Stream thermal responses to wildfire in the Pacific Northwest

Wildfire disturbance is one of the most prevalent forces of ecological change in freshwater environments. Studies of stream temperature responses to wildfire disturbance are often limited to individual watersheds or burn events, resulting in an incomplete understanding of: 1) how wildfires affect stream temperature at broader spatial scales and 2) which environmental and pyrological factors are important predictors of thermal responses. We analyzed long-term temperature monitoring data from 18 fire-affected streams across the Pacific Northwest We used a nonparametric effect-size approach to quantify response patterns among 26 metrics describing the thermal regime (magnitude, variability, frequency, and timing of thermal events) in the 1st postfire year. We used multivariate ordination and spatially explicit data on pyrological, hydrological, and meteorological conditions to identify potential drivers of thermal sensitivity to wildfire effects in disturbed watersheds. Thermal responses were highly variable at the site level, but, in general, streams of the Pacific Northwest are prone to increased frequency of warmer temperatures and decreased frequency of cooler temperatures in the year after a wildfire. Redundancy analysis showed that 30 to 55% of the variation in stream temperature responses was equally explained by pyrological, hydrological, and meteorological conditions. Mean annual precipitation, % watershed burned, and mean annual flow were primary factors shaping thermal response to wildfire, but the relative importance of these potential drivers varied among aspects of the thermal regime. Our study provides new insight into the regional influence of wildfire on stream thermal regimes and indicates that wildfire could exacerbate climate-induced warming in streams. Further investigation of the interactions among wildfire, hydrology, and climatic processes and enhanced stream network monitoring are needed to manage fresh waters in a future characterized by more severe and frequent wildfire activity.