Effect of contemporary forest harvesting practices on headwater stream temperatures: Initial response of the Hinkle Creek catchment, Pacific Northwest, USA

We investigated the effect of contemporary forest harvesting practices on warm-season thermal regimes of headwater streams using a Before-After-Control-Intervention (BACI) design within a nested, paired watershed study. We applied harvesting treatments to four headwater tributaries of Hinkle Creek, designed in accordance with the Oregon Forest Practices Act. Therefore, fixed-width buffer strips containing overstory merchantable trees were not left adjacent to the four non-fish-bearing streams. The summer following harvesting, we observed a variable temperature response across the four harvested streams. Mean maximum daily stream temperatures ranged from 1.5 °C cooler to 1.0 °C warmer relative to pre-harvest years. We also observed significantly lower minimum and mean daily stream temperatures, and recorded particularly low temperatures in treatment streams on days that minimum stream temperatures in reference streams were high. At the watershed scale, we did not observe cumulative stream temperature effects related to harvesting 14% of the watershed area in multiple, spatially-distributed harvest units across four headwater catchments. At the watershed outlet, we observed no change to maximum, mean, or minimum daily stream temperatures. We attribute the lack of consistent temperature increases in headwater streams to shading provided by a layer of logging slash that deposited over the streams during harvesting, and to increased summer baseflows. Comparisons of canopy cover before and after harvesting indicate little change in canopy cover when logging slash is considered, indicating that logging slash affected energy balances within the streams. However, other ecosystem-level effects of logging slash in forest streams are difficult to predict. In order to devise optimum practices related to in-stream slash management, forest scientists and policymakers should continue to study and consider the potential impacts of logging slash to stream ecosystems.