Long-term history and synchrony of mountain pine beetle outbreaks in lodgepole pine forests

Aim: A subcontinental-scale outbreak of mountain pine beetle (MPB; Dendroctonus ponderosae) has affected millions of hectares of forest in the western USA and Canada over the past 15 years. Little research has examined the long-term and broad-scale history of MPB outbreaks, which is necessary to provide a baseline for comparing current and future outbreak extents. We examine the long-term history of MPB outbreaks in western Colorado, and analyse the synchrony of previous outbreaks across western North America. Location: Western Colorado for tree-ring analysis; western North America for comparative analysis. Methods: We used dendroecological methods to reconstruct the history of MPB outbreaks since the 1700s in lodgepole pine (Pinus contorta) forests in Colorado. We then combined these new records with previously published data on MPB outbreaks to examine the historical synchrony of outbreaks across western North America. Results: The tree-ring record indicated that multiple cross-site MPB outbreaks occurred in Colorado lodgepole pine forests, initiating c. 1760s, 1780s, 1820s-1830s, 1860s, 1910s, 1960s and 1980s. Comparative analysis indicated that these outbreak dates coincide with documented and reconstructed MPB outbreaks in British Columbia, Colorado, Idaho, Montana, Wyoming and Utah. Main conclusions: Several subcontinental-scale MPB outbreaks have occurred in lodgepole pine forests over the past three centuries. Although the current study does not address the intensity of past outbreaks, it does indicate that previous outbreaks appear to have been highly synchronous across western North America. The subcontinental, rather than purely local, nature of these outbreaks is important for putting recent and future outbreaks of MPB into a broader ecological context. This study presents the longest tree-ring reconstruction of MPB outbreaks in the southern Rocky Mountains available to date, as well as the first subcontinental long-term comparative analysis of MPB outbreaks in western North America.