Exposure of the Canadian wildland–human interface and population to wildland fire, under current and future climate conditions

Fire can move readily between forest and structural fuels in the wildland–Human interface (WHI), which is defined as the areas where forest fuels intermingle with or abut housing, industry, and infrastructure (Johnston and Flannigan 2018). Among the major Canadian WHI fires within the last decade, the 2016 Fort McMurray wildfire remains one of the most impactful, resulting in the evacuation of 88 000 people and the loss of 2400 structures (McGee 2019). It is also the costliest natural disaster in Canadian history, with $3.7 billion in insured losses (MNP LLP 2017). Although the Fort McMurray fire is an extreme example, the Canadian WHI experiences disturbances every year, causing direct structure loss or community evacuations (Sankey 2018). Most wildland fire-related evacuations occur in sparsely populated boreal regions, where wildland fires are the most active (Beverly and Bothwell 2011). Many First Nations reserves are located in forests prone to wildland fire. They are often particularly vulnerable to wildland fire emergencies due to their remoteness and limited access (Christianson 2015). Reserves account for almost one-third of all evacuees and evacuation events from recent decades (Christianson 2015).

Wildfire impacts include any possible losses and gains for people and communities resulting from a wildfire, whether they are direct (e.g., damage to property, evacuations, and suppression costs) or indirect (e.g., business closure, unemployment rise, and psychological stress). The level of fire impacts depends on both local fire exposure and the intrinsic characteristics of the community, such as the types of assets or the distribution of population. Thus, the potential for a community to be impacted by a wildfire varies greatly over Canada. For example, southern regions are generally less exposed to wildfire but can be seriously impacted by even low intensity fire occurrences due to their higher built-up and population densities. However, extremely damaging impacts can be experienced in northern boreal areas that, although less populated and built-up, are often more physically exposed to more extreme fire behaviour (e.g., fire intensity and crowning). Moreover, many important industrial sites and infrastructure, such as hydroelectric facilities and power lines, are based in remote northern areas, and disruption to these services can have wide-ranging impacts at the regional or even provincial scale.

Fire activity is primarily driven by climate, through its control on weather conditions or on fuel productivity and flammability (Flannigan and Wotton 2001; Macias Fauria and Johnson 2008; Girardin and Wotton 2009; Parisien et al. 2011). Fine-scale fuel characteristics, such as the arrangement, moisture, quantity, and continuity of vegetation, exercise significant control over local fire behaviour and occurrence (Forestry Canada Fire Danger Group 1992; Erni et al. 2017). In particular, hardwood species and young stands (0–30 years) are associated with reduced fire ignition and fire spread in the boreal and temperate forests (Krawchuk et al. 2006; Bernier et al. 2016; Marchal et al. 2017; Erni et al. 2018). Humans also affect overall fire activity directly through ignitions (Stocks et al. 2002; Hanes et al. 2019; Mietkiewicz et al. 2020), fire management (Martell et al. 1984; Stocks and Simard 1993; Hirsch and Martell 1996; Magnussen and Taylor 2012), and fuel management (Mouillot and Field 2005; Gralewicz et al. 2012; Parisien et al. 2016; Boulanger et al. 2017), as well as indirectly through land-use alterations (Bowman et al. 2011) and climate change (Flannigan et al. 2009).

Negative impacts due to wildland fires are expected to worsen throughout the 21st century, particularly in the boreal forest. Wildland fires are projected to increase in number, size, and intensity due to climate-warming-caused increases in extreme fire weather and fire season length (Flannigan et al. 2009; Wotton et al. 2017; Coogan et al. 2019). Changes in annual area burned (AAB) have already begun (Hanes et al. 2019), and rates could increase by 1.5 to 4 times before 2100 (Boulanger et al. 2014). By the end of the century, extreme fire years and their associated suppression costs are projected to become more common for most provinces (Hope et al. 2016). Such an increase would require the doubling of agency capacity to maintain current performance levels (Wotton and Stocks 2006; Podur and Wotton 2010).

