Previous fires and roads limit wildfire growth in Arizona and New Mexico, U.S.A.

Fire shapes landscapes long after the flames are extinguished by leaving legacies of heterogeneous fuel mosaics, species composition patterns, and age classes. Fire perimeters have received little research attention, but their locations have implications for both landscape patterns and processes, including vegetation structure and subsequent disturbances. In this study, we focused on the role of previous wildfires and roads in limiting wildfire growth and influencing the pattern of fire at a regional scale. Using fire perimeter data from the U.S. Southwest, we asked (1) to what degree previous wildfires and roads limit the spread of subsequent fires, (2) what the temporal patterns are in fire perimeter limitations, in terms of time-since-fire and stability of patterns over time, and (3) whether limitations to fire spread differ across National Forests and topographic variables. We found strong evidence that previous fires and roads play a role in limiting subsequent fire progression. Of fires that spatially intersected previous wildfires, 8.7% of fire perimeters aligned only with previous wildfire perimeters. On average, 25.7% of fire perimeters aligned only with roads, compared to 11.6% when fires were randomly shifted, and road alignments tended to be on less steep slopes than wildfire alignments. More than 60% of fire perimeter alignments occurred when time since the previous fire was 5 years or less. Finally, results varied by National Forest; the Coconino and Kaibab National Forests, which have fairly flat terrain, had high percentages of fire-road alignments while the Gila National Forest, which contains a large amount of Wilderness, had the most fire-fire alignments. As more fires burn, fire interactions are likely to increase, and previous fire footprints may have more opportunity to act as fuel breaks or control points for subsequent fires.