Does conserving roadless wildland increase wildfire activity in western US national forests?

We investigated the influence of management regime on fire ignitions, extent, and severity within national forests in the 11 conterminous western US states (WA, ID, MT, OR, CO, CA, NV, UT, CO, AZ, NM; figure 1). We limited our study to this geographic scope because the vast majority of national forest lands (82%) are found within these states and because the USFS has adopted lower size thresholds for protecting roadless areas and wilderness areas in the eastern states. Also, many eastern US national forests consist of formerly intensively managed private lands acquired by the federal government, which complicates inference about the effects of management regime on fire pattern. Most USFS lands in the west have remained in the public domain since Euro-American colonization (Bramwell 2012). We delineated national forest lands using USFS spatial data layers and divided them into two categories: roadless and roaded areas (figure 1, table 1). Roadless areas fall within the administrative boundaries of lands protected by the Wilderness Act or inventoried roadless areas protected by the Roadless Rule. Roaded areas include the remainder of the national forest lands managed for multiple use objectives.

We evaluated the influence of roadless management on fire ignitions and probability of those ignitions escaping initial suppression efforts using the Fire Occurrence Database (FOD) maintained by USFS fire management staff. We defined escaped fires as those ignitions that grew to be larger than 121 ha, a size at which the USFS generally expects wildfire to exceed initial attack capabilities (Fried and Fried 1996, Calkin et al 2005). At the time of our analysis the FOD included information about fire ignition location and ultimate fire size for ∼2 million ignitions in the United States between 1992 and 2017 (Short 2017). Although other data sources, including the Monitoring Trends in Burn Severity (MTBS) program (see below), contain information about fires prior to 1992 we used the FOD because 90% of area burned in the study area occurred after 1991 and because the FOD contains detailed information about the spatial and temporal origin of fires (including fires that did not grow to the minimum size mapped by other data sources) essential for evaluating the influence of land use and biophysical variables on the fate of wildfire ignitions.

To evaluate the influence of management regime on fire extent we downloaded fire perimeter data from the MTBS program (www.mtbs.gov), which at the time of our analysis mapped all large (>405 ha) fires in the United States between 1984 and 2017 (Eidenshink et al 2007, Finco et al 2012). We added fire perimeter data compiled by the Geospatial Multi-Agency Coordination (GeoMAC; www.geomac.gov) to extend our fire extent data through the year 2018. We excluded GeoMAC fire perimeter data for fires less than 405 ha for consistency with MTBS fire perimeter data.

To evaluate fire severity we created seamless maps of the relative differenced normalized burn ratio (RdNBR) at 30 m resolution using Landsat satellite time-series within each MTBS fire perimeter (Kennedy et al 2018). RdNBR is an objective measure of fire-induced changes in vegetation (i.e. tree mortality) and soil reflectance that accounts for differences in vegetation across different regions (Miller and Thode 2007). We provide a detailed account of our fire severity mapping methods as supplementary materials (see supplementary materials S1 (available online at stacks.iop.org/ERL/16/084040/mmedia)).

We summarized the total number of ignitions documented in the FOD across both roadless and roaded portions of the western states national forest study area. Then we estimated the influence of biophysical variability and management regime on the probability of fire escape using generalized additive models (GAMs) with a binomial distribution implemented with the mgcv package in the R statistical environment (Wood 2004, 2006, R Core Team 2018). Many of the ignitions on national forest lands catalogued in the FOD that occurred between 1992 and 2017 were close together in time and space and may be temporally and/or spatially autocorrelated. We included individual national forest and year of fire as random effects in models, reasoning that variability in escape probability could be explained in part by the resources available to individual administrative units of the national forest system and the year in which ignitions occurred.

We summarized fire extent across roadless and roaded portions of the study area using the complete inventory of area burned within our western states study area provided by MTBS/GeoMAC data. To better understand variability in fire extent across national forest lands, we summarized total area burned in roadless and roaded areas within each of 23 different level III ecoregions delineated by the US Environmental Protection Agency located within the study area (Omernik 1987). We do not report results from 12 ecoregions that are composed of less than 2% national forest land or in which fire burned less than 1000 ha during the study period.

