The spatially varying influence of humans on fire probability in North America

Humans affect fire regimes worldwide, sometimes causing profound changes to ecosystem structure and function (McWethy et al 2013). In many parts of the world, people increase fire activity beyond what is naturally expected by deliberately or accidentally setting fire to natural vegetation (Carcaillet et al 2009, Bowman et al 2011). This is the case, for example, in African savannas, where cultural practices of frequent burning make it the archetype of a 'human-tended' fire regime (Archibald et al 2012). Conversely, in other regions or ecosystems of the world, such as in parts of North America, land-use change and a culture of aggressive fire suppression may limit fire activity (Cumming 2005, Finney et al 2009). Globally, humans are usually responsible for the majority of ignitions; however, many regions where human ignitions are prevalent are also subject to intense fire management programs aimed at extinguishing fires before they become large (Stocks et al 2002, Stephens 2005). In addition to these direct effects on fire activity, humans may exert pervasive and enduring indirect effects on fire regimes by altering, or removing, the natural vegetation (e.g., change in forest structure, invasive annuals, logging, conversion to irrigated agriculture) (Cochrane 2003, Pausas and Keeley 2009, Perry et al 2012, Fréjaville and Curt 2015).

Several investigations of wildland fire in North America show a dramatic effect on fire activity following the arrival of Europeans. An initial surge in area burned at the time of settlement due to slash-and-burn practices in some areas (Weir and Johnson 1998) was followed by a large-scale reduction in fire activity that was largely the result of fire suppression and human modifications to the natural vegetation cover (Marlon et al 2008). Over the past three decades, however, substantial increases in wildfires have been reported, notably in the western United States (Westerling et al 2006) and in the boreal zone (Kasischke and Turetsky 2006). Although this phenomenon has been largely attributed to a warmer and drier climate, it is increasingly recognized that, in some areas, humans may have played a role (Stephens et al 2007, McWethy et al 2013). Paradoxically, some of the recent increases in fire activity in parts of the continent, notably in the west, may be due to a legacy of fire management policies that were too successful (Stephens and Ruth 2005). Almost a century of active fire suppression has led to unintended biomass accumulation that consequently increased the average size and intensity, as well as ecological impact, of modern fires (Keane et al 2002). The impact of humans on fire is thus inherently complex: some human activities yield more fire, whereas the opposite is true of others (Sturtevant and Cleland 2007, Parks et al 2015), thereby challenging our assessment of the net effect of humans on wildland fire activity in North America and elsewhere.

In North America, the influence of humans on fire activity is assumed to be highly variable, ranging from extreme in areas of high population and altered land use to seemingly negligible in unmanaged wildlands. Areas with historically active fire regimes (whether natural or anthropogenic), notably in the northeastern United States and adjacent Canada, now experience virtually no wildland fire (Clark and Royall 1996, Nowacki and Abrams 2008). In contrast, the heavily populated regions of southeastern North America, where a culture of prescribed burning has existed for centuries, have highly active fire regimes that are largely human dominated (Waldrop et al 1992, Slocum et al 2007). In the center of the continent, in the American Midwest and Canadian Prairies, where vast grasslands were converted to agriculture, previous high fire activity has been greatly diminished or even excluded altogether (Collins and Wallace 1990, Brown et al 2005). Currently, most fire activity in North America occurs in the western United States and in the boreal forests of Canada and Alaska, where, in spite of large pockets of urbanization and agriculture, fire is promoted by expansive wildlands combined with fire-conducive climates and vegetation (Swetnam 1993, Westerling et al 2003). However, though quite active, it has been argued that fire regimes in most forests of western North America are far from natural, having been shaped by a century of fire suppression and forest management (Stephens and Ruth 2005). The most natural fire regimes of North America are presumed to occur in the northern forests of Canada and Alaska (Flannigan et al 2009). Not only are these regions sparsely populated, but, given their regimes of low-frequency, high-intensity fires, some have suggested that fire suppression is less effective here than it is in other forest types (Johnson et al 2001).

