Three-decade record of contiguous-U.S. national forest wildfires indicates increased density of ignitions near roads

Background

Roads play an important role in managing fire on the national forests. But roads also are known to increase ignitions and damage ecosystems. Roads may limit the size of wildfires, which may be viewed as desirable where fires endanger human life and property or undesirable if roadlessness allows more land to experience the ecological benefits of fire. In this paper, we examine a large, nationwide dataset to determine whether roads on the national forests are associated with higher ignition density, and we examine patterns of fire size to see whether wilderness and roadless areas are associated with larger fires.

Results

From 1992 to 2024, in all 8 contiguous-US Forest Service regions combined, wildfire-ignition density was lowest in designated wilderness areas (1.75 fires/1000 hectares), followed closely by Inventoried Roadless Areas (1.97 fires/1000 ha). The highest wildfire-ignition density was in lands within 50 m of roads (7.99 fires/1000 ha), and the second highest wildfire-ignition density was in other national forest lands outside of the 50-m road buffers but not in wilderness or roadless areas (3.50 fires/1000 ha). For human-caused, natural, and undetermined fires, wildfire-ignition density decreased as distance to road increased, irrespective of designation categories such as “wilderness” or “roadless.” In lands between 0 and 250 m from roads, 6 fires ignited per 1000 ha, whereas fewer than 2 fires ignited per 1000 ha at a distance class of over 2000 m from roads. Mean fire size varied by where the fire started: it was greatest in wilderness areas (239 ha), followed by Inventoried Roadless Areas (135 ha), roaded national forest lands outside of Inventoried Roadless Areas, wilderness, and the 50-m buffer (62 ha), and lands within the 50-m road buffer (49 ha). We found, however, that the largest 2% of fires had similar mean sizes and ignition densities regardless of where they started.

Conclusions

Our results suggest that building roads into roadless areas is likely to result in more fires. These fires will, on average, be smaller than fires farther from roads, but there will be more of them, and some of them will grow to become large fires. Nevertheless, both roads and roadless areas can contribute to strategic, landscape-scale fire management.