Description of case studies
The available case studies were predominantly located in forest ecosystems of the western U.S. (Fig. 1). All of the case studies were focused primarily on forest vegetation types, although some included areas of shrubland or grassland (Table 1). We found no case studies from ecosystems in the eastern or southeastern U.S. Six case studies took place in forests of the Sierra Nevada Mountains, two were in forests of the southern Rocky Mountains, three were in a mix of chaparral shrublands and conifer forests in southern California, two were in the Arizona/New Mexico Mountains, two were in the Cascade Mountains, and one was in the Idaho Batholith. Additionally, the case studies included one study in the Superior National Forest in northern Minnesota and one in the northwestern Great Plains; these two studies are clearly identified in the following description of findings, given their different environmental contexts, ecology, and management history as compared to the studies in the western U.S.
The wildfires described in the case study reports burned between 1999 and 2013 and ranged in size from approximately 1000 to more than 500,000 acres (400 to 200,000 ha; Table 1). Each case study described a unique combination of a wildfire event and previous fuel treatments or fires, and given the large size of most of the wildfires, the reports described the varied effects of many units within multiple treatment types. Several types of fuel treatments were evaluated, including mechanical thinning, prescribed fire, mastication, and lop and scatter, as well as previous wildfires, timber stand improvement, commercial timber harvest, and tree plantations. Although not all of these “treatment types” were intentionally implemented to modify fuels and fire behavior, the manager case studies commonly considered the effects of a broad range of previous events that had the potential to alter the fuel profile and subsequent wildfire. The treatments varied widely in their timing (i.e., how recently they were completed before the wildfire), size, and location in relation to other treatments or landscape features (e.g., ridges). While many of the case studies discussed the influence of these factors on treatment effectiveness, the reports were descriptive and did not analyze trends or produce specific criteria determining effective treatments.
Fuel treatment effects on fire behavior, suppression tactics, and fire effects
The case studies evaluated multiple indicators of fuel treatment effectiveness, including various measures of fire behavior, suppression tactics, and fire effects. It is important to note that fuel treatment effects were different across all the case studies because each set of fuel treatments was exposed to a unique fire event. The manager perspectives generally reflected an understanding that treatment effectiveness depended on the resulting distribution and abundance of fuels in various fuel strata, effects that can differ within a specific treatment type based on treatment intensity, recency, and design criteria (Jain et al. 2012).
Multiple studies documented that wildfire exhibited less extreme fire behavior in treated stands than in untreated stands. Although fuel treatments are not necessarily intended to stop a wildfire without accompanying fire suppression (Prichard et al. 2021), there was some evidence that very recent prior fire (prescribed and wildfire occurring within 1 year or less) could stop fire progression locally (Graham et al. 2003). Multiple case studies reported slower rates of fire spread in treated stands (Murphy et al. 2007; Rogers et al. 2008) or within recent fire perimeters (Ewell et al. 2012). Other studies stated that treated areas ignited easily and did not inhibit fire progression, particularly where treatments were small or narrow, did not reduce surface fuels sufficiently, or where implementation was not complete (Graham et al. 2009, 2012). Managers generally considered fuel treatments successful at changing fire behavior, reducing spotting distances (distance between the main fire and new fires started by wind-transported sparks or embers), and reducing convective and radiant heat. In several reports (Fites et al. 2007a; Keller et al. 2011; Murphy et al. 2007), fire transitioned from very high intensity in untreated stands to low or moderate intensity as it entered stands where fuel reduction work had occurred. Treated stands were more likely to experience surface fire behavior rather than crown fire (Graham et al. 2009; Murphy et al. 2007). Treated areas also were reported as experiencing less torching (ignition of a tree or group of trees) and spotting than outside treatment areas (Murphy et al. 2010).
