Contemporary fires are less frequent but more severe in dry conifer forests of the southwestern United States

Wildfires in the southwestern United States are increasingly frequent and severe, but whether these trends exceed historical norms remains contested. Here we combine dendroecological records, satellite-derived burn severity, and field measured tree mortality to compare historical (1700-1880) and contemporary (1985-2020) fire regimes at tree-ring fire-scar sites in Arizona and New Mexico. We found that contemporary fire frequency, including recent, record fire years, is still <20% of historical levels. Since 1985, the fire return interval averages 58.8 years, compared to 11.4 years before 1880. Fire severity, however, has increased. At sites where trees historically survived many fires over centuries, 42% of recent fires resulted in high tree mortality. Suppressed wildfires tended to burn more severely than prescribed burns and fire use wildfires. These findings suggest that expanded use of low-severity prescribed and managed fire would help restore forest resilience and historical fire regimes in southwestern dry conifer forests.

Methods
To quantify tree mortality from contemporary fires, we sampled fire effects at 74 of the 406 fire history sites used in the study. We located the field sites across a gradient of contemporary burn severity and fire management strategies. For example, proportion of fires in the full suppression category ranged from 100% in the Pinaleño and Santa Catalina Mountains to 14% on the Kaibab Plateau. Similarly, the proportion of sites which burned with high probability of tree mortality (CBI > 1.61) ranged from 73% in the Santa Catalina Mountains to 0% on the Kaibab. Data collection focused on six key geographic areas where networks of fire history sites had been established prior to wildfires occurring over the past ten years (2011-2020): the Jemez, Rincon, Santa Catalina, Pinaleño, and Chiricahua Mountains, and the Kaibab Plateau. In the Jemez Mountains, sites are in Bandelier National Monument (including the Bandelier Wilderness), the Valles Caldera National Preserve, and the Santa Fe National Forest. The Rincon Mountain sites are mostly located in the Saguaro Wilderness (within Saguaro National Park), and about half of the sites on the Kaibab Plateau are in proposed wilderness in Grand Canyon National Park; the remaining sites sampled in Arizona are within the Coronado and Kaibab National Forests.

We relocated each fire history site and established a 10-m radius plot. If we found a tree or stump sampled in the original fire history data collection, the plot was centered at its location; if a sampled tree was not located (typically due to high-severity fire effects), we centered the plot at the coordinates provided by the original researcher. Where we found multiple sampled trees or stumps at least 20 m apart, we installed a plot at each, for a total of 91 plots at 74 distinct fire history sites. For all trees in the field plots, we recorded tree diameter at breast height (dbh), species, and status (live or dead). We measured diameter and assigned species for downed logs and recorded an overall count of trees both live and dead, standing and down. A qualitative description of site conditions and photographic documentation completed our site characterization.