To account for environmental differences between roadless and roaded areas that potentially influenced fire extent, we analyzed 100 000 points randomly located across the study area. We constrained random location procedures so that points were at least 500 m apart, a distance that the literature suggests avoids spatial autocorrelation in fire effects that could bias coefficient estimates (van Mantgem and Schwilk 2009, Lanorte et al 2013, Kane et al 2015). We examined autocorrelation function plots and performed Moran's I tests to ensure that there was no significant autocorrelation among points. We estimated the influence of management regime (roadless or roaded) and a wide variety of biophysical variables (table 2) on the probability that these points would fall within a 1984–2018 fire perimeter using a GAM with a binomial distribution. We graphed the probability of fire along 30 year average precipitation and maximum temperature gradients because these climate variables are key indirect controls of natural fire regimes and good surrogates for environmental variability at the broad spatial scales we considered (Westerling 2008, Guyette et al 2012, Liu et al 2013, Whitman et al 2015).

To evaluate differences in fire severity attributable to management regime, we summarized mean RdNBR for the roadless and roaded portions of fire perimeters between 1985 and 2017, leveraging pre- and post-fire Landsat imagery spanning 1984–2018. Like our summaries of fire extent, our summary of fire severity represents a comprehensive census of fire across the study area. To account for environmental differences between roadless and roaded areas that potentially influence fire severity, we modeled the influence of biophysical variability and management regime on RdNBR with a GAM using the subset of the 100 000 random points (20% of points) that fell within 1985–2017 MTBS fire perimeters.

Our goal with statistical models was to determine if management regime had a significant influence on fire escape, fire extent, and fire severity given environmental variables that influence fire. All continuous variables were modeled as smoothed functions in order to capture non-linear relationships between the fire response and environmental gradients. All categorical variables, including management regime, were modeled as parametric variables. We tested a variety of different vegetation classifications in wide use as predictor variables in models (table 2) and used the vegetation classification that explained the most variability in the response. We report the influence of management regime to be significant when confidence intervals (alpha = 0.05) for the effect of roadless and roaded management on the fire response examined do not overlap.

A number of the continuous environmental variables that we tested as explanatory variables for fire escape, fire extent, and fire severity responses were correlated. For instance, maximum temperature was strongly correlated with elevation and latitude. We evaluated full models that included all biophysical variables and parsimonious models created using forwards and backwards model selection procedures that maximized deviance explained and minimized Akaike Information Criterion while eliminating terms with concurvity indexes that exceeded 0.60. Previous research suggests that this threshold provides a conservative approach to eliminating variables that describe similar environmental gradients (Johnston et al 2019). Because there was little difference in the estimated effect of management regime between full and parsimonious models, we report results from parsimonious models to improve interpretability. Model specifications and a detailed summary of our statistical methods are found in supplementary materials (see supplementary materials S2).

Of the 194 151 total ignitions within the western US national forest study area currently documented in the FOD, 3902 (2% of the total) escaped initial suppression efforts and grew to be >121 ha. The majority of ignitions (74% of the total) occurred within the roaded portion of the national forest landscape, but an almost equal number of fire ignitions (1925) escaped initial control efforts within roadless areas as escaped within roaded areas (1977). Of fire ignitions in roadless areas, 4% escaped control compared to 1.4% of ignitions in roaded areas. GAM models indicated that management regime exerted a significant influence on probability of fire escape even after accounting for topographic ruggedness, slope steepness, vegetation type and other factors that influence the ability of managers to suppress fires. Escaped fires that began in roadless areas grew to be one-third larger than escaped fires that began in roaded areas (table 3). Of the 20 largest fires on national forest land from 1984 to 2018, more than three-quarters began within roadless areas, which account for slightly less than half of the total area of western US national forests (table 4).

