Influence of climate and weather on observed spatiotemporal patterns of mountain pine beetle outbreaks in Washington and Oregon

Background/Question/Methods

The growing impacts from mountain pine beetles in conifer forests of North America have drawn considerable attention from scientists, forest managers, and the public, especially where outbreaks have threatened human and economic values.  There is strong evidence that climate change has contributed to the extent and severity of recent outbreaks, and growing realization about the relationship between bark beetle population dynamics and trends in climate. Although process models that capture climate influences on mountain pine beetle physiology and development have advanced our understanding of beetle populations, there are few studies that have assessed their influence across multiple outbreaks or at larger spatial scales or quantified the changes in damage level due to climate. In this study, we used observations of mountain pine beetle outbreaks in Oregon and Washington State over the past three decades to examine whether climate variables could explain outbreak activity over longer time periods and larger scales than previously studied.

Results/Conclusions

The results showed that new mountain pine beetle attacks are more likely to occur at locations with climatological mean August temperatures greater than 15^oC. Once an outbreak was established in a location, the probability of an outbreak was most affected by the beetle pressure in the region in the previous year.  After controlling for beetle pressure, the annual weather variables with the largest effect on outbreaks were minimum winter temperature and drought condition 0-4 years prior. Among the process-based climatological variables, cold tolerance was the strongest indicator of an attack becoming epidemic size. However, a combination of the simple temperature and precipitation variables through a regression model had a stronger influence on the probabilities of outbreaks than process-model variables.  Overall, the results confirm the importance of cold winters and drought on the regulation of bark beetle epidemics. The regression models developed were found useful for predicting expected amounts of damage due to mountain pine beetle outbreaks and for quantifying the contribution of climate change to the total damage.