Several case studies focused on the potential for fuel treatments to facilitate fire suppression efforts, and in many cases, fuel treatments were considered to have made suppression resources more effective. For example, reduced rate of spread and shorter flame lengths in treatment areas provided opportunities for fire line construction, anchor points (barriers to fire spread from which to start building a fire line), safety zones (cleared areas used for firefighter escape if the line becomes unsafe), structure protection, and spot fire suppression (Fites et al. 2007b; Graham et al. 2009; Harbert et al. 2007; Reiner et al. 2014; Rogers et al. 2008). There were reports from fireline personnel stating that burnout operations (setting fire to consume fuel between the edge of the fire and the control line) were more successful where stand density and fuel load had been reduced (Fites et al. 2007b; Keller et al. 2011; Murphy et al. 2010), and previously treated areas along roads and trails greatly reduced the time needed to prepare for burn operations (Fites et al. 2007a; Henson 2007). Although fireline decisions are dynamic and are thus difficult to evaluate empirically, multiple studies reported that the presence of large fuel treatments contributed to firefighters’ perception of safety and presented suppression opportunities that otherwise may not have been available.
Fire effects, or the impacts of fire on the environment, were less severe in treated than in untreated areas in many, though not all, of the case studies. Several case studies included quantitative analyses comparing at least one metric of fire severity in treated and untreated stands based on post-fire severity assessments, but high variability made statistical comparisons challenging, and many of the effects were interpreted qualitatively (see McKinney et al. for a review of empirical fuel treatment studies). Treated areas were reported to have lower tree crown consumption (Dailey et al. 2008; Fites et al. 2007b) and higher survival of large diameter trees (Jain et al. 2007), presumably due to treatment-related changes in tree canopy base height and consequent reductions in fire intensity. Similarly, in shrub-dominated ecosystems, treatments that reduced surface fuel load and shrub heights resulted in lower severity of fire effects on soils and vegetation (Reiner et al. 2014). However, reduced fire intensity did not always translate to reduced tree mortality: even where thinning treatments were credited with restricting fire behavior to a surface fire, extreme burning conditions and the presence of surface fuels sometimes resulted in near-total tree mortality (Graham et al. 2003). Managers also reported that treatments that were smaller, poorly maintained, or designed to modify only a single fuel layer were less effective at reducing fire severity than larger, recent treatments designed to modify multiple fuel layers (Crook et al. 2015; Graham et al. 2003).
Factors influencing fuel treatment effectiveness
Fuel treatments include a wide range of management approaches intended to alter fuels, each with its own effect on the distribution and abundance of fuels across multiple strata. Treatments also interact with the heterogeneous landscapes on which they are implemented, and outcomes are affected by temporal variability in fuels and weather at annual, seasonal, and daily scales. These complex interactions were consistently recognized in the manager case studies as influencing the effectiveness of fuel treatments and the outcomes of wildfire. The primary factors identified by the case studies included treatment effects on fuel layers, treatment recency, treatment size and placement in relation to topography and adjacent features, and weather conditions. Overall, the managers’ perspectives of the factors influencing treatment effectiveness agreed with accepted fuel layer principles of forest fuel reduction treatments (Agee and Skinner 2005): reduction of surface fuels, increased height of the live crown, and decreasing crown density.
In the case studies, the effectiveness of treatments in mitigating wildfire outcomes was strongly related to the extent to which the treatments reduced surface, ladder, and crown fuels. Specifically, fuel treatments that reduced the load in multiple fuel layers were considered more effective in reducing fire intensity and severity than those designed to modify only a single layer. For example, several case studies reported that thinning and prescribed fire combined were most effective in combination compared to thinning or prescribed fire alone (Crook et al. 2015; Dailey et al. 2008; Fites et al. 2007a; Jackson et al. 2011; Murphy et al. 2010), consistent with a prior data-driven review of site-level treatment effects (Kalies & Kent 2016). Treated areas with abundant surface fuels, including incomplete treatment implementation (e.g., piles not burned), lop and scatter, or mastication without prescribed fire, did not result in adequate fuel reductions and often experienced severe fire effects on soil and vegetation (Fites et al. 2007a; Graham et al. 2003, 2012; Murphy et al. 2007, 2010). In other words, treatments that redistributed fuels were generally considered less effective than treatments that reduced fuels, consistent with common principles of fire science (e.g., Agee & Skinner 2005).
