Assessing the widescale efficacy of vegetation management to mitigate bark beetle-caused tree mortality in the western United States

Forests in the western U.S. have experienced widespread mortality due to recent bark beetle outbreaks. While vegetation management, primarily basal density reduction, has mitigated bark beetle-caused tree mortality in small stands, larger-scale analyses of treatment efficacy remain lacking. This study aims to determine if thinning treatments reduce bark beetle-caused mortality at larger spatial scales, especially in that of forests and geographic regions. We focused on two of the most prevalent and productive conifer species in western forests: ponderosa pine (Pinus ponderosa) and douglas-fir (Pseudotsuga menziesii) which are chiefly attacked by mountain pine beetle (Dendroctonus ponderosae) and douglas fir beetle (Dendroctonus pseudotsugae), respectively. The study was conducted within national forests in eight states (AZ, CO, ID, MT, NM, SD, UT, WY) in the western U.S. Within each forest, four sites were selected in areas that were exposed to bark beetle pressure, free of fire damage, and treated within the last 15 years. At each treated and corresponding untreated site, tree mortality and growth data were measured in seven sample plots. A total of 96 pairs of sites within 24 forests were sampled during 2021-2022. We found that in drier areas of the southwestern U.S. there were minimal differences between treated and untreated sites, while in the remainder of sampled western states treatments reduced focal tree mortality. Our results show that for the majority of the western U.S. forest thinning treatments reduce bark beetle-caused tree mortality, although additional environmental factors in the southwest may reduce the efficacy of such treatments.