HEADWATER CATCHMENTS RESPOND TO INSECT-INDUCED FOREST MORTALITY WITH REDUCED STREAMFLOW AND MULTI-SCALE ATTENUATION OF CARBON AND NITROGEN

Forested montane catchments are critical to downstream water resources and the regulation of biogeochemical fluxes. In the Intermountain West, increased tree stress resulting from longer growing seasons and higher vapor pressure deficits has contributed to unprecedented forest die-off from mountain pine beetle (MPB) infestation. Reduced snow interception and transpiration are expected to increase streamflow, while increased organic matter decay is expected to increase biogeochemical stream fluxes. Tree-scale observations have documented the expected changes, but there has been little significant change to streamflow or water quality at the larger scales relevant to water resources. A critical gap exists in our understanding of why tree-scale process changes have not led to the expected, large-scale increases in streamflow and biogeochemical fluxes.

We address this knowledge gap with observations of water and biogeochemical fluxes at nested spatial scales including tree, hillslope, and catchments from 3 to 700 ha with >75% mortality. Catchment discharge and eddy covariance fluxes showed reduced streamflow and increased vapor losses following MPB. Stable isotope fractionation in snowpack, soil water and streams indicated greater abiotic evaporation offsetting decreased interception and transpiration. Soil water DOC and DON were similar beneath killed and healthy trees, but concentrations were elevated 2-10 times in groundwater. Stream DON in small catchments (3-10 ha) was lower than in groundwater, while DOC was similar to groundwater. Both DOC and DON showed further reduction of ~50% within 5 km downstream in a 700-ha catchment with similar MPB forest mortality. Soil water NO³ up to 500 µeq l^-1 was attenuated to ~10 µeq l^-1 before reaching the stream. These observations show how increased evaporation and processing of carbon and nitrogen in headwater catchments can mitigate the larger-scale impacts of MPB.