How does water yield respond to mountain pine beetle infestation in a semiarid forest?

Abstract. Mountain pine beetle (MPB) outbreaks in western United States result in widespread tree mortality, transforming forest structure within watersheds. While there is evidence that these changes can alter the timing and quantity of streamflow, there is substantial variation in both the magnitude and direction of responses and the climatic and environmental mechanisms driving this variation are not well understood. Herein, we coupled an eco-hydrologic model (RHESSys) with a beetle effects model and applied it to a semiarid watershed, Trail Creek, in the Bigwood River basin in central Idaho to evaluate how varying degrees of beetle-caused tree mortality influence water yield. Simulation results show that water yield during the first 15 years after beetle outbreak is controlled by interactions among interannual climate variability, the extent of vegetation mortality, and long-term aridity. During wet years, water yield after beetle outbreak increases with greater tree mortality. During dry years, water yield decreases at low to medium mortality but increases at high mortality. The mortality threshold for the direction of change is location-specific. The change in water yield also varies spatially along aridity gradients during dry years. In relatively wetter areas of the Trail Creek basin, water yield switches from a decrease to an increase when vegetation mortality is greater than 40 percent. In more water-limited areas on the other hand, water yield typically decreases after beetle outbreaks, regardless of mortality level. Results suggest that long-term aridity can be a useful indicator for the direction of water yield changes after disturbance.