Conditions inside fisher dens during prescribed fires; what is the risk posed by spring underburns?

The use of spring prescribed fires to reduce accumulated fuel loads in western forests and facilitate the return of natural fire regimes is a controversial topic. While spring burns can be effective at reducing fuel loads and restoring heterogeneous landscapes, concerns exist over the potential impacts of unnaturally-timed fires to native species. To protect native wildlife from disturbance during critical periods, limited operating periods (LOPs) are often implemented. However when LOPs for multiple species are combined into an integrated management plan, very few time windows for implementing prescribed fires remain. The use of spring burns is often effectively eliminated, thereby reducing land managers' opportunities to implement what can be their most effective tool for forest restoration. To help guide the design of LOPs for fishers in the western United States, and to help identify opportunities to mitigate the risks posed by spring prescribed burns, we evaluated conditions within tree cavities during five prescribed fires in the Sierra National Forest and Yosemite National Park, CA. This relatively simple experiment was designed to provide much-needed and timely answers to crucial questions regarding the short-term impacts of prescribed fire on fishers and other wildlife species using cavities. Temperatures were remarkably stable within cavities, averaging 20.03 °C during burns. Carbon monoxide accumulation posed a greater threat, averaging a maximum of 170.8 ppm during burns and remaining elevated for >30 min, conditions potentially hazardous to fisher neonates. We discuss how these risks can be interpreted, and recommend that measures to mitigate smoke accumulation in tree cavities be implemented where spring burns are conducted in areas potentially occupied by fishers.