Mechanical thinning restores ecological functions in a seasonally dry ponderosa pine forest in the inland Pacific Northwest, USA

An increasingly important goal of federal land managers in seasonally dry forests of the western US is restoring forest resilience. In this study, we quantified the degree to which a thinning treatment in a dry forest of eastern Oregon restored aspects of forest resilience by focusing on key functional attributes of our study system. First, we measured several physiological responses of overstory trees that are associated with resilience, including radial growth, resin production, abundance of non-structural carbohydrates (NSC), and leaf area. Second, we investigated understory vegetation responses including species diversity, composition, and cover by growth form that influence fire behavior and resilience to disturbance. We found that tree radial growth was greater in trees in thinned stands beginning three years post-treatment. The abundance of key chemical compounds found in trees, including resin, starch, total NSC and sucrose did not differ between treatments; however, abundance of glucose plus fructose was lower in treated stands, suggesting mobilization and use of carbon reserves for foliar and wood production following thinning. We observed an increase in species richness and diversity within thinned stands three to four years after thinning, primarily due to the release of forbs and reestablishment of graminoids following ground disturbance. Here, we demonstrate that elements of forest resilience can be restored in dry forest systems via selective thinning to promote historical forest structure. In forests where thinning reduces stand density, vigorous overstory trees and increased herbaceous cover can help facilitate the re-establishment of low intensity surface fire regimes that maintain stable and persistent vegetative states. Understanding the ecological effects of fuel reduction treatments allows land managers to assess potential forest resiliency and adapt future treatments based on the observed results of previous activities.