Landscape and climatic influences on size and severity of large fires on Western United States, 1984 to 2010

Background/Question/Methods

Although many studies have examined wildfire activity and its drivers, few have focused on the temporal variability of climate and spatial variability of landscape controls on individual fire size and severity. In this study, we obtained perimeter and burn severity for large fires (>1000 acres) in the western US from 1984 to 2010 from Monitoring Trends in Burn Severity data, which is derived from remote sensed spectral indices. We used boosted regression trees to quantify the relative influence of climate, topography, human, and vegetation type on fire size and severity for western US and six subregions. Climate variables (max temperature, wind speed, and precipitation) were summarized for different days preceding to ignition date and days during spread for each fire. Topography, human, and vegetation type were overlaid with each fire patch and calculated as mean value for each fire. 

Results/Conclusions

Results showed that vegetation type, topography, human variables often had the biggest influences on fire size although their ranks differed among ecoregions. Topography had strongest influence on the percent of high severity for each fire. Vegetation type, climate, topographic, and human variables collectively determined area and percent of high severity fire. Prolonged periods of extreme weather conditions preceding the fire event enabled the growth of high severity fires. Our results indicated that the spread and severity of large fires were strongly constrained by landscape controls. Our analysis suggested that fires may become larger and more severe under future, warmer climate. However, large and severe fires are likely to be clustered in landscapes with particular vegetation and topographic characteristics. Understanding the locations and determinants of these hot spots can help in adapting fire management strategies for future climates.