Despite the future outlook and recent dramatic impacts of wildland fire in Canada, there is no national standardized spatial assessment of wildland fire exposure and, thus, risk in the WHI. This basic information is critical to enhance resilience, by informing the development of risk reduction strategies and prioritizing mitigation and preparedness activities (McFarlane 2006; McGee et al. 2009; McFarlane et al. 2011). A full quantification of fire exposure across the country is challenged by data availability and impact uncertainty (Johnston et al. 2020). As a result, in this paper we employ several sources of data to examine components of fire exposure. Population density and WHI areas are used to indicate potential impacts. Fire return intervals (FRI; i.e., the inverse of annual burn rate) are used to indicate the frequency at which these values may be exposed to wildland fire. We investigated this “frequency of exposure” under current conditions and under future climate projections. Results are presented spatially across the country and compared between differing interface types (i.e., communities, industrial areas, and infrastructure), interface density (i.e., densely populated interface communities versus more sparsely populated areas), and between First Nations reserve and non-reserve populations.

The study area encompasses the forested regions of Canada, as represented by the Homogeneous Fire Regime (HFR) zones of Boulanger et al. (2014); it excluded the Western Subarctic region due to the absence of communities (WS; Fig. 1B). Fire activity is highly heterogeneous within this area, with the highest burn rates (i.e., regional calculation of the mean proportional area burned estimated from historical records) occurring in western Canada’s Mackenzie River watershed and on the eastern side of the James Bay (Boulanger et al. 2014). Climate is considered continental, with short warm summers, long cold winters, and generally less than 800 mm total annual precipitation across the area, except for the easternmost part of the study area where the maritime influence of the Atlantic Ocean is strong. Forest cover is dominated by boreal conifer species but is frequently mixed with deciduous species. The landscape mosaic is complex with permanent landscape structure (e.g., wetlands, peatlands, water bodies, exposed rocks) and topography influencing fire ignition and spread. Active fire suppression takes place within the southern part of the boreal forest where most human activities (e.g., forest harvesting, mining, agriculture, urban development) are concentrated.

A combination of census data and recent Canadian Forest Service spatial products were used to define the exposure to wildland fires for communities, infrastructure, and industrial areas (Fig. 2). The detailed methods to define FRI and WHI areas have been published (Boulanger et al. 2014; Bernier et al. 2016; Johnston and Flannigan 2018) but are briefly reported here.

This study is the first assessment of current and future wildland fire exposure for the WHI, over the forested areas of Canada. Our results indicate that the exposure of WHI areas and human population living within the WUI will likely increase considerably by the end of the 21st century, with a notably more dramatic increase expected for First Nations communities. Therefore, we can expect increasing impacts of wildland fires on people and their built environment. This will bring increased pressures on fire management agencies in the future, challenging their capacities (Wotton and Stocks 2006; Podur and Wotton 2010; Reimer et al. 2019) and escalating their suppression costs (Hope et al. 2016).

Impacts of wildland fire will not only affect WUI areas and populations, but also areas within the WII and the INF. The WII covers a much smaller area than the INF, as Canada has many roads and other infrastructure spread across the landscape. Much of this critical infrastructure (e.g., hydroelectric generation) and industrial activity (e.g., mining and harvesting areas) are in remote areas of the boreal forest where FRI are the shortest, at the scale of the country. Wildland fires in these nonresidential areas can have impacts on industrial operations and infrastructure with potentially far-reaching impacts that go beyond direct damage of infrastructure. For instance, the Horse River wildland fire in May 2016 resulted in local oil and gas production being shut down near Fort McMurray. This affected many workers livelihoods and measurably caused impacts on Canada’s national gross domestic product for that year (MNP LLP 2017). Similarly, in 2017, British Columbia’s numerous fires not only damaged parts of the provincial electrical grid but also forced lumber and mining companies to shut down or cut back on operations, with substantial economic losses for the companies and communities (British Columbia 2018). The potential impacts on INF areas can be quite dramatic and may even affect people living far away from highly exposed infrastructure. For example, a July 2013 fire in Quebec’s James Bay area caused smoke and heat that short-circuited high-voltage power lines. This event caused widespread outages in the province; notably, it affected the subway system in Montréal, a university hospital, several industries, and shopping centers located more than 1000 km away from the burn (Globe and Mail 2013). The projected increase in fire activity in WII and INF areas could result in even more pressure to protect industrial areas and infrastructure that is critical to communications, transportation, and power generation and transmission. These changes will be a significant challenge to policies and planning for both land development and fire management.

