Summary/Abstract
Recent increases in Douglas-fir (Psuedotsuga menziesii var. menziesii) mortality in the Klamath Mountains ecoregion of Oregon raise concerns about the long-term resilience of Douglas-fir, and increased potential for uncharacteristic wildfire. This peer-reviewed study uses data from the USDA Forest Service Aerial Detection Survey and ninety-six field plots to explore the relationships between regional characteristics, climate variables, and Douglas-fir mortality. The study’s 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 beetles, fungi, and other 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.
The authors then develop 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. The authors predict that these “decline spirals” will become more common in an increasing number of sites as hotter climate-driven droughts continue and intensify.