Although previous wildfires are not intentionally designed treatments, many managers considered their presence on the landscape to be an important driver of large wildfire events, and decisions around suppression tactics often considered previous fire boundaries within a mosaic of treated areas. Wildfires can be intentionally managed as a way to remove fuels (or achieve other resource benefits) when burning under conditions where low to moderate fire severity and intensity can be expected, and this approach can simultaneously accomplish fuel reductions and restore fire as an ecological process (Crook et al. 2015; Ewell et al. 2012). Previous wildfires, along with treatments such as higher-severity prescribed fires that experienced intensive reductions in surface fuels, had less severe fire effects than lower-intensity treatments. For example, Crook et al. (2015), Fites et al. (2007b), and Graham et al. (2003) found that recent wildfires appeared to be more effective at reducing fire severity than mechanical treatments. Previous wildfires and prescribed fires that created heterogeneous forest structures and composition tended to produce mosaics of fire severity following subsequent fires (Crook et al. 2015; Graham et al. 2009), an outcome that may meet both fuel reduction and forest restoration objectives simultaneously (Prichard et al. 2021; Stephens et al. 2021).
The abundance and distribution of fuels within treated areas are known to be directly related to time since treatment or past fire (Cochrane et al. 2013), and many of the case studies described a decline in the effectiveness of older treatments. Treatments are only effective for a finite time because fuels accumulate over time as vegetation regrows. Recent treatments, if completed, were consistently more effective at mitigating fire behavior and reducing fire severity than older treatments that had experienced vegetation regrowth (Crook et al. 2015; Graham et al. 2003, 2009; Harbert et al. 2007; Reiner et al. 2014). The case studies indicated that the length of time needed before retreatment depended on site productivity, plant species traits, and initial fuel removal, consistent with a broad-scale analysis of reburning potential after wildfires (Buma et al. 2020). More intensive fuel reductions within stands typically last longer than less intensive treatments, particularly after reductions in multiple fuel layers (Agee and Skinner 2005). For all treatment types, the long-term reduction of wildfire hazard requires maintenance of fuel treatments as they age (Agee and Skinner 2005; Reinhardt et al. 2008).
Thinning to low overstory densities can promote the regeneration and growth of young trees and shrubs, particularly if this regeneration is not controlled through the intermittent use of prescribed fire (Jain et al. 2020). This effect was documented in some of the case studies, where prior timber harvests without surface fuel removal resulted in relatively high mortality of the residual trees (Dailey et al. 2008; Graham et al. 2003). Recent tree plantations, which are often characterized by young forests and spatially homogenized fuels (e.g., Zald and Dunn 2018), were described as burning with greater fire severity than nearby unmodified fuels, while older plantations experienced lower fire severity (Graham et al. 2003). Treatments designed to introduce fire-resistant species rather than to control surface fuels, such as shelterwoods followed by overstory removal, may create ladder fuels in the short-term but more fire-resilient conditions in the long term (Agee and Skinner 2005).
Fuel treatments interact with spatial heterogeneity in topography, landscape features, and existing vegetation to influence post-fire outcomes. In many of the case studies, fuel treatment effectiveness was influenced by prevailing winds and how they interacted with topography. Fuel treatments on steep slopes were less effective in changing fire behavior or reducing fire severity than those on flatter ground, especially under high wind conditions (Harbert et al. 2007; Murphy et al. 2007). One case study (Henson 2007) found that areas previously treated with thinning and prescribed fire successfully slowed fire as it was backing downhill but did not impede rapid uphill runs. Fuel treatments strategically located along ridge tops, in which fuel reduction impacts coincide with topography-related reductions in the rate of spread, were considered particularly useful in facilitating fire suppression efforts (Harbert et al. 2007; Murphy et al. 2010) or reducing the probability that past fires reburned (Gray & Prichard 2015).
Large treatments were consistently considered more effective than smaller treatments. At landscape scales, networks of larger treated areas were more effective than smaller, disconnected treated areas at reducing fire effects (Jackson et al. 2011). Many of the case studies described how the momentum produced by large fires overwhelmed small fuel treatments (Crook et al. 2015; Dailey et al. 2008; Fites et al. 2007a; Murphy et al. 2010) and produced spots that easily breached narrow fuel breaks (Graham et al. 2012). Thus, treatment placement adjacent to previous treatments, wildfires, land uses that reduce vegetation, or natural firebreaks of non-flammable features (e.g., wetlands, rock outcrops) can increase the footprint of the treated area and amplify effectiveness. Alignment with prevailing winds places treatments in the path of where fires are most likely to occur, and orienting treatments to maximize the distance a fire travels through the treated landscape can increase the effect of the treatment on fire progression. Managers described the strategic placement of fuel treatments on the windward side of resource values such as housing developments and discussed the importance of aligning linear treatments parallel with prevailing winds to inhibit spotting across the treatment (Graham et al. 2009; Reiner et al. 2014). Placement in relation to suppression needs is critical in wildland-urban interface areas, taking into consideration access, egress, and communities at risk (Rogers et al. 2008). Overall, landscape-scale treatment designs that integrate fuels, topography, prevailing winds, fire or treatment history, and available infrastructure are more effective than opportunistically implementing small, disconnected treatments.
