OOS 64-6 - Forest disturbance effects on water yield in the western US: A review and large-sample analysis combining forest inventory and curated hydroclimatic data

As the climate warms in the western U.S., we are seeing and expecting more forest disturbances. Disturbance is generally expected to lead to increased streamflow, because reduced cover typically leads to less transpiration and interception, yet recent studies have found no change or even decreased streamflow following disturbances due to drought and insect epidemics. We reviewed 78 papers published during 2000–19 to determine the effects of forest disturbance on streamflow in western coniferous forests. Although some studies observed increases in water yield, in many cases water yield did not change or even decreased. Decreases were generally observed in areas with high radiation (i.e., arid watersheds at low latitudes and south-facing aspects); rapid growth of post-disturbance vegetation; and non-stand replacing disturbances, e.g., drought and insect-caused mortality. Contrary to the expectation that disturbance reduces evapotranspiration (ET), making more water available as runoff, post-disturbance ET sometimes increased because of (a) increased evaporation due to more sunlight reaching the subcanopy, and (b) increased transpiration due to rapid post-disturbance growth. To test the hypothesis that arid watersheds are more likely to produce less runoff following disturbance, we quantified streamflow response to forest cover change using hydrologic, climatic, and Forest Inventory & Analysis (FIA) data for 159 watersheds during 2000–2019. Annual streamflow decreased in many watersheds, even as forest cover decreased. Although change in streamflow was significantly related to tree mortality, we found that the direction of this effect depends on aridity. Specifically, forest disturbances in wet, energy-limited watersheds (i.e., where annual potential ET [PET] is less than annual precipitation) tended to increase streamflow, while post-disturbance streamflow more frequently decreased in dry water-limited watersheds (where the PET to precipitation ratio exceeds 2.35).