Wildfire decisions in Grand Canyon National Park high elevation forests with a mixed severity fire regime

Background/Question/Methods

Understanding and managing mixed severity fire regimes is a challenge for contemporary land managers. Managers must balance the goals of restoring and maintaining natural ecosystem processes with the goals of protecting threatened species and cultural resources and preserving forests for recreation and other human values. Each year in the North Rim area of Grand Canyon National Park, managers decide whether to allow lightning-ignited wildfires to burn the high elevation forests with a mixed severity fire regime or to suppress these fires. Decades of fire suppression in these forests have altered the mixed severity mosaic of forest structure such that large patches of high severity fire that could negatively impact other park values become possible. To better understand the conditions under which large patches of high severity fire have been generated in the high elevation forests over the past 10 years, and to help predict when they might occur in the future, we compiled data from six previous wildfires in this area. These fires all burned in forests with a mixed severity fire regime for 20 or more days, had records of daily fire progressions, and had Monitoring Trends in Burn Severity (MTBS) one-year post burn severity data.  We analyzed the daily progression and MTBS severity data using vegetation variables, topographic variables, and daily weather data from the nearest Remote Automatic Weather Stations (RAWS) to determine whether these variables could predict large patches of high severity fire on the landscape.

Results/Conclusions

The results indicate that daily weather variables can predict large patches of high severity fire in the high elevation forests with a mixed severity fire regime.  Large patches of high severity fire occurred only on the days when maximum temperatures exceeded 82° F, minimum daily relative humidity dropped below 9%, or energy release component values exceeded 80. In addition, preliminary analyses show that topographic variables such as slope, aspect, and the interaction of the two also contribute to promoting large patches of high severity fire.  These analyses provide an important decision support tool to fire managers and tracking these variables when fires are in forests with mixed severity fire regimes will improve the park’s wildfire decision-making process.