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

Abstract

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 Within fires that have burned, severity distributions are disconnected from historical references.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.

Changing wildfire, changing forests: the effects of climate change on fire regimes and vegetation in the Pacific Northwest, USA

“…Empirical models do not account for these potential changes in vegetation and fuels (among other limitations; see McKenzie and Littell 2017). In the near term, high stem density as a result of fire exclusion and past management may increase fire severity in dry, historically frequent-fire forests (Haugo et al 2019).…”

A System Dynamics Model Examining Alternative Wildfire Response Policies

“…In other areas where suppression has persisted, landscape conditions may present greater challenges for reintroducing fire. Expanding rates of burning to reduce the fire deficit is not a risk-free strategy; if the return of fire results in fire severity outside the range of adaptation, then it can serve to further reduce rather than restore landscape resilience [18].…”

We’re Not Doing Enough Prescribed Fire in the Western United States to Mitigate Wildfire Risk

“…In the US Pacific Northwest, Haugo et This trend towards increased understanding of prescribed fire should theoretically support its increased application, and such application is sorely needed. In the US Pacific Northwest, Haugo et al [31] reported that for the 31-year period between 1984 and 2015, wildfire burned just 1.6 mill ha across a subset of forests in Washington and Oregon. This total was an order of magnitude less than the 15-21 mill ha expected under historical fire regimes and in the absence of fire suppression and exclusion.…”

“…This total was an order of magnitude less than the 15-21 mill ha expected under historical fire regimes and in the absence of fire suppression and exclusion. This deficit included 7-10 mill ha of the type of low-severity fire that prescribed burning can replace or be used to restore ecological function [31]. Other more general studies have similarly reported widespread wildfire deficits across western US forests [32].…”