By Eric Walberg and Si Balch
The extensive mountain pine beetle (MPB) outbreak in the western U.S. and Canada is a fascinating and instructive case study of the role of climate change in expanding the range and destructive potential of forest pests. The unprecedented forest damage associated with this outbreak is the result of the combined effects of forest conditions, MPB outbreak cycle and warming climate. The worst of the outbreak occurred in British Columbia (BC), starting in 2001 and peaking in 2007. While the outbreak is ongoing, the severity is tapering off as the MPB reaches the end of this outbreak cycle. The epidemic is estimated to have killed approximately 279,000 cords of commercially valuable pine in BC, over 50% of the resource.1
Forest Conditions: Fire suppression efforts in BC prior to the start of the epidemic resulted in a high density of mature pine in the interior of the Province. MPB preferred tree species are ponderosa, lodgepole, western white and limber pines. Prior to the implementation of fire suppression efforts, 157,000 cords of mature pine were estimated in this region. As a result of fire suppression efforts the volume of pine skyrocketed to approximately 526,000 cords prior to 2000.1 The high density of mature pine provided an ideal environment for the massive MPB outbreak to begin.
Climate Change: Climate change is a major factor in the geographic range and severity of the outbreak (Figure 1). Winter weather in BC has historically included periods that were consistently cold enough to limit MBP activity. Due to a combination of warming climate and regional weather patterns BC has been without a killing frost since 1996.1 A week or more of temperatures at or below -31 F are required to kill overwintering MBP.
Modeled Projections of MPB Range Expansion and Implications for Other Pests: In 2001 Jesse Logan and James Powell modeled the life cycle of the MBP and theorized that climate change driven expansion in MPB range could threaten previously unaffected jack pine populations.2 The projected range expansion was verified by in 2011 by Cathreine Cullingham, et al. Their research found that the projected range expansion is occurring and that a hybrid zone of lodgepole and jack pine in Alberta is under attack at the leading edge of the expansion.3
Figure 1: Change in mean temperature by season, 1950 – 20074
The work of Logan and Powell provides guidance for evaluating the likelihood of climate-enabled range expansion of pests similar to the MPB. They differentiate between pests in which diapause (a period of delayed development in response to environmental conditions) inhibits response to warming temperatures and those pests such as MPB that are directly responsive to warming temperatures. They theorize that pests not inhibited by diapause will be more adaptable to a warming climate. Southern pine beetle is similar to MPB in this respect and in fact the range of the southern pine beetle is moving north as climate warms.
Given the complexities of forest/pest interactions it will be necessary to take a multipronged approach to identifying and responding to changing threats. Modeling such as that carried out by Logan and Powell can provide valuable insight on possible range expansion. Once climate-related pest thresholds are identified, enhanced monitoring can be deployed to provide early warning and maximize management response time. Organizing and delivering information on evolving climate-related threats by geographic region is one of the primary goals of the Climate Smart Land Network.
1. A History of the Battle Against the Mountain Pine Beetle, 2000 to 2012. at <http://www.for.gov.bc.ca/hfp/mountain_pine_beetle/Pine%20Beetle%20Response%20Brief%20History%20May%2023%202012.pdf>
2. Logan, J. & Powell, J. Ghost Forests, Global Warming, and the Mountain Pine Beetle (Coleoptera: Scolytidae). Am. Entomol. 47, 160–173 (2001).
3. Cullingham, C. I. et al. Mountain pine beetle host-range expansion threatens the boreal forest. Mol. Ecol. 20, 2157–2171 (2011).
4. Zhang, X. et al. Canadian climate trends, 1950-2007. (Canadian Councils of Resource Ministers, 2011). at <http://www.biodivcanada.ca/default.asp?lang=En&n=137E1147-0>