Coupled Hydrologic and Biogeochemical Response to Insect-Induced Forest Disturbance

Abstract

Forest disturbance is expanding in rate and extent and is affecting many montane catchments critical to water resources. Western North America is experiencing an epidemic of mountain pine beetle (MPB) that has affected 20 million hectares of forest in Canada and the United states. This epidemic may have long-lasting consequences for coupled cycles of water, energy, and biogeochemicals. While impacts of forest disturbance by fire and harvest have been studied for more than a half-century, insect-driven mortality differs from these events in the timing and accompanying biophysical impacts. In this work, we quantified catchment hydrologic and hydrochemical response to severe MPB infestation in a lodgepole pine ecosystem. 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 through the ground surface and the rooting zone to the interface with groundwater. We quantified responses manifest in snowpack, the primary hydrologic input to this montane ecosystem, in water partitioning between vapor flux and streamflow, and in biogeochemical patterns across the landscape. Key findings of this study include 1) Loss of shelter from the atmosphere caused compensatory sublimation of snowpack to offset decreased interception losses after MPB-driven canopy loss; 2) Vaporization at multiple scales increased over time and in comparison to control forest, reducing water available for streamflow; 3) Nitrogen (N) concentrations were elevated in hillslope groundwater, but attenuation in the riparian zone protected streams from major N influx; and 4) headwater streams rapidly attenuated dissolved carbon (C) and N inputs. Collectively these results demonstrate compensatory negative feedbacks which help explain the lack of strong response to streamflow and stream chemistry observed in the recent MPB epidemic.