Abstract
BackgroundFrequent-fire forests of the western United States have undergone remarkable changes in structure, composition, and function due to historical exclusion of naturally occurring fire. Mechanized tree thinning to reduce forest density and fuel loads tends to be expensive and cannot be effectively implemented across all lands, and there is increasing interest in managing naturally ignited wildfires for meeting forest restoration objectives. To investigate general effectiveness of resource objective (RO) wildfires for restoring frequent-fire and associated forests of the western United States, we conducted a review of the related peer-reviewed literature.ResultsFormal analysis of ecological responses to RO fires is relatively recent, and 21 of the 37 papers (57%) we reviewed were published between the years 2010 and 2018. We found 17 studies that investigated RO fire outcomes in Sierra Nevada forests, while other ecoregions that were represented in the literature included Arizona–New Mexico Mountains, Middle Rockies–Blue Mountains, and the Colorado Plateau. Yosemite National Park was utilized in 14 of the studies we reviewed. We noted several ecoregions where frequent-fire forests occur, but for which published studies on RO fires are lacking. The main focus of research (14 studies) was related to RO fire effects on forest structure, and next in importance was research (12 studies) related to fire severity, extent, and type, with studies of understory vegetation responses and landscape-scale patterns or dynamics also prevalent. Research findings indicated that RO fires were effective for reducing tree density and fire behavior; however, densities often remained above known historical ranges of variation in several studies. Understory responses reported in the literature were mixed with respect to effects on abundance and species richness. Increases in invasive species abundance in areas of high burn severity were consistently reported. Research on landscape patterns indicated that RO fires can increase heterogeneity with respect to vegetation distribution.ConclusionRO fires can be generally effective for restoration of frequent-fire forests, particularly on landscapes with long histories of fire use and in areas of moderate burn severity. More research is needed to test fire outcomes against specific restoration targets, and additional studies are needed concerning important ecological processes and functions.
Highlights
Interruption of historical fire regimes and decades of fire suppression during the twentieth century were primary factors leading to major shifts in structure, composition, and function of dry forest ecosystems in the western United States (Covington et al 1994; Allen et al 2002; Hessburg et al 2019)
Because we were interested in a broad synthesis of current state of knowledge, and due to variability in methodologies and metrics across studies included in the review, we did not pursue meta-analysis; we identified and tallied directional responses of attributes commonly used to evaluate restoration success in frequent-fire forests of the western US
Literature demographics public land managers have allowed lightningignited wildfires to play their natural role in large national parks and wilderness areas beginning in the 1960s (National Park Service) and 1970s (US Forest Service), research on ecological responses to these events has only recently been pursued
Summary
Interruption of historical fire regimes and decades of fire suppression during the twentieth century were primary factors leading to major shifts in structure, composition, and function of dry forest ecosystems in the western United States (Covington et al 1994; Allen et al 2002; Hessburg et al 2019). Changes related to tree population irruptions have included increases in hazardous fuels and uncharacteristic fire behavior, reduced understory diversity, and homogenization of stand and landscape heterogeneity (Covington et al 1994; Hessburg et al 2019). Concern over these changes has motivated a series of natural resource policy mandates aimed at expanding and accelerating ecosystem restoration on public lands (Schoennagel et al 2009; Franklin and Johnson 2012; Schultz et al 2012; Stephens et al 2016). To investigate general effectiveness of resource objective (RO) wildfires for restoring frequent-fire and associated forests of the western United States, we conducted a review of the related peer-reviewed literature
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