Most evaluations of the influence of human activity on North American fire regimes have been conducted for specific countries, subregions, or landscapes (e.g., Cardille et al 2001, Sturtevant and Cleland 2007, Syphard et al 2007, Gralewicz et al 2012, Hawbaker et al 2013, Liu and Wimberly 2015). Moreover, published studies have been carried out at various temporal and spatial scales and using different data types and modeling techniques, thereby preventing an unbiased comparison among regions (but see Bistinas et al 2013). A continent-wide assessment that uses a consistent approach would complement previous efforts with the added benefit of providing a baseline for large-scale policy or management plans. As such, the overarching goal of this study was to conduct a comprehensive evaluation of the effect of humans on fire activity across North America. Our specific objectives were to (1) assess the importance of human influence on fire activity and (2) evaluate the shape of this relationship (positive, negative, unimodal). This was achieved by partitioning the United States and Canada into 16 hexagonal polygons (size = 1.38 × 10⁶ km²) and, for each hexagonal polygon, creating a statistical model of fire probability (based on area burned) for the 1984–2014 time period as a function of climate, enduring features (topography and percent nonfuel), lightning, and indices of anthropogenic influence. These models allowed us to (1) statistically control for the effect of other factors when assessing the influence of humans on fire activity and (2) evaluate the spatial variability in these relationships at a continental extent.

The study area covers the United States (US) and Canada (hereafter North America; 19 437 101 km²) (figure 1(a)), excluding Hawaii and the Caribbean islands. Full coverage of North America was not possible due to lack of data for Mexico. The study area was partitioned into 16 hexagonal polygons (hereafter hexels) (hex 1 to hex 16) having a width of 1260 km and an area of 1 374 902 km² (figure 1(b)). Explorations showed that this size and placement of hexels represent a good tradeoff between capturing subcontinental variability in fire activity and producing robust models of fire activity across the study area. The response variable is sampled within burned areas, and consequently the modeled response quantifies the probability that any given pixel burned over the 1984–2014 time period (hereafter fire probability). For each hexel, we built a statistical model of fire probability as a function of climatic normals, enduring features (topography and nonfuel), lightning, and measures of anthropogenic influence.

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The fire probability models were fully comparable among hexels, as they were built using the same set of explanatory variables and used the same model settings. We evaluated the importance of indices of anthropogenic influence to determine the human impact on fire activity by controlling (statistically) for the effect of climate, enduring features, and lightning patterns in each hexel. We then plotted the relationship of each of these metrics to assess whether fire probability decreased, increased, or had a nonlinear (e.g., unimodal) relationship to the anthropogenic variables in each hexel.

Models of fire probability were built as a function of several explanatory variables likely to influence fire activity for each hexel. Although a large set of independent variables was initially considered for modeling, we selected a subset of variables that were not highly correlated with one another and also had good explanatory power for predicted area burned across North America (table 1; figure B1). The description that follows focuses on the variables selected for the modeling. The full list of variables is found in table A1. We processed all model variables using a North America Albers equal-area conic projection at a 1 km pixel resolution.

Fire

Climate

Enduring features

Lightning

Anthropogenic influence

Variable selection

Assessment of anthropogenic influence

Overall, models of fire probability as a function of all variables (i.e., the full model) performed well (figure 2). The test AUC averaged 0.776 among hexels, and ranged from 0.672 (hex13) to 0.891 (hex8). AUC values increased, sometimes substantially, when adjusted for prevalence (mean 0.804).

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The relative importance of climate, enduring features, lightning, and anthropogenic variables varied substantially among hexels (figure 2). For all hexels, the two most influential categories of variables were climate and anthropogenic. The anthropogenic category was ranked as the most important in four hexels (hex2, hex7, hex9, hex12) and the second-most important in the remaining 12 hexels. Climate was the most frequent dominant control on fire activity, having the highest variable importance in all but four hexels (i.e., the ones dominated by anthropogenic variables). Enduring features ranked third in terms of importance in all hexels except four (hex1, hex5, hex8, hex10), where this category was the least important and lightning had the third-highest contribution. Lightning was otherwise the least important variable. The importance of each anthropogenic variable, calculated both from the full model and models with a single anthropogenic variable, is reported by hexel in table 2. All three anthropogenic variables were useful in explaining fire probability models, but their relative importance varied substantially among hexels.

The relationship between fire activity and human influence varied as a function of the specific anthropogenic variable, but was overall fairly coherent among hexels. Area burned usually decreased as a function of increasing LogPopDens, except in hexels where there was a strong (hex8) or moderate unimodal response (hex1, hex2, and hex3) (figure 3). The HumanFoot variable had a decaying form in all hexels, though the range of slopes varied considerably (figure 4). The response to the RdlsVol was the most diverse of the three anthropogenic variables (figure 5). Fire activity increased monotonically as a function of RdlsVol in some hexels (hex3, hex4, hex5, hex6, hex7, hex13), decreased in some hexels (hex10, hex12, hex15, hex16), and was unimodal in others (hex3, hex8, hex9, hex14). The bivariate relationship between fire activity and anthropogenic variable show broadly similar patterns to the partial dependence plots (figure D1).

