Douglas-fir Tree Mortality Caused by the Douglas-fir Beetle in Thinned and Unthinned Stands in Montana, USA

Abstract Bark beetles are casual agents of tree mortality, often affecting landscapes with diverse topography. The Douglas-fir beetle (Dendroctonus pseudotsugae Hopkins) (DFB) uses Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco), as its primary host. Populations often increase following disturbances such as defoliation, fires, or blowdowns and then transition into live healthy trees. Mitigating tree mortality where high mortality levels are incompatible with land manager objectives is of relevance. In this study, we compared mortality levels in eighteen thinned and twelve unthinned pure Douglas-fir stands in the Helena National Forest in Montana. Managed stands were thinned during the summers of 2014 and 2015 and tree mortality evaluated in 2017 and 2018. Douglas-fir beetle populations were at endemic levels in the study area, as represented by low observed tree mortality. Even under endemic populations, 3.6% of Douglas-firs > 12 in diameter at breast height (dbh) were killed in unthinned stands compared to 1.6% in thinned stands. Basal area of trees killed in unthinned stands was 5.4 (± 2.4) ft2/ac compared with 0.5 (± 0.3) ft2/ac in thinned stands. Although insect pressure was low, this study indicates that vegetation management could be a tool to mitigate DFB-caused tree mortality. Additional studies conducted under higher insect populations are needed. Study Implications: Douglas-fir beetle (DFB) can cause extensive mortality of Douglas-fir, particularly in dense stands of large-diameter trees with a high percentage of hosts. Vegetation management through silvicultural thinnings has been proposed as a long-term strategy to mitigate tree mortality from bark beetles. In this study, we compared tree mortality caused by DFB in thinned and unthinned Douglas-fir stands. Douglas-fir beetle populations were at low levels post-thinning and mortality levels were limited. Regardless, we observed increased mortality in denser, unthinned stands compared with thinned stands with lower tree density, suggesting that stand density reductions may mitigate DFB-caused tree mortality.