Elevated Annual Runoff Ratios in Pacific Northwest Catchments Impacted by Epidemic Foliage Disease of Douglas-fir

Catchments in the Western United States are undergoing unprecedented levels of tree die-off and/or reduced vigor due to increased severity of wildfire, drought, insect outbreaks, and disease. In the U.S. Pacific Northwest, Swiss needle cast (SNC) is the most damaging foliar disease of Douglas-fir (Pseudotsuga menziesii), physically obstructing stomata and preventing CO2 uptake and transpiration. A recent analysis in coastal Oregon indicated a substantial increase in area affected by the disease, from 530.5 km2 in 1996 to 2,387.1 km2 in 2015. Deforestation or reduced tree vigor can have profound impacts on catchment hydrology, in theory, producing increased streamflow due to reduced interception and transpiration. However, these increases have not always been detectable as impacts also depend on factors such as climate and vegetation composition. Moreover, press disturbances, such as insect outbreaks or disease, often do not result in complete removal of understorey or canopy vegetation. We analyzed trends in annual runoff ratios (quotient of discharge divided by precipitation) from 1990-2015 in 12 catchments (183-1,744 km2) in western Oregon. In general, runoff ratios increased by 10-27% in catchments with a total area of SNC >10%, with the most substantial runoff increases in catchments with SNC impacting >25% of the area. Interestingly, the most severely impacted catchment ( 90.5% SNC) showed a decrease in runoff. This is consistent with a potential compensatory response from understory western hemlock (Tsuga heterophylla) trees, a phenomenon observed in the most severely impacted sites. Findings from this study are important for assessing the impacts of biotic forest disturbances on water supply and aquatic ecosystem health.