Quantifying wildfire risk to the built environment in rural rangelands of the US Interior West

Authors: McGranahan, Devan; WONKKA, CARISSA - University Of Florida

Submitted to: Philosophical Transactions of the Royal Society B
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/29/2024
Publication Date: N/A
Citation: N/A

Interpretive Summary: The Wildland-Urban Interface (WUI) refers to areas where the proximity of vegetation to human development increases the vulnerability of human lives and property to greater wildfire hazard. In the western US, most WUI (54%) occurs in rangeland. Overall, the majority of the western US is rangeland (60%) and 4.3% of that -- over 1 million square kilometers -- is WUI. Most land classified as WUI is rural: 59% is located greater than 10 km from a town of 2,500 persons. Tribal areas are even more remote, with 76% of land classified as WUI greater than 10 km from town. Importantly, rangeland categorized as WUI is approximately twice as likely to be degraded by woody encroachment than non-WUI rangeland. This means that conventional fire suppression tactics for rangeland fuel types might be insufficient or even unsafe when attempted to protect property. Although there is a lot of focus on WUI in forest areas, rangeland WUI is clearly an issue across the entire western US.

Technical Abstract: Fire is a natural process in ecosystems around the world, but patterns of human development can create conflict with the built environment. The Wildland-Urban Interface, or WUI, describes areas where the proximity of natural or semi-natural vegetation to the built environment increases the vulnerability of human lives and property to greater wildfire hazard. Although these zones only encompass 5% of land surface worldwide, they affect half of the global human population. In the United States, especially, attention frequently concentrates on WUI issues in forested areas. But amenity migration—the extension of human populations into rural areas—is also occurring in rangelands, which are characterized by grass, other herbaceous vegetation, and occasionally shrub species. One of the primary drivers of rangeland degradation is greater woody plant abundance as fire suppression and other management-induced changes alter disturbance regimes. Thus, the combination of WUI expansion and woody plant encroachment might present a novel challenge to wildfire management, especially given the rural nature of rangeland ecosystems in the US, which extends the response time of emergency services. But WUI patterns in rangeland-dominated regions have not yet been examined. We use publicly available data in a complete census of four ecoregions of the US Interior West to describe the abundance, distribution, type, and overall wildfire risk in rural rangelands. Most of the WUI in the US Interior West (54%) occurs in rangeland: The majority of the US Interior West is rangeland and 4.3% of that—over 1 million km2—is classified as WUI. Most land classified as WUI is rural: 59% is located greater than 10 km from a town of 2,500 persons. Tribal areas are even more remote, with 76% of land classified as WUI greater than 10 km from town. Importantly, rangeland categorized as WUI is approximately twice as likely to be degraded by woody encroachment than non-WUI rangeland, suggesting that conventional fire suppression tactics for rangeland fuel types might be insufficient or even unsafe when attempted to protect property. Although the number of properties in an area generally declines as the proportion of rangeland in the area increases, overall wildfire risk does not change, which suggests that jurisdictions with a high proportion of rangeland face no less of a wildfire hazard to the built environment than WUI in other land cover types. At the same time, nothing in our data suggests that greater proportion of rangeland is associated with lower rural capacity at the county level. Greater awareness of the prevalence of rural rangeland WUI might help officials leverage existing community-level adaptive capacity to address the novel challenges of protecting human lives and property beyond the urban and peri-urban zones conventionally assumed to be most vulnerable to WUI hazards.