Fuel Treatments of Ponderosa Pine (Pinus ponderosa) in the Blue Mountains of Eastern Oregon: A Mycorrhiza Perspective

Graduation date:2017Over the course of the last century, a successful history of fire suppression has contributed to unsuccessful present day control over wildfire. In the absence of fire and the janitorial and ecological services it provides, drier inland forests are shifting in species composition and exceeding densities that cannot survive and persist in current fire patterns occurring on the landscape. In the past two decades, managers have introduced restoration use of fire and thinning for their ecological benefits and to convert fuel-heavy forests to fuel-lean landscapes to lessen the threat of stand-replacing wildfire. In this study, we evaluated the long-term impact of forest restoration practices on soil biochemistry and the mycorrhizal fungi associated with ponderosa pine (Pinus ponderosa). Study sites were located in the Blue Mountains of northeastern Oregon where treatments implemented in 1998 and 2000 included mechanical thinning of forested areas, prescribed fire, a combination of thinning followed by fire, and an untreated control. Soil sampling for this study occurred in 2014 and included four replications of each treatment for a total of 16 experimental units. Fungal-specific sequence analysis of morphotyped root-tips from ponderosa pine was used to assess the species diversity and root-tip density abundance of each fungal taxon within each sample unit. Additionally, litter depth, pH, and soil carbon (C), nitrogen (N), and Bray phosphorus (P) were measured from soil cores that were stratified into depths of 0-5cm and 5-10cm. Bray-P, pH, and percent C differed among treatments. Bray-P and percent C and N differed between soil depths. The results indicated that given a decade-plus period of recovery, mycorrhizal fungi in dry inland forests dominated by ponderosa pine returned to levels similar to the untreated controls. Similar litter depths across treatments suggest that litter depth stabilizes over time in these forests. Soil C and nutrient differences may have been driven by the thinning treatments and the resultant deposition of residual slash following harvesting or the consumption of slash by prescribed fire. Elevated ectomycorrhiza biomass in the thinning and burning treatment may be a response of the host trees that creates a larger nutrient acquisition network in a less fertile environment. The results of this study demonstrate the resiliency of these forests to disturbances associated with restoration treatments, providing managers increased flexibility of options if maintaining abundant and persistent fungal communities is a concern. Given that the mean fire return interval for these forests is 15 years, a second reintroduction of prescribed fire may be timely