Modern pyromes: Biogeographical patterns of fire characteristics across the contiguous United States

Authors: CATTAU, M - BOISE STATE UNIVERSITY; Mahood, Adam; BALCH, J - UNIVERSITY OF COLORADO; WESSMAN, C - UNIVERSITY OF COLORADO

Submitted to: Fire
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/4/2022
Publication Date: 7/10/2022
Citation: Cattau, M.E., Mahood, A.L., Balch, J.K., Wessman, C.A. 2022. Modern pyromes: Biogeographical patterns of fire characteristics across the contiguous United States. Fire. 5(4). Article e95. https://doi.org/10.3390/fire5040095.
DOI: https://doi.org/10.3390/fire5040095

Interpretive Summary: In recent decades, wildfires in many areas of the United States have become larger and more frequent with increasing anthropogenic pressure. This includes interactions between climate, land use change, and human ignitions, which together define a fire regime "pyrome". We characterized seven different pyromes and the spatio-temporal patterns of contemporary fire characteristics that are associated with these pyromes across the contiguous United States. Human ignition pressure provides a key explanation for the East-West patterns of fire characteristics. Human-dominated pyromes covered 54% of the U.S., primarily in the East. Wildland pyromes occurred in 13% of the U.S., primarily in the West. The percent of anthropogenic ignitions increased over time in all pyromes. Higher fire frequency was related to smaller events with lower intensity. These relationships were moderated by vegetation, climate, and ignition type. This effort to delineate modern U.S. pyromes based on fire observations provides a national-scale framework of contemporary fire regions. These results may help elucidate patterns of change in an uncertain future.

Technical Abstract: In recent decades, wildfires in many areas of the United States have become larger and more frequent with increasing anthropogenic pressure, including interactions between climate, land use change, and human ignitions. We aimed to characterize the spatio-temporal patterns of contemporary fire characteristics across the contiguous United States. We derived fire variables based on frequency, fire radiative power (FRP), event size, burned area, and season length from satellite-derived fire products and a government records database on a 50km grid (1984-2016). We used k-means clustering to create a hierarchical classification scheme of areas with relatively homogeneous fire characteristics, or modern‚ pyromes™, and report on the level with seven major pyromes. Human ignition pressure provides a key explanation for the East-West patterns of fire characteristics. Human-dominated pyromes (95% mean anthropogenic ignitions), with moderate fire frequency, area burned, and intensity, covered 54% of the U.S., primarily in the East. Wildland pyromes (27% mean anthropogenic ignitions) characterized by relatively large (average 543 mean annual ha per 50km pixel) and intense (average 70 mean annual megawatts/pixel) fires occurred in 13% of the U.S., primarily in the West. The percent of anthropogenic ignitions increased over time in all pyromes (0.2-1.7% annually). Higher fire frequency was related to smaller events and lower FRP, and these relationships were moderated by vegetation, climate, and ignition type. Notably, a spatial mismatch between our derived modern pyromes and both ecoregions and historical fire regimes suggests other major drivers for modern U.S. fire patterns than vegetation-based classification systems. This effort to delineate modern U.S. pyromes based on fire observations provides a national-scale framework of contemporary fire regions and may help elucidate patterns of change in an uncertain future.