Bennett, Max, Shaw, David C., Lowrey, Laura. 2023. Recent Douglas-fir Mortality in the Klamath Mountains Ecoregion of Oregon: Evidence for a Decline Spiral. Journal of Forestry fvad007, 07 March 2023.

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Abstract: Recent increases in Douglas-fir (Psuedotsuga menziesii var. menziesii) mortality in the Klamath Mountains ecoregion raise concerns about the long-term resilience of Douglas-fir in the ecoregion and increased potential for uncharacteristic wildfire. We used data from the USDA Forest Service Aerial Detection Survey and ninety-six field plots to explore the relationships between physiographic and climate variables and Douglas-fir mortality. Our results provide strong evidence for a decline spiral in which Douglas-fir growing on hot, dry sites (predisposing factor) are further stressed by drought (inciting factor) and are then exploited by the flatheaded fir borer (Phaenops drummondi) and other secondary biotic agents (contributing factors), resulting in decline and mortality. At the landscape scale, Douglas-fir mortality increased as average annual precipitation declined and average climatic water deficit increased. We developed a risk score integrating several environmental variables associated with drought and heat stress to predict the likelihood and intensity of mortality at the stand scale.

Study Implications: Douglas-fir mortality in the Klamath Mountains ecoregion commonly occurs during and following drought on hot, dry sites that are already climatically marginal for the species. Landowners and managers can use climatic water deficit to identify high mortality risk sites at the landscape scale and our risk score integrating topographic and site factors for risk assessment at the stand scale. Steering management toward oak-pine restoration may be warranted in high risk locations. Projections of future climatic water deficit suggest that the area of marginal, high risk habitat for Douglas-fir will increase substantially by 2055

Bennett, Max and Chris Adlam, OSU Extension Service. 2023. Trees on the Edge: Understanding Douglas-fir Decline and Mortality in Southwestern Oregon. OSU Extension Service Manual 9406. Douglas-fir is the dominant tree species in many low- to mid-elevation forests across southwest Oregon. Douglas-fir trees provide vital wildlife habitat, are an important source of timber, and capture and store carbon. However, a rising number of Douglas-firs in southwest Oregon are dying. People want to know: Can this dieback be reduced or prevented? What are the long-term prospects for Douglas-fir in this region? How do we manage the dead trees? What about the risk of wildfires? This publication answers frequently asked questions about why and where Douglas-firs are dying and what actions we can take.

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