A socio-ecological framework for understanding mountain pine beetle outbreaks in North America

Background/Question/Methods

The role of socio-ecological drivers on recent large-scale tree mortality in coniferous forests caused by beetle outbreaks in western North America remains unclear. Climate change and forest governance have played different roles, and need further investigation within a socio-ecological framework. Climate directly affects beetle behavior and indirectly impacts beetles by affecting their hosts, parasites and predators. Fire suppression over the last century has resulted in an increase in trees susceptible to beetle development, thereby generating concerns about ecological effects of policy on forest landscapes. However, there is limited understanding of the relative importance of climate change and forest governance in affecting beetle outbreaks across different spatiotemporal scales. In this study, we applied backcasting and Box-Jenkins methods to reconstruct the spatial pattern of the mountain pine beetle (Dendroctonus ponderosae, MPB) in the western US and in the past century, using bioclimatic and social variables identified in the literature that are crucial for beetle development - including beetle behavior, tree growth, and fire occurrence. We also compared spatial patterns of MPB outbreaks with fire regime and vegetation departure data from LANDFIRE and fire occurrence from tree-ring and fire atlas data to determine the impact of fire exclusion at a local scale.

Results/Conclusions

We have inferred the historical beetle range and evaluated the significance of different variables selected using mainly logistic regression models. Preliminary results support the hypothesis that long-term climatic changes have had a stronger impact on MPB outbreaks at a regional scale, whereas long-term fire suppression has changed landscape and fire regimes and further influenced beetle activity at a local scale. Fire suppression is more effective in the regions where forests have adapted to frequent and low-intensity fires. Forest stands with fire-return intervals greater than a century and that adapt to replacement severity fires are less subject to the impact of fire exclusion, and climate remains the primary driving force in beetle outbreaks. In contrast, climate change has loosened the winter temperature constraints and provided suitable heat conditions for MPB outbreaks at a larger scale, which further facilitated the recent unprecedented outbreak. The paper will draw upon our ongoing data analysis for updated results on the species distribution modeling of MPB and the modeling uncertainty.