Our work refined the WUI product of Johnston and Flannigan (2018) by delineating the interface and intermix areas. Each represents a distinct environment and each requires specific approaches to risk reduction, given the differences in characteristics such as road access (Ronchi et al. 2019), structure spacing, fuel continuity (Caballero et al. 2007; Hammer et al. 2007; Lampin-Maillet et al. 2010; Galiana-Martin et al. 2011), and fire protection complexity and capacity (Hammer et al. 2007). Furthermore, specific preventive fuel management options may be prioritized depending on the WUI type. For example, densely populated and built-up interface areas could cost-effectively benefit from local fuel removal or fuel reduction in their immediate vicinity, because firebrands, landscaping vegetation, and other buildings are common sources of ignition (Partners in Protection 2003; Cohen 2004; Scott et al. 2016; Kramer et al. 2019). In comparison, large-scale fuel treatments, such as prescribed burns, would be an appropriate option in intermix areas. This strategy should be implemented while coping with the preferences and perceptions of the wildland landowners (Faulkner et al. 2009; McFarlane et al. 2011; Girardin and Terrier 2015).

Northern communities, which are primarily Indigenous or associated with resource-extraction industries, are particularly exposed to wildland fires and their impacts. Despite large AAB (averaging ∼2 Mha annually from 1959 to 2015; Hanes et al. 2019) and numerous evacuations (Beverly and Bothwell 2011; Canadian Forest Service 2021), fatalities in the civilian population directly attributed to wildland fires are rare in Canada. In 1938 there were 17 deaths in the Dance Township Fire in northwestern Ontario (Alexander 2010; Alexander and Buxton-Carr 2011). This was followed by over 80 years without civilian fatalities, until the July 2021 fire in Lytton, British Columbia, resulted in the loss of two civilian lives. In comparison, two other WUI events, the Black Saturday bushfires in Australia (February 2009) and the Camp Fire in the United States (November 2018) caused 173 and 85 fatalities, respectively. With the projected increase in WHI and population exposure, tragic situations may become more common in Canada. The Canadian population exposed to FRI ≤100 years is expected to increase considerably by the end of the 21st century. Our estimates are conservative, since they are based on a static population (Census 2011). In fact, the Canadian population is increasing every year, at a rate close to 1% per year for the period 2011–2016 (from 33 476 688 people in 2011 to 35 151 728 people in 2016; Statistics Canada 2018a), and to 3% per year when considering First Nations people only (from 851 560 people to 977 230 people in 2016; Statistics Canada 2018b). This population growth will be accompanied by an increase in built-up areas (i.e., urban, industrial, and infrastructure) that will likely expand the WHI area as humans encroach further into forested lands.

First Nations inhabitants on reserves are more exposed to wildland fires than other communities in the country, both in terms of living areas and population. Our results suggest that the extent of losses and the number of evacuations that First Nations populations experience could drastically increase in the coming decades. This may translate into major consequences, including more structural and cultural losses, more land alterations, and more inherent social disruptions due to evacuation (Beverly and Bothwell 2011; Christianson 2015). Unfortunately, the organization behind emergency management for some communities can be complex and unclear. Unclear lines of responsibility as well as multiple and different agreements made among the communities, provincial agencies, and federal government can generate unnecessary vulnerability and confusion in the face of a disaster (Asfaw et al. 2019). As a result, it is imperative to develop new procedures in partnership with First Nations for emergency fire management (Dodd et al. 2018). It is also important to recognize and value traditional knowledge in the design and implementation of mitigation practices, such as the historical practice of controlled burning (Christianson et al. 2014; Christianson 2015), to reduce fire risk and to enhance the long-term resilience of communities (White et al. 2011; Mistry and Berardi 2016).