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Because of its extensive wildlands and low human densities over much of its territory, North America is often viewed as having fire regimes that are closer to natural than those in other parts of the world (Lavorel et al 2007). Although this may hold true to some degree, Cardille and Lambois (2010) have shown in the conterminous US that, even in seemingly 'intact' areas, very few landscapes are completely free of some human imprint. The results of this study similarly suggest that the human impact on fire regimes is widespread and pervasive. Climate is nevertheless the dominant environmental control on fire in most of North America, thereby supporting the claim that its fire activity today—as it has been for millennia—is mainly regulated by top-down controls (Swain 1973, Flannigan et al 2005, Guyette et al 2012). There are clearly, however, other 'non-natural' controls that exert an influence on fire, even overriding the effect of climate in some portions of the continent, as shown by Hawbaker et al (2013) in the conterminous US. We found a non-negligible impeding influence of humans on fire, even in some of the least densely populated areas. Though our results show a remarkably coherent negative influence of humans on fire, the specific nature—and hence outcome—of the human–fire relationship is far from uniform. A potentially interesting exception to the negative effect of people on fire is observed in some of North America's most remote areas where, despite the lack of humans, we saw little fire activity.

One of the most surprising aspects of this study's results is the relatively strong influence of humans on fire activity across North America. Anthropogenic effects were usually substantially greater than enduring features and lightning, and even greater than climate in some areas. Our expectations were that more sparsely populated parts of the continent would have a relatively lower anthropogenic effect on fire. As shown in all of the boreal hexels (hex10, hex12, hex13, hex14, hex15, hex16), this expectation did not bear out. More aligned with our expectations was the high anthropogenic impact of the Midwest (hex4, hex8), Great Plains (hex5, hex9), and Gulf of Mexico (hex2, hex4), areas that have undergone widespread land conversions and that are at a more advanced stage of anthropogenic fire-regime transformation (Guyette et al 2002, Bowman et al 2011). By contrast, some heavily human-altered hexels of the conterminous US show an unexpectedly moderate effect of humans on fire activity, perhaps because climate is simply much more dominant than other environmental controls. This may be the case in coastal California (hex3), where a clear anthropogenic effect (Syphard et al 2008) may be masked by strong climate forcing (Moritz et al 2010).

Any large-scale effect of population density on fire activity can be confounded by the type of human activities on the landscape (Hawbaker et al 2013). For example, the culture of prescribed burning in the southeastern US and, more recently the tolerance of lightning-ignited fire in some of the protected areas in western North America, may be idiosyncratic and not found elsewhere at similar population densities. Overall, our results agree with those of Knorr et al (2014) and Hantson et al (2015), who found a globally negative relationship between fire and population density, and are coherent with the conclusions of Marlon et al (2008) and Mouillot and Field (2005), who reported a decrease in area burned in relation to increased population trends in boreal and temperate biomes. As observed in this study, Bistinas et al (2013) found this relationship to be spatially variable, but there are discrepancies in the direction of relationships (positive and negative) between the two studies. In some parts of the continent (southwestern US, Florida, and the Great Lakes area), we observed the unimodal relationship reported by Syphard et al (2007), whereby the peak in fire ignitions occurred at intermediate population densities, suggesting that both intense human activity and the near absence of humans is associated with fewer fires. We also observed that the per capita effect of humans on fire activity varies widely across North America. Notably, in the boreal zone we found a non-negligible human impact on fire activity in spite of that region's low population densities. This may be due in part to the spatially expansive nature of human features and policies of aggressive fire suppression (Cumming 2005, Martell and Sun 2008). In this region, the effects of humans on fire are spatially structured, in that most fires are ignited by humans close to roads, but it is the remote lightning-ignited fires that are responsible for the majority of the area burned (Gralewicz et al 2012).

While many large-scale wildland fire studies may include a single variable describing anthropogenic impacts on fire (e.g., Krawchuk et al 2009, Bistinas et al 2013), we included three conceptually different variables to capture varying aspects of the fire–human relationship. We were expecting the Human Footprint Index, which accounts for land use and infrastructure in addition to population density, to have a different relationship with area burned compared to population density. Results, however, show that population density and the Human Footprint Index have a remarkably similar (negative) relationship with fire. This provides further support to the idea that, at the spatiotemporal frame of this study, the anthropogenic factors that impede fire activity (e.g., suppression, landscape fragmentation) outweigh those that promote it (e.g., ignitions) in North America. Our third index, the roadless volume, tells a somewhat different story, and therefore we caution against using a single index to measure human influence on fire regimes. Even our use of multiple indexes can probably not describe all important human impacts. For instance, the invasion of exotic cheatgrass species (Bromus spp.) have profoundly modified fire regimes in parts of the western US (Balch et al 2013, Parks et al 2015), but are not fully captured by our three indexes. The same could be said for the effect of past fire management policies on current fire activity, given that these have likely altered the nature of the fire−environment relationship in some areas (Higuera et al 2015, Ruffault and Mouillot 2015).

