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