Climate, weather, and recent mountain pine beetle outbreaks in the western United States

Abstract

Recent outbreaks of mountain pine beetle (Dendroctonus ponderosae) have impacted large areas of western North America. Climate and weather conditions influence beetle population dynamics, and managers and policymakers are concerned about the potential effects of climate change on outbreaks. Here we studied five locations with extensive outbreaks in lodgepole pine (Pinus contorta) forests across the western United States. Using observations and modeling, we quantified means and changes relative to prior years of three climate or weather factors associated with outbreaks: (1) year-round temperatures that affect adaptive seasonality; (2) low temperatures that induce mortality of overwintering beetles; and (3) drought stress of host trees. Climate variable means varied among locations, indicating the beetle’s tolerance to different climate during outbreaks. Analyses of climate or weather factors as outbreaks progressed revealed that year-round temperatures during outbreaks were typically higher than in prior years, and outbreak years lacked very low winter temperatures that often occurred in prior years. Drought was present at each location during some time of an outbreak, and increases in beetle-caused tree mortality at lower beetle population levels (as indicated by killed trees) were usually coincident with drought. Furthermore, drought was not required to maintain large outbreaks; in several locations, relief from drought during periods of high tree mortality did not cause subsequent declines in tree mortality. We did not find strong evidence that maladaptive seasonality, cold-induced mortality, or drought stress was responsible for decreases in tree mortality, suggesting the role of host depletion. Large variability in the relationships between climate or weather variables and outbreaks suggests that different climate and weather factors may have been limiting outbreaks at different times and that these factors did not influence beetle-caused tree mortality similarly among locations. Our results increase understanding of the climate and weather factors that influence beetle outbreaks and their variability in space and time and will lead to more accurate predictions of future patterns of outbreaks that consider future climate.