The roadless volume represents a more accurate depiction of the degree of isolation from human activities than the population density and the Human Footprint Index. That is, the 'stretch' of values in this metric is more heavily weighted towards the distance from roads (i.e., low-impact areas), translating into a more diverse set of human−fire relationships. Much of the northern and western parts of North America, for instance, show a unimodal response of fire to roadless volume, which suggests less fire in the most isolated areas (mainly protected areas) compared to areas of moderate-to-high remoteness. These results are consistent with what Parisien et al (2012) reported in the western US; however, our results show that a single response for the western US masks the variability in the fire responses across such a large area. Notwithstanding, there is a relative decrease in fire probability in some of the most remote areas of some hexels. Reasons why some remote areas may have experienced relatively less fire than adjacent anthropogenized areas may be due to differences in fire environments, past fire-management policies, or simply that there have experienced comparatively fewer ignitions (Cardille et al 2001, Miller et al 2012, Haire et al 2013).

We suspect that the unimodal response of fire to human influence (roadless volume) in some areas of North America is the result of ignition limitation, as observed in some studies in California (Syphard et al 2007, Keeley et al 2011) and other parts of the globe (Bradstock 2009, Kraaij et al 2013). Flannigan et al (2005) hinted that this may be the case in the largely uninhabited parts of the lightning-poor boreal zone. Some of the most remote areas of the western US may also be ignition limited simply because they encompass some of the highest mountains and because lightning ignitions are deficient at very high elevations (Dissing and Verbyla 2003) (figure E1). It must be noted that some remote areas may have experienced more fire in the past if they were subjected to frequent burning by humans, but the extent to which these past fire regimes were 'natural' or not is debated and likely varies greatly among areas (Vale 2002, White et al 2011). Nevertheless, in an era of rapid and sometimes drastic change, it is important to evaluate the potential effect of changes in ignition loads, especially given the sensitivity of some ecosystems to changes in fire activity (McWethy et al 2013).

Limitations

The results of this study suggest that there may be few truly natural fire regimes in North America today. While this study points to a general impeding effect of people on fire across North America, it also paints a complex picture of anthropogenic effects on fire across the continent—one where fire is as variable as the biophysical environment that defines it. Because of this complexity, the specific mechanisms by which humans alter fire ignition and spread may be difficult, or even impossible, to identify. Further, humans can indeed distort or mask fire–climate relationships, thereby undermining our ability to predict current and future fire activity in a changing climate (Parks et al 2014). Consequently, this study's results should be viewed as another building block towards more in-depth investigations of the spatial variability of human impacts on North American fire regimes. The statistical relationships reported here could, for example, be incorporated into a process-based model to enhance our understanding of changing human–fire relationships (e.g., Thonicke et al 2010). As adaptation and mitigation measures are developed for fire-prone areas, it will become imperative that wildland fire scientists and land managers account for human influences in projections of future fire.

This study was partially funded by the Adaptation Program of Natural Resources Canada. We are grateful to Nicolas Mansuy and two anonymous reviewers for constructive comments.

Model evaluation erlaa3227t4followed the method of Parisien et al (2012) and was performed on each of the 100 subset models and subsequently averaged for each hexel. As a measure of overall of model performance, the AUC was computed from the true positives and false positives. AUC values may range from 0.5, where prediction accuracy is no better than if samples were randomly selected, to 1, which indicates perfect classification accuracy. However, in a presence-only framework, as in this study, it is impossible to achieve an AUC value of 1 because absences (hence false positives) are unknown. The maximum achievable AUC in a presence-only framework is equal to 1 – a/2, where a is generally the fraction of the study area covered by fire (i.e., the prevalence). For the sake of adjusting the AUC value, we considered a to be the percentage of pixels where fire was observed. This provides a fair, yet underestimated, approximation of prevalence. Also calculated were the estimated fraction of the suitable (i.e., burnable) area, which represents an approximation of the false positive rate, and the omission, which is the false negative rate. These two metrics were measured at the fire probability threshold that minimizes the sum of these errors of the two metrics (Liu et al 2005). As such, they can be interpreted as the expected rate of false negatives for a given predicted suitable area.