Subwatershed-Level Lodgepole Pine Attributes Associated with a Mountain Pine Beetle Outbreak

Abstract

Mountain pine beetle (Dendroctonus ponderosae Hopkins; MPB) is an aggressive bark beetle that attacks numerous Pinus spp. and causes extensive mortality in lodgepole pine (Pinus contorta Douglas ex Loudon; LPP) forests in the western United States and Canada. We used pre-outbreak LPP attributes, cumulative MPB attack severity, and areal extent of mortality data to identify subwatershed-scale forest attributes associated with severe MPB-caused tree mortality that occurred across the Northern Rockies, USA from 1999–2014. We upscaled stand-level data to the subwatershed scale to allow identification of large LPP areas vulnerable to MPB. The highest mortality occurred in subwatersheds where LPP mean basal area was greater than 11.5 m2 ha−1 and LPP quadratic mean diameter was greater than or equal to 18 cm. A coarse assessment of federally-owned LPP-dominated forestland in the analysis area indicated about 42% could potentially be silviculturally treated. Silvicultural management may be a suitable option for many LPP forests, and our hazard model can be used to identify subwatersheds with LPP attributes associated with high susceptibility to MPB across landscape spatial scales. Identifying highly susceptible subwatersheds can help prioritize general areas for potential treatments, especially where spatially extensive areas of contiguous, highly susceptible LPP occur.

Does the legacy of historical thinning treatments foster resilience to bark beetle outbreaks in subalpine forests?

“…2003, Williams et al. 2018, Negrón 2019), with mean diameters of susceptible lodgepole pine trees smaller than 20 cm and no trees exceeding 40 cm diameter (Appendix : Table S3).…”

“…In addition to treatment timing relative to outbreaks, our finding of a stand-scale effect only in treatments that approximated a clearcut suggests a threshold of thinning intensity needed to foster MPB outbreak resistance. Preoutbreak, the heaviest historical treatments in our study were the only stands below suggested thresholds for MPB outbreak resistance (Mata et al 2003, Williams et al 2018, Negr ón 2019, with mean diameters of susceptible lodgepole pine trees smaller than 20 cm and no trees exceeding 40 cm diameter (Appendix S1: Table S3).…”

Climate‐induced outbreaks in high‐elevation pines are driven primarily by immigration of bark beetles from historical hosts

“…Numbers represent order of relative importance and are provided to facilitate comparisons across models. Performance metrics for each model are in Table S1 [Colour figure can be viewed at wileyonlinelibrary.com] where epidemic populations preferred vigorous trees, successively removing the largest diameter trees until few suitable hosts remained (Safranyik & Carroll, 2006;Safranyik et al, 1974;Waring & Pitman, 1985;Williams et al, 2018), and at the landscape scale, where epidemic populations preferred older stands and colonized younger, less optimal stands when few unattacked stands remained (Maclauchlan et al, 2015). Finally, our results suggest that the density-dependent dynamics of short-and long-range dispersal modulated expansion into whitebark pine (Logan et al, 1998;Safranyik et al, 1992).…”

“…Thus, irruptive beetle species are usually limited to relatively rare, severely stressed trees during lengthy low‐density periods in which their populations remain relatively stable (Bleiker et al, 2014; Safranyik & Carroll, 2006). However, when populations become high, mass attacks by beetles, a positive density‐dependent change in host selection behavior, can overcome the defenses of nearly all trees, which greatly increases the quantity and quality of available hosts (Boone et al, 2011; Waring & Pitman, 1985; Williams et al, 2018). Mass attacks are made possible by aggregation pheromones that allow rapid recruitment of enough beetles to overcome otherwise efficacious tree defenses (Blomquist et al, 2010).…”