Roadless areas were associated with a far greater extent of fire relative to roaded areas. Between 1984 and 2018, an area equivalent to 30% of the total area of roadless lands experienced fire, whereas an area equivalent to 18% of roaded areas experienced fire (table 1). This difference is striking because roadless areas within the western states national forest study area contained a larger proportion of cooler and moister environments than roaded areas (figure 2). Management regime exerted a strong influence on the probability of fire even after accounting for vegetation type and biophysical variables. The general shape of the fire probability response while holding other vegetation and biophysical variables constant was similar across climate gradients in roadless and roaded areas (figure 3). In both roadless and roaded landscapes, probability of fire was lowest in very cool and moist settings where climate is less conducive to fire and in very hot and dry settings where fuel is sparse. Probability of fire was highest in areas with precipitation sufficient for fuel to accumulate and dry conditions suitable for fire spread (Krawchuk et al 2009, Pausas and Ribeiro 2013, Whitman et al 2015). However, the probability of fire was significantly higher in roadless areas across these gradients. There was little overlap in confidence intervals for estimates of probability of fire in roadless and roaded areas except within temperature and moisture regimes where fire was relatively rare (figure 3).

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Although roadless areas were associated with a substantially greater extent of fire than roaded areas, there was enormous variability in the extent of fire within different ecoregions (figure 4). Some ecoregions with extensive national forest land such as the Northern Rockies and Wasatch and Uinta Mountains have experienced relatively little fire, with fire evenly distributed between roadless and roaded areas. Other ecoregions with extensive national forest land such as the Idaho Batholith and Arizona/New Mexico Mountains have experienced extensive fire and a notably greater extent of fire in roadless areas. Several ecoregions stand out for the dramatic extent of fire in roadless areas, particularly when including multiple overlapping fires (i.e. reburn). An area equivalent to 86% of the large (1090 342 ha) national forest area managed as roadless wilderness in the Klamath Mountains/California High North Coast Range ecoregion burned between 1984 and 2018, while only 40% of the roaded portion of this ecoregion experienced fire during the same period. Similarly, an area equivalent to 100% of the relatively small (280 782 ha) roadless national forest land in the Central California Foothills and Coastal Mountains ecoregion burned during the same time period compared to 67% of the remaining roaded lands. Approximately 15% of total fire extent across the western states study area between 1984 and 2018 occurred within the roadless portion of the Idaho Batholith ecoregion. We provide a detailed summary of fire extent and severity in different ecoregions as supplementary materials (supplementary materials table S3.1).

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Management regime had no influence on fire severity in any of the statistical models we tested. The difference in mean RdNBR between roadless and roaded areas was negligible given the wide range of RdNBR values in our dataset (figure 5, table 1). Vegetation type and biophysical variables that influence the distribution and abundance of vegetation including elevation, annual precipitation, and maximum summer temperature explained as much as 17% of the variance in the severity response compared to approximately 1% of the variance explained by management regime. Confidence intervals for the estimated effect of management regime on RdNBR overlapped zero in all models we tested that included both vegetation and biophysical variables. There were no consistent differences in fire severity between roadless and roaded areas at the scale of individual ecoregions (see supplementary materials table S3.1).

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The greater incidence of fire ignitions in roaded areas but the greater extent of fire in roadless areas that are generally less conducive to fire is most likely related to three factors. First, proximity to developed areas is associated with more ignitions (Balch et al 2017). Second, a lack of road access limits deployment of the full range of suppression tools available to managers to control fires in roadless areas when they are small (Dunn et al 2017). Third, large remote landscapes allow managers flexibility to allow fires to burn when these fires are compatible with resource management objectives or when suppression resources are strained by numerous large fire events (van Wagtendonk 2007, Haire et al 2013, Thompson et al 2017). Both human infrastructure and human ignitions are increasing rapidly within the wildland urban interface (Mietkiewicz et al 2020). These trends, in conjunction with spiraling fire suppression costs, may ultimately result in divergent fire regimes, with fires in roaded areas influenced primarily by the exigencies of suppression efforts while fire extent and severity in roadless areas become increasingly self-organized (Riley et al 2018).

