Hydrological Response to Mountain Pine Beetle Infestation in Western Subalpine Watersheds

Water supply in western North America is controlled primarily by snow accumulation and melt in forested headwater basins. Trees impact runoff through wintertime canopy interception losses of snowfall and summertime transpiration losses. The mountain pine beetle (Dendroctonus ponderosae) epidemic attacking western pine forests will produce an estimated 90% mortality in lodgepole (Pinus contorta) stands, and will likely impact other tree species at significant levels over large areas of the US and Canada. Management studies suggest that changes in water quantity and quality will occur in response to beetle induced tree mortality. Hydrological responses in beetle killed forests are dependent on local climatology, forest age and species composition, understory response, and severity of infestation. Changes in discharge measured at the watershed level are typically quantified using statistical methods applied to time series data. Critical analysis elements are stationarity, and a sufficient data record for statistically significant detection of change. Short-term studies comparing statistical properties of flow often lack these critical elements and should be examined with caution. We show why short-term studies related to pine beetle impact on hydrology are unreliable. During the past five years, significant forest mortality has resulted from the current bark beetle epidemic, yet double mass plots using control basins and analysis of covariance (ANCOVA) show no significant response to date.