Bark Beetle Outbreaks in Lodgepole Pine

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. Past research suggests that when climate conditions are favorable, forest age is the most important factor influencing host susceptibility and outbreak severity. However, stand age is a difficult forest attribute to measure and not available over large spatial extents compared to other host tree characteristics such as tree size and density. We used available pre-outbreak LPP characteristics and mountain pine beetle attack severity data to test which host characteristics were most influential on attack severity during the 2000s MPB outbreak in the Northern Rockies region at the subwatershed scale. Classification analysis of the simplest model indicated that the highest severity attack level was most likely to occur in subwatersheds where mean basal area was greater than 50 11.5 m2/ha and quadratic mean diameter was greater than or equal to 18 cm. Prior to the outbreak, 28% of subwatersheds contained LPP is this high susceptibility state (as defined by model results), and 51% of the susceptible subwatersheds were severely impacted by MPB-caused mortality in at least 10% of the LPP forests in that subwatershed. The results from this study indicate that during the 2000s Northern Rockies MPB outbreak the most important stand characteristics for describing attack severity in lodgepole pine were basal area and quadratic mean diameter. Other studies suggest that the extent of available host trees may have been facilitated by forest management history and fire exclusion. Research that tests the validity of this assumption could provide valuable information to guide management practices and mitigate LPP host characteristics that foster large-scale, severe MPB outbreaks.