Although differences in fire escape and fire extent are strongly associated with management regime, our statistical analyses demonstrate that the primary drivers of fire severity across the national forest landscape are differences in the fire environment and not land use designations per se. The small difference observed in fire severity between roadless and roaded areas (figure 5, tables 1 and S3.1) likely reflects differences in the abundance of different tree species between these management regimes. Trees growing in roadless sites with generally greater moisture availability and lower temperatures are less fire tolerant and more susceptible to mortality from fire than species found in drier, lower-elevation landscape settings (Dunn and Bailey 2016, Johnston et al 2019, Stevens et al 2020). Even without accounting for differences in mortality attributable to differences in species composition and the fire environment, the difference in fire severity between roadless and roaded areas is minor. For example, one study that calibrated Landsat-derived RdNBR with plot-based fire severity data in the Pacific Northwest found that the mean fire RdNBR values for roadless vs roaded areas across our study corresponded to 58% and 52% mortality of overstory trees from fire (Reilly et al 2017).

Limiting smoke exposure to vulnerable populations and reducing risk to water supplies, habitat, and human infrastructure from uncontrolled 'mega-fires' are important goals of policy makers (McKenzie et al 2014, Vaillant and Reinhardt 2017). Mechanical fuel treatments are a commonly used tool to accomplish these objectives (Stephens et al 2012), but more than half of all fires, including most of the largest fires in western US national forest lands, burn primarily in roadless areas where mechanical treatments are generally prohibited (tables 1 and 4). The extent of fire in landscapes where management options are limited drives home the need to adapt to fire rather than overcome fire in the American West (Dunn et al 2020).

Feedbacks between biophysical and policy systems may compound divergent trends in fire extent between roadless and roaded areas. The USFS is placing an increased emphasis on firefighter safety, which will result in fewer fire engagements in remote, complex terrain that are associated with firefighter casualties and which are more prevalent in roadless areas (Page et al 2019). Additionally, the greater extent of fire in roadless areas means that new fires are more likely to occur within previous fire perimeters. Suppressing these new ignitions may be extremely difficult and unsafe because of extensive patches of dead trees and difficult medical evacuation, potentially leading to a greater number of fires escaping initial suppression efforts in the future (Dunn et al 2019).

The disproportionate extent of fire in roadless areas may present challenges for managers depending on their objectives. For instance, roadless areas in coastal southern California are associated with large and costly fires that pose significant risk to adjacent densely populated communities (Keeley et al 2009). In other regions, repeated fire in dry mixed-conifer forests may result in new vegetation states with fundamentally different habitat potential (Coppoletta et al 2016, Serra-Diaz et al 2018, McCord et al 2020). Managers may need to adjust conservation strategies for sensitive species that rely on closed canopy forests within areas that are dominated by roadless areas given the extent of fire associated with this management regime (Spies et al 2006). For instance, in the Klamath ecoregion, a network of late-successional reserves designed to protect and restore habitat for the northern spotted owl (Strix occidentalis caurina) and other old-growth associates is anchored within roadless areas (Reilly et al 2018, Spies et al 2019). The extent of fire in these areas may require managers to conserve additional complex older forest within the roaded portion of the landscape or to develop a dynamic reserve system in which reserves are reallocated based on disturbance-mediated changes to habitat (Spies et al 2018).

Despite these challenges, in many areas throughout the western states, fire associated with management for roadless and wilderness characteristics has the potential to confer resilience in the context of climate change. Fire suppression has sharply reduced wildfire activity on national forest lands over the last 100–150 years, and most western forests are in a 'fire deficit' relative to the natural fire regime (Marlon et al 2012, Parks et al 2015). Lack of fire has resulted in increased forest density, shifts in species composition, and loss of resiliency to fire, drought, and insect outbreaks (Hessburg et al 2005, Stephens and Fulé 2005, Collins et al 2011). A number of recent studies have shown that forests in wilderness and roadless areas that have experienced multiple fires are less likely to experience stand-replacing fire and are recovering structural and compositional characteristics that were prevalent prior to Euro-American colonization (Larson et al 2013, Parks et al 2014b, Coop et al 2016). Climate change will increase flammability of most forests in the American West, and recent fire occurrence has a strong potential to moderate future fire effects and promote more diverse landscapes (Parks et al 2016, Hurteau et al 2019).