In interpreting our findings, some elements must be kept in mind. Mapped FRI values in our study are local depictions of regional burn rates expressed as a function of vegetation characteristics (Bernier et al. 2016). Accordingly, they do not represent “true” spatially explicit local burn probabilities, as many local features that may influence fire spread and ignition (e.g., forest species composition, stand age, topography, water bodies, dominant wind direction) were not considered. Moreover, the attribution of the population data to the interface portion of the WUI only provides rough spatial estimates of the current population across the study area. In addition, future population growth, either in number or in interface areas, are not incorporated in our projections. Both population factors could amplify the exposure to wildland fires, including increased numbers of human caused fires, and could furthermore have effects on wildfire suppression costs (Mietkiewicz et al. 2020). WHI areas in various parts of the world are continuously expanding. This is largely fed by an increasing number of homes, which is associated with population growth and current trends in residential preferences (Radeloff et al. 2018). The urban sprawl phenomenon is progressing in most regions of Canada, particularly among peripheral municipalities (+6.9% between 2011 and 2016 versus 5.8% among central municipalities; Statistics Canada 2017), as well as in rural areas located close to census metropolitan areas. Our estimations of future FRI do not consider future change in vegetation. Climate change may have impacts on vegetation type and distribution, which is further complicated when considering potential feedback of altered vegetation on fire regimes (Gauthier et al. 2014; Syphard et al. 2018; Marchal et al. 2020). Further work on producing spatial projections in population growth and vegetation changes is needed to improve our assessment of future fire activity in relation to human land use. This will be particularly important when making complex decisions to reduce the potential catastrophic impacts to the built environment, while promoting the positive impacts of fire and considering fire impacts beyond humans and their structures.

Our work presents a first investigation of the current and future exposure of residential areas, industrial structures, infrastructure, and populations to wildfires. By showing where and in what period regions will become more exposed to wildland fires, it provides information needed by fire managers, land planners, community members, and business owners to evaluate the appropriateness of existing suppression policies and to adopt optimal mitigation strategies for the future (Partners in Protection 2003; Eiser et al. 2012; McFarlane et al. 2011; Sherry et al. 2019). Although coastal and southern regions show relatively low wildland fire exposure, numerous northern and central regions will likely experience more intense fire activity, with dramatic consequences on social and economic environments. Appropriate management approaches, whether in terms of mitigation or suppression, are required in WHI areas. These approaches depend on the local vegetation conditions and fire activity, as well as on the configuration of individual communities (e.g., distribution of intermix or interface, industrial zones, evacuation roads) and potential changes in local fire regimes due to climate warming. Our estimates indicate that over 5% of interface inhabitants live in areas with FRI ≤250 years and are potentially at risk of negative impacts from wildland fires. Among them, First Nations peoples on reserves are overrepresented, as they constitute 10% of the interface population living under FRI ≤250 years and 39% of the population when considering FRI ≤ 100 years.

We hope that our findings will raise awareness of the impacts of climate change on humans living and working in areas exposed to wildland fire. Our results show that a steadily increasing proportion of WHI area will experience more frequent wildland fires in response to a changing climate. The impacts of these climate effects will be amplified with continued population growth and land development across the country. Our findings emphasize the need to consider wildland fires in land-use planning policies to support the sustainability and resilience of humans in their built environment.

The authors acknowledge Jacqueline Oliver for her skillful editing of the manuscript. We also thank two anonymous referees for helpful comments on the earlier version of the manuscript.