Climate-driven mountain pine beetle outbreaks in British Columbia, Canada

Mountain pine beetle (Dendroctonus ponderosae) is one of the most destructive biotic agents in western Canadian forests. A large mountain pine beetle (MPB) outbreak has occurred in British Columbia during the last decade – 2 to 7x10⁶ ha affected annually -. MPB outbreaks are caused by a combination of 1) a succession of years with favorable climatic conditions and 2) large areas of susceptible mature pines. Area affected by MPB outbreaks in British Columbia from 1959 to 2004 was found to be positively correlated with the Pacific Decadal Oscillation (PDO) at interdecadal timescales of 7-10 years. The onset of the outbreak appears to be correlated with the PDO shift in 1976/77. Higher winter temperature minima, summer temperatures >18ºC, and spring drought during positive PDO phases favor MPB populations survival and growth. Also, winter temperature minima have shown a significant positive trend during the 20^th century. In addition, large areas in British Columbia are in the susceptible age classes because of large fires at the end of the 1800s and early 1900s: wildfire occurrence in western Canada is also associated with the PDO. Results suggest that MPB outbreaks are controlled by large scale climate dynamics (i.e. PDO) that control 1) the frequency of occurrence of climate events related to beetle population processes and 2) the landscape age mosaic (and hence the availability of suitable host trees). The recent unprecedented major outbreak of the last decade has taken place due to the combination of a positive phase of the PDO since 1976/77 and a long-term positive trend in minimum winter temperatures related to global warming.