Trends in forest structure restoration need over three decades with increasing wildfire activity in the interior Pacific Northwest US

Abstract

Wildfire is a keystone ecological process in many forests worldwide, but fire exclusion and suppression have driven profound shifts in forest structure (e.g., increased density, canopy cover, biomass) that have contributed to increases in large, high-severity fire in many seasonally dry forests and woodlands of the western United States. Comparisons between contemporary and historic range of variability (HRV) in forest structure can quantify the amount and types of restoration that shift landscapes toward structural conditions that have historically fostered resilience to fire. However, landscapes are dynamic over time and conditions reflect the net effects of planned actions (e.g., fuel reduction treatments) and unplanned actions (e.g., wildfire). How wildfire activity may shift landscapes toward or away from the HRV and correspondingly affect the need for restoration, has not been widely tested. Here, we quantify long-term (1986–2017), and continuous (annual resolution) trends of forest restoration need and ask how wildfire activity during this period has affected restoration need trends at three nested spatial extents: across the eastern Washington (USA) ecoregion, among fire regimes within this ecoregion, and between watersheds of a frequent/low-severity fire regime that experienced contrasting amounts of wildfire. At the broadest scale, restoration need did not change substantially during the study period, with approximately 35 % of forest area in need of disturbance restoration—despite 16.6 % of the total forested area (593,000 ha) burning from 1986 to 2016. At intermediate spatial extents (among fire regimes), forests characterized by historically frequent/low-severity fire experienced the greatest decrease in disturbance restoration need following recent fire activity. Although > 50 % of forests within this fire-regime remained in need of disturbance restoration at the end of the study period, we found a strong correspondence between forested area burned and decreased restoration need in this fire regime; relationships were equivocal or non-existent in other fire regimes. At the finest spatial scale (watersheds dominated by historically frequent/low-severity fire-regime forests), we found sharp contrasts between areas that experienced high fire activity in recent years and those that did not. At this scale, recent large fires have decreased disturbance restoration need by > 25 %. Our findings suggest that recent large wildfires have reduced the amount of forest in need of restoration, but have done so modestly and primarily at local or sub-regional extents. Overall, our approach can be applied to understanding how wildfires or other disturbances contribute to affecting forest structure and management targets in other ecosystems through time and space.