The missing fire: quantifying human exclusion of wildfire in Pacific Northwest forests, USA

Western U.S. wildfire area burned has increased dramatically over the last half-century. How contemporary extent and severity of wildfires compare to the pre-settlement patterns to which ecosystems are adapted is debated. We compared large wildfires in Pacific Northwest forests from 1984 to 2015 to modeled historic fire regimes. Despite late twentieth-century increases in area burned, we show that Pacific Northwest forests have experienced an order of magnitude less fire over 32 yr than expected under historic fire regimes. Within fires that have burned, severity distributions are disconnected from historical references. From 1984 to 2015, 1.6 M ha burned; this is 13.3-18.9 M ha less than expected. Deficits were greatest in dry forest ecosystems adapted to frequent, low-severity fire, where 7.2-10.3 M ha of low-severity fire was missing, compared to a 0.2-1.1 M ha deficit of high-severity fire. When these dry forests do burn, we observed that 36% burned with high-severity compared to 6-9% historically. We found smaller fire deficits, 0.3-0.6 M ha, within forest ecosystems adapted to infrequent, high-severity fire. However, we also acknowledge inherent limitations in evaluating contemporary fire regimes in ecosystems which historically burned infrequently and for which fires were highly episodic. The magnitude of contemporary fire deficits and disconnect in burn severity compared to historic fire regimes have important implications for climate change adaptation. Within forests characterized by low- and mixed-severity historic fire regimes, simply increasing wildfire extent while maintaining current trends in burn severity threatens ecosystem resilience and will potentially drive undesirable ecosystem transformations. Restoring natural fire regimes requires management that facilitates much more low- and moderate-severity fire.