Recent Douglas-fir Mortality in the Klamath Mountains Ecoregion of Oregon: Evidence for a Decline Spiral

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.