Climate change, mountain pine beetles, and whitebark pine forests of the Greater Yellowstone Ecosystem

Background/Question/Methods

Whitebark pine (Pinus albicaulis) is an important, high-elevation tree species that provides critical habitat for wildlife and supplies valued ecosystem services.  These trees currently face multiple threats, including attack by mountain pine beetles. They consequently have been listed as warranted for protection under the Endangered Species Act, but precluded due to funding constraints.  Historically, mountain pine beetle outbreaks were rare in whitebark pine forests.  However, a widespread outbreak recently occurred in the Greater Yellowstone Ecosystem.  Our goals were to increase the understanding of the causes of this outbreak, and to estimate future outbreak potential given future climate change.  To accomplish this, we used an empirical approach to develop generalized additive models of the probability of tree mortality from mountain pine beetles, and then applied the best model to future climate projections.  We used observations from USDA Forest Service aerial surveys to determine the presence of whitebark pine mortality from mountain pine beetles.  Our explanatory variables represented processes affecting mountain pine beetle development, host tree susceptibility, the number of attacking beetles, and stand structure.  We applied the best model to future climate projections from multiple global climate models for three 30-year time periods and three emissions scenarios.

Results/Conclusions

In the Greater Yellowstone Ecosystem, area with mortality from mountain pine beetles increased from below-observable levels in the late 1990s to a peak of just over 3,500 km² in 2008, followed by a slight decline in 2009.  Our top model of mortality probability predicted the observed annual pattern of mortality well, indicating confidence in the interpretation and predictions of the model.  We found that summer precipitation, winter minimum temperature, and fall temperature were important variables for explaining the observed outbreak pattern.  The probability of whitebark pine mortality increased with increasing winter minimum temperature, increasing average fall temperature, and decreasing summer precipitation, in agreement with previously published studies.  The future climate projections showed trends of increasing winter and fall temperatures, with the greatest increases towards the end of this century and under the highest emissions scenario.  Anticipated future precipitation trends varied among and within models, and across emission scenarios and time periods.  Across all climate models, scenarios, and time periods, the average probability of whitebark pine mortality was greater in the future than 1950-2006.  Our model indicated future weather suitability similar to what occurred during the 2000s, suggesting the potential for severe future outbreaks, given there is sufficient whitebark pine present.