In the Pacific Northwest, hotter and drier conditions driven by climate change and a history of fire suppression are increasing the frequency of larger and more severe wildfires. While the environmental responses to wildfire are well-studied in terrestrial settings, the effects of these disturbances in aquatic systems are less understood. In particular, stream temperatures are thought to be sensitive to wildfires and post-fire conditions following alteration of channel morphology and increased incipient solar radiation, yet the magnitude and scale of this sensitivity has only been documented for individual watersheds or burn events. Freshwater habitats provide a broad range of ecosystem services and are home to numerous species of management concern in the Pacific Northwest, and the importance of temperature in driving the distribution and phenology of these species illustrates the need to elucidate the role of wildfire disturbance in influencing stream thermal regimes. This study combines historical stream temperature monitoring with satellite-imagery burn severity data to understand the response of stream thermal regimes to wildfire, and explores the hydrological and pyrogeographical covariates driving this response. By decomposing temperature time series into a suite of metrics summarizing the magnitude, variability, frequency and timing of thermal events throughout the year, we measured the effect of wildfires on various facets of the thermal regime relative to pre-fire conditions. We then used redundancy analysis and spatially-explicit environmental datasets to identify drivers of thermal sensitivity to wildfire conditions.
Our results indicate that while maximum daily temperatures may increase in the immediate post-fire period, impacts on stream temperatures in the year following wildfire disturbance are generally small and transient. Over 50% of sites experienced a higher frequency of warm events and a lower frequency of cold events relative to standardized annual temperatures, indicating that post-fire conditions may exacerbate thermal stress on already threatened cold-water species, while providing more opportunities for migration and development of warm-water species. Multivariate analyses suggest that environmental covariates such as fire severity and topography account for up to 40% of variation in thermal metrics in the year following wildfire. Our work suggests that at broader spatial and temporal scales, stream temperatures are generally resilient to post-fire conditions. Nevertheless, these results can inform management of freshwater systems by identifying the environmental settings driving thermal sensitivity to disturbance in streams, and help preserve critical ecosystems services provided by these habitats in the face of heightened wildfire occurrence predicted in future climates.