decaying material) levels to (i) seasonal burning (fall vs. spring) and (ii) alternative fuel reducing restoration treatments (thinned only, prescribed burned only, thinned and prescribed burned). In both studies, treatments significantly reducing duff depth (e.g. fall burning and both restoration burning treatments) negatively impacted EMF species richness and live fine root biomass. The EMF community is characterized by a large number of species scattered at low frequencies across the sites. The frequent occurrence of a few species (e.g. Rhizopogon salebrosus and Wilcoxina rehmii), in both studies before and after treatment applications, demonstrates that some EMF species survive or rapidly reestablish after disturbance. The initial reduction of EMF species richness, fine root biomass, and duff levels after prescribed fire has important implications for whether managers can achieve the desired future condition of stands with large-tree retention and low fuel loads. The impacts of prescribed fire on the soil microbial community, along with the recovery potential of a site and the impending risk of stand-replacing wildfire in stands differing in structure from historic conditions, bear consideration when developing restoration prescriptions.