A common theme of the case studies was that, during periods of extreme fire weather, fuel treatment effectiveness declined regardless of other factors. Short-term fire weather is often the primary driver of fire activity (e.g., Hart & Preston 2020), making quantitative inferences about fuel treatment effectiveness challenging in large fires that span a range of fire weather conditions. Under more moderate burning conditions, treatments were generally considered effective at mitigating fire behavior and supporting suppression efforts; however, half of the case studies reported that at least some fuel treatments were less effective or ineffective because they were overwhelmed by extreme fire behavior during periods of extreme fire weather conditions (Crook et al. 2015; Dailey et al. 2008; Fites et al. 2007a; Graham et al. 2003, 2009, 2012; Harbert et al. 2007; Henson 2007; Rogers et al. 2008). Fuel treatments are typically designed for moderate weather conditions, yet to maintain effectiveness into the future, fuel treatment designs may need to incorporate the increasing probability of extreme fire weather conditions (Stavros et al. 2014, Abatzoglou & Williams 2016).
At shorter temporal scales, seasonal variation in vegetation phenology can have important effects on fire outcomes, especially in ecosystems with an important component of shrubs and other deciduous species. For example, the case study from the Great Lakes Region (Fites et al. 2007b) reported that treatments were more effective at moderating fire behavior in a summer-season wildfire compared to a spring-season wildfire, presumably because the summer fire occurred after understory plants had leafed out, helping to reduce fire behavior in treated areas. Although we had only one study from the Great Lakes Region, this observation suggests an important role of understory phenology that could inform fuel treatment effectiveness monitoring in similar vegetation types.
Barriers to implementation and research needs
Many barriers to implementing effective treatments were identified, including limited resources and competing objectives. Declining or variable funding levels for fuel treatments have impeded consistent long-term planning, implementation, and maintenance of fuel treatments (Reiner et al. 2014). Because resources for fuel treatments are limited, small treatments targeting high-value resources are often prioritized over landscape-scale treatment designs, despite evidence that strategic treatment designs that include adjacent wildlands increase protection opportunities in the wildland-urban interface (Jackson et al. 2011; Rogers et al. 2008). Targets for accomplishing treatments across large areas can also compete with retreatment needs, leading to the deterioration of fuelbreaks and other investments (Henson 2007). In some cases, fuel reduction goals directly conflict with other resource management objectives (Reinhardt et al. 2008). For example, in northern California, the protection of dense forest habitat for species such as spotted owl (Strix occidentalis Xántus de Vésey, 1860) is sometimes viewed as in conflict with fire risk reduction efforts (Fites et al. 2007a). Lastly, managers identified the need for increased communication with community cooperators and agency partners about the risks and gains of completing fuel treatments to improve engagement and coordinate planning efforts (Reiner et al. 2014).
Although these case studies cumulatively advance our understanding of landscape fuel treatment effectiveness, critical knowledge gaps remain, and several opportunities for future research were identified by the case study authors. Specific research needs included assessing the relationship between treatment scale and fire size (e.g., the potential for fuel treatments to prevent small fires from increasing in size), evaluating fire effects across forest and rangeland mosaics in complex topography, and the need for site-specific pre- and post-treatment data. Given the finding that higher-intensity treatments (e.g., combination of thinning and prescribed burning) were generally found to be most effective, there was an expressed need to quantify the cost and benefits of different landscape strategies, such as the tradeoff between treating more acres with less intense, low-cost treatments versus targeting fewer acres with more intense, high-cost treatments. Research is also needed to determine the environmental factors that drive the longevity of treatment effectiveness to inform the development of appropriate maintenance schedules. Finally, there is a need to better understand the interaction of topography and winds with treatment effectiveness, which would inform the placement of treatments across large landscapes.
