Across the Pacific Northwest, streams are feeling the heat. For years, ecologists have recognized the importance of water temperature in driving ecological patterns in freshwater streams, from shaping the spatial distributions of fish species to affecting the emergence timing of aquatic insects. In the Pacific Northwest, higher air temperatures and changes in precipitation patterns due to climate change are expected to result in warmer streams, potentially increasing the stress imposed on cold-water stream species. Additionally, higher temperatures can facilitate the expansion of invasive species that are more tolerant of warmer conditions, increasing the risks native species will face from competition and predation. The compounding threats to freshwater biodiversity posed by increased stream temperatures makes understanding natural thermal dynamics an essential goal for the management and conservation of stream ecosystems.
Climate change also plays a role in affecting physical habitat in terrestrial systems, and warmer, drier summers in the Pacific Northwest are predicted to fuel more extreme wildfires. In collaboration with the Forest Service, my research focuses on the effect of wildfire disturbance in stream ecosystems, and in particular looks to identify patterns and drivers of thermal response in streams following wildfire. By combining data from the NorWeST monitoring program (USFS) and the Monitoring Trends in Burn Severity Project (USGS), I’ve collated temperature data from numerous streams in Washington, Oregon and Idaho that have recently been affected by wildfire disturbance. Using a multi-metric approach, I decomposed temperature time series into a suite of ecologically-motivated thermal metrics to quantify the impact of wildfire to the magnitude, variability, frequency and timing of thermal events. I then collected spatially-explicit data on wildfire, watershed and climatic characteristics in a GIS and used multivariate ordination techniques to determine the environmental conditions driving thermal response following wildfire disturbance.
Results of my analysis indicate that there is substantial variability in stream thermal response to wildfire, and that facets of the thermal regime may respond differently depending on the environmental context. Unlike previous studies, we found that not all streams experienced higher average and maximum temperatures immediately following wildfire. However, there was a significant response across sites in the frequency of extreme thermal events, with warmer periods occurring more often and cooler periods occurring less often in the post-fire year. Redundancy analysis showed that 33-47% of variation in thermal responses was due to differences in fire activity, watershed characteristics and precipitation patterns, and indicated that different facets of the thermal regime are more susceptible to local pyrological and environmental conditions than others. Fire severity, fire proximity, and precipitation were the most important predictors of changes in stream temperatures across sites, indicating that fine-scale burn patterns and broad climatological patterns are most important in driving the response of stream temperatures.
So will wildfires turn up the heat in stream ecosystems? Perhaps not, but they could exacerbate already stressful conditions faced by cold-water species. Although organisms in streams have been able to adapt to wildfire events for millennia, the pressures of climate change and anthropogenic impacts could reduce their resilience to disturbance. Proper management of wildfires and watersheds will depend on recognizing the environmental conditions leading to thermal sensitivity, continuing temperature monitoring efforts and restoring natural levels of variability from wildfire disturbance into the landscape. This work is currently in review for publication, so stay posted!