Characterizing Wildland-Urban Interface Fire Typology and Climate Associations Across California, USA

Abstract. California experiences some of the most intense wildfire activity globally, with substantial human casualties and economic losses that show accelerating trends. Existing research predominantly quantifies the relative contributions of anthropogenic factors and climate forcing to WUI fires at the aggregate level, yet overlooks the heterogeneity in fire initiation and propagation mechanisms arising from differences in ignition locations and dominant spread areas. Using multi-source data from California (2002–2023), we classified WUI fires into four behavioral modes based on whether ignition sites and primary spread areas were located in the WUI: I-I (WUI ignition, WUI spread), I-W (WUI ignition, wildland spread), W-I (wildland ignition, WUI spread), and W-W (wildland ignition, wildland spread). We systematically analyzed size characteristics, inter-annual trends, fuel composition, and climate sensitivity across modes. Results revealed: (1) WUI fires accounted for 96.6% of total burned area from large fires, although only 12.2% of burned area fell within the WUI; both total and mean burned area increased significantly over the two-decade period. (2) Naturally-ignited WUI fires showed significantly delayed ignition dates, whereas human-caused fires occurred significantly earlier, with peaks in fire frequency during Independence Day, Labor Day, and Thanksgiving. (3) I-I fires were predominantly driven by anthropogenic factors with the highest proportion of shrubland fuel and the smallest mean size; W-W and I-W fires exhibited significant climate sensitivity, with I-W showing a higher rate of increase than W-W over the two decades. These findings reveal differentiated driving mechanisms of anthropogenic and climatic factors across WUI fire behavioral types, providing scientific evidence for stratified fire management strategies.