Assessing stream temperature response and recovery for different harvesting systems in northern hardwood forests using 40 years of spot measurements

Stream temperature is a critical control on aquatic habitat and a key forest management concern in many jurisdictions. Most research on stream temperature response to forest harvesting is from coniferous forests in rain-dominated watersheds and focused on the first few years following harvesting. In contrast, we know less about the harvesting impacts on stream temperature for silviculture approaches typically used in northern hardwood forests that are influenced by snow. We addressed this knowledge gap by using four decades (1980 to 2020) of spot water temperature measurements recorded at three treatment and two reference catchments (areas 4.5 to 69 ha) as part of a long-term water quality monitoring programme at the Turkey Lakes Watershed study near the eastern shores of Lake Superior. We were able to control for diel and seasonal biases in the spot temperature measurements and found that clearcut harvesting showed a summer temperature increase that persisted for 5 to 7 years after harvesting. Shelterwood and selection harvest did not exhibit a detectable change in stream temperature. These responses are consistent with observed changes in forest canopy through time and between harvesting approaches. In addition, the stream temperature responses were likely muted due to the streams being short and characterized by intermittent flow conditions, as well as the potential moderating influence of increased subsurface runoff following harvesting. Our results highlight how insights can be extracted from routine water quality programmes that were hitherto unrecognized.