QUANTIFYING THE THREAT OF UNSUPPRESSED WILDFIRES REACHING THE ADJACENT WILDLAND-URBAN INTERFACE ON THE BRIDGER-TETON NATIONAL FOREST, WYOMING, USA

An important objective for many federal land management agencies is to restore fire to ecosystems that have experienced fire suppression or exclusion over the last century. Managing wildfires for resource objectives (i.e., allowing wildfires to burn in the absence of suppression) is an important tool for restoring such fire-adapted ecosystems. To support management decisions that allow wildfires to burn unsuppressed, land managers need a quantitative assessment of the potential for such wildfires to reach nearby fire-susceptible resources and assets. We established a study area on a portion of the Bridger-Teton National Forest near Jackson, Wyoming, USA, where land managers wish to restore fire by managing wildfires, but are concerned about the threat to residential buildings. We modeled the ignition and unsuppressed growth of wildfires starting in a remote portion of the study area using FSim, a fire occurrence, growth, and suppression simulation model. We then characterized annual area burned and the likelihood that wildfires would reach a nearby wildland-urban interface (WUI) defense zone. Early-season fires burned longer and grew larger than late-season fires, and thus had a higher likelihood of reaching the WUI zone (3% of May fires compared to 0.1% of October fires). Because fire managers do not anticipate managing all fire starts for resource objectives, we applied a simple rule set termed RO rules, indicating the fraction of starts by month to be managed for resource objectives. This reduced the expected number of fires reaching the WUI zone by 70%, and the expected WUI zone area burned by 61%. From 1990 to 2009, a mean of 207 ha yr(-1) had been burned by wildfires starting in the remote portion of the study area. By contrast, we estimated that 14 431 ha yr(-1) could burn if no fire starts were suppressed, and 4861 ha yr(-1) after applying the RO rules. Our analysis approach can be extended to determine which parts of the landscape are most likely to produce fires that reach specific targets on the landscape.