Forest disturbance is expanding in rate and extent and is affecting many montane catchments critical to water resources. Western North America is experiencing a recent bark beetle epidemic that has killed forests over more than 500,000 km2. Potential impacts include altered amount, timing and quality of water resources deriving from beetle-impacted catchments. While forest disturbance by fire and harvest is well studied, insect-driven mortality has different temporal and biophysical characteristics. In this work, I quantified hydrologic and hydrochemical response to beetle infestation in catchments dominated by lodgepole pine in the central Rocky Mountains. Observations were organized laterally in a nested fashion from soil observations to nested headwater catchments. Vertical observations encompassed what is often termed the critical zone, from atmospheric interactions at the top of the forest, across the ground surface, and through the rooting zone to the groundwater interface.
I quantified responses of snowpack, water partitioning between vapor flux and streamflow, and hydrochemical patterns across the landscape to beetle-driven forest mortality. Key findings of my study include 1) In contrast to expected snowpack increases, loss of shelter from the atmosphere caused compensatory sublimation of snowpack to offset decreased interception losses; 2) Despite major loss of overstory transpiration, abiotic evaporation and total vaporization (ET) increased, reducing water available for streamflow; 3) Nitrogen concentrations were elevated in hillslope groundwater, raising new concerns about the duration and extent of contamination; 4) The riparian zone protected streams from major N influx to a greater degree than reported for other forest disturbances; and 5) Headwater streams rapidly attenuated biogeochemical inputs, consistent with results from other forest disturbances. Collectively these results demonstrate coupling between the hydrologic and biogeochemical changes. The emerging picture is that process responses often compensate for one another, contributing to resilience in affected catchments.