The study site in the Wallowa-Whitman National Forest, Oregon, that was thinned in 1998 and then burned in 2000. Photo by Jane E. Smith.
Results from a 15-year study in the Blue Mountains of eastern Oregon demonstrate the resiliency of these forests to disturbances associated with restoration treatments, providing managers with increased flexibility.
To increase ecosystem resilience, and achieve the desired future condition of stands with large-tree retention and low fuel loads, federal agencies have implemented fuel reduction and forest restoration projects in low-elevation, dry conifer forests throughout the Western United States. Questions existed about the effects of these disturbances on critical belowground mycorrhizal fungi. These beneficial soil fungi colonize the roots of trees and shrubs and bring them key nutrients in exchange for the carbon produced by the plants during photosynthesis.
To learn more, scientists conducted a study on a site that is also part of the nationwide Fire and Fire Surrogate study. They assessed mycorrhizal fungi in the dry-climate region of eastern Oregon where fuels reduction treatments were implemented more than 15 years prior. Specifically, they wanted to know if mechanical thinning, prescribed burning, or a combined effect of the two treatments, as compared to untreated forest stands, had a transient or long-term effect on soil biochemistry and mycorrhizal fungi associated with ponderosa pine.
They found that differences in soil nutrients among treatments 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. After more than a decade of recovery, mycorrhizal fungi returned to levels similar to those in the untreated controls, and litter depth across treatments stabilized in these forests.
These findings will be used to address public comments to forest plans on the Wallowa-Whitman National Forest and other forests in the Blue Mountains with fire-suppressed low-elevation stands of ponderosa pine. Increased understanding of biochemical processes in forest restoration treatments will assist in the development of best management practices aimed at lessening the threat of stand-replacing wildfire.