Climate and Mountain Pine Beetle-Induced Tree Mortality in the Selway-Bitterroot Wilderness Area

Kipfmueller, Kurt; Swetnam, Thomas; Morgan, Penelope

Dendrochronological methods are used to date past beetle-induced whitebark pine mortality in three watersheds of the Selway-Bitterroot Wilderness Area, Idaho and Montana. Death rates were determined for 84 of 110 (76%) sampled whitebark pines. Whitebark pine mortality due to mountain pine beetle epidemics is clustered around a dew short time periods, with the largest peak centered around 1930, a period of widespread beetle-induced mortality documented elsewhere. Mortality events also occured in 1960s and 1980s, but are limited to only two watersheds. The cessation of the 1930s outbreak may be related to prolonged cold temperatures during the winter of 1930. Aerial survey data (1966-1999) for mountain pine beetles was obtained to develop regression models to predict beetle-induced tree mortality. Two region-wide mountain pine beetle outbreaks have occured since about 1966. The first outbreak began gradually around the late 1970s and peaked during the mid 1980s. A second epidemic started around 1994 but has not peaked. Pearson correlations between the beetle mortality (total area killed) and climate relates to July precipitation (P≤0.001). Variance explained in four species-specific regression models in between 22-24%. A fifth model that incorporates past beetle mortality explains 75% of the variance. Whitebark pine mortality due to mountain pine beetles may be relatively infrequent occuring only during periods when beetle populations are at epidemic proportions at lower elevations, or only when drought is prolonged. Rising global temeratures may increase the risk of whitebark pine habitats to more severe or prolonged mountain pine beetle epidemics by creating an environment more conducive to the life history of the beetle.