Mesic mixed-conifer forests are resilient to both historical high-severity fire and contemporary reburns in the US Northern Rocky Mountains

Abstract

High-severity fires and short-interval reburns strongly influence forest structure and composition and may overwhelm forest ecosystem resilience and catalyze persistent shifts to non-forest conditions. Recent increases in annual area burned and severity in the western United States (US) highlight the need to better understand the long-term effects of high-severity fire, including interactions with subsequent fires. In the early 20th century, the northern US Rocky Mountains experienced several fire seasons with widespread, high-severity fires, under fire weather comparable with extreme conditions today. The Selway-Bitterroot Wilderness in north-central Idaho has remained an active fire regime with limited suppression and management, making it an ideal location to investigate long-term effects of initial high-severity fires, as well as ecosystem resilience to contemporary reburning. Using field sampling informed by fire history data from 1870 to 2020, we investigated the influence of fire frequency (once, twice, and thrice burned from 1910 to 2017) on forest structure, conifer regeneration, and fuel loading in mesic mixed-conifer forests that burned at high severity in either 1910 or 1934. Tree regeneration was abundant across all three burn histories, and 99% of sample sites were <200 m from the nearest conifer seed source when sampled in 2021. Abundance of snags and coarse woody material was less affected by fire frequency and more impacted by time since last fire. High shrub biomass occurred only on steep southwest aspects with low overstory basal area and was not related to burn history. Live tree composition and density differed across forests with contrasting recent fire histories, but even thrice-burned sites supported abundant conifer tree regeneration, indicating that northern Rocky Mountain mesic mixed-conifer forests that experienced fire during the twentieth century currently remain resilient to wildfire. Wildfire as an ecological process in the Selway-Bitterroot Wilderness likely contributed to ecosystem resilience.