Northern range limit mountain pine beetle outbreak dynamics in mixed sub-boreal pine forests of British Columbia

Recent widespread mountain pine beetle (Dendroctonus ponderosae) (MPB) outbreaks in western North America have caused unprecedented tree mortality across much of the beetle's known historic range and into novel habitats near the range margins. Our current understanding of outbreak dynamics and their relationship to forest structure is based almost wholly on research from climatically optimal habitats where infestations have historically been most severe. As range expansion continues marginal habitats may become increasingly susceptible to outbreaks, yet little is known about the factors driving outbreak development in these forest types or how the disturbance regime differs. This study uses dendroecology and forest structure analysis to reconstruct historic outbreak dynamics at the northern MPB distribution limit in British Columbia to extend the disturbance record beyond the scope of documented history and to evaluate the long-term relationships between stand structure, host suitability, and regime characteristics at the range margin. We sampled 22 sites with evidence of past infestations, and using mortality dates, survivor growth releases, and tree vigor analyses we reconstructed five probable outbreak periods predating the current epidemic. At least two of these events fit the profile of large epidemics occurring in the 1900s-1910s and again in the 1940s-1950s, with evidence of two 19th century infestations and chronic localized mortality beginning around the 1980s. Stand age appears to be the most important factor limiting outbreaks prior to 1870, indicating a strong relationship with fire history at centennial time scales. Outbreak return intervals were comparable to those reported in other locations, yet beetle preference for larger and older trees suggests that provincial susceptibility models may not accurately represent northern outbreak risk, particularly in older stands. Following stand maturation, host availability does not appear to limit outbreak severity or recurrence. Here density-independent factors associated with additional climate pressures are likely more important. With enhanced climate suitability, outbreak scale and severity should increase dramatically in this landscape which is currently not limited by host distribution. Our reconstruction may inform management strategies as this occurs and outbreaks move into previously uninfested forests that share many structural and ecological similarities with our sites.