Changing fire regimes raise concerns about increasing vulnerability of dry conifer forests in the southwestern United States. However, the extent to which contemporary fire regimes may have diverged from historical patterns and processes remains the subject of considerable uncertainty, and consequently, active scientific debate. At issue is the historical role and extent of high severity fire. Here, we contrast the frequency and severity of historical (1700-1880) and contemporary (1985-2020) fires across a network of 408 tree-ring fire-scar sites in Arizona and New Mexico. We combine dendroecological records, satellite-derived metrics of burn severity, and field measures of tree mortality. Historically, low- to moderate-severity fires burned at these sites with a mean fire return interval (MFI) of 16.9 (± 10.7) years. These fire regimes ended by 1880 at most sites, initiating a long fire-free period. Over the satellite record (1985-2020) nearly half of the sites did not experience fire, underscoring a still-growing fire deficit in large portions of these landscapes. Of the sites analyzed, 26.7% burned once while 24.3% burned two or more times. In first-entry contemporary fires, 42.4% of sites burned at severity more likely than not to kill trees (CBI>1.61), anomalous for sites where trees historically survived fire for centuries. Contemporary burn severity was linked to climatic variables including maximum temperature and vapor pressure deficit, but not to historical MFI or the length of time between historical and contemporary fire events. We did not directly evaluate pre-fire fuel conditions as a driver of burn severity, but suspect variability therein was overcome by long fire-free intervals across the region. These results underscore the consequences of climate change on the severity of forest fires, but also suggest that management interventions can achieve promising outcomes given the right climatic opportunities.
