Quantifying the Effects of Climate Change on Mountain Pine Beetle Outbreaks and Subsequent Threats to Whitebark Pine

Whitebark pine is a high-elevation, important tree species that provides critical habitat for wildlife and supplies valued ecosystem services. These trees currently face multiple threats, including attack by the mountain pine beetle, which has recently killed whitebark pines over much of the western U.S. Climate is an important factor in these outbreaks, and future warming is expected to affect epidemics. Our project developed statistical models of outbreaks in whitebark pine for three regions: the Greater Yellowstone Ecosystem, the Northern U.S. Rocky Mountains, and the Cascade Range. We used these models to understand climate/beetle outbreak relationships, evaluate climatic causes of recent outbreaks, and estimate the potential for future outbreaks given projections of climate change. The models fit the observations well, indicating confidence in their reliability. Climate influenced mountain pine beetle outbreaks through fall and winter temperatures, which are direct effects on beetles, as well as via reduced summer precipitation which increases drought stress on trees. Recent outbreaks were caused by warming and drought in the early 2000s. We found that, compared with a baseline of 1985-1994 when little beetle activity occurred, future climate will be more favorable for mountain pine beetle outbreaks in whitebark pine. In the Greater Yellowstone Ecosystem, our preliminary results indicate that some projections were similar to or exceed the climate favorability of conditions during the recent severe and extensive outbreak (2000-2009). Variability existed among outbreak projections as a result of the amount of future warming and changes in precipitation, which are functions of climate model, emissions scenario, and decade. Our project provides evidence of the importance of climate for influencing recent and future beetle outbreaks in whitebark pine forests. We recommend that land managers and decision makers consider the impacts of expected climate change on mountain pine beetle outbreaks in whitebark pine when planning conservation actions.