A Classification of Recent Widespread Tree Mortality in the Western US

Abstract:

Widespread tree mortality has been documented across the western United States in recent decades. Climate change has been implicated in these events, in particular warming and associated effects on tree stress and biotic disturbance agents. Given projected future warming, the capability of accurately predicting future tree mortality is critical. However, sufficient ecological understanding is needed to do so. Here we describe differences in various mortality types associated with spatial characteristics and climate drivers. We loosely classify mortality types into four categories: 1) widespread but low severity background mortality that has been increasing mainly because of greater stress associated with rising climatic water deficit; 2) tree die-offs that are driven by severe, hotter drought in which biotic agents play minor roles, such as sudden aspen decline; 3) tree die-offs in which hotter droughts combined with outbreaks of biotic agents, often less aggressive bark beetles, to cause mortality, such as piñon pine mortality in the Southwest; and 4) tree die-offs that were initiated or facilitated by droughts but which were associated with aggressive biotic agents that can kill healthy trees at high populations, such as mountain pine beetle outbreaks. An important use of this classification is the different pathways by which climate change can cause tree mortality. For some classes (background and primarily drought-driven mortality), predictions may be sufficiently accurate based on climate (drought) metrics. For classes in which biotic agents play a role, the direct warming effect on insects may occur through mechanisms not related to drought, and therefore predictions may need to include mechanisms other than drought. We note that this is a simplistic classification designed to facilitate understanding of tree mortality, and that overlap occurs among categories.