Human and biophysical factors influencing modern fire disturbance in northern Wisconsin

Abstract

Humans cause most wildfires in northern Wisconsin, but interactions between human and biophysical variables affecting fire starts and size are not well understood. We applied classification tree analyses to a 16-year fire database from northern Wisconsin to evaluate the relative importance of human v. biophysical variables affecting fire occurrence within (1) all cover types, and (2) within forest types in each of four different fire size groupings (all fires; fires ≥0.4 ha (1 acre); fires ≥4 ha (10 acres); fires ≥16 ha (40 acres)). Housing density was the most important indicator of fire observations. Increasing minimum fire size increased the relative importance of biophysical variables. Key biophysical variables included land cover type, soil moisture indicators, and an index of presettlement fire rotation associated with glacial landforms. Our results indicate the likelihood of fire starts is primarily influenced by human activity in northern Wisconsin, whereas biophysical factors determine whether those fire starts become large fires. Important interactions between human and biophysical variables were observed for nearly all fire types and size thresholds examined. Our results have implications for both ecological restoration and the management of fire risk within historically fire-prone systems currently experiencing rapid rural development.