Wildfire Effects on Stream Temperature and Flow in Humid Mountain Forests: Insights from the H.J. Andrews Experimental Forest

Wildfires can alter hydrology and stream temperature (Ts) in headwater catchments, yet their effects in humid forests remain poorly understood. In the Western U.S., warming and declining snowpacks are increasing fire frequency and severity, threatening small streams that provide critical aquatic habitat. We examined how catchment storage mediates hydrologic response—both summer flows and Ts—following wildfire in the H.J. Andrews Experimental Forest (Oregon, USA), an old-growth conifer system. We leveraged long-term hydrometric records for watersheds that burned in 2020 Holiday Farm Fire and using 75 sensors, we monitored Ts in six headwater streams before, during, and after the 2023 Lookout Fire. While low flows appear to be increasing post-fire given reductions in evapotranspiration, effects of the fire on high flows are not evident. Pre-fire Ts ranged from 7–14 °C, driven by solar radiation and subsurface storage. Post-fire, burned streams warmed by 0.5–3 °C, with the largest increases in low-storage catchments. Diel amplitudes also rose more in these systems. Spatial models linked burn severity and storage to both thermal and flow shifts, offering predictive insight into future fire impacts. Our findings underscore the buffering role of subsurface storage and the interaction between fire severity and storage in shaping stream resilience. This research advances understanding of wildfire impacts on thermal regimes and hydrology in humid forests and informs management of fire-affected headwater ecosystems under a warming, fire-prone climate.