For species that are reliant on mature forest conditions, larger fires have negative short-term effects through reduction in available habitat. In NWT, the two largest fire seasons on record (2014 and 2023; 2.9 M and 4 M ha, respectively; Walker et al., 2018; Jain et al., 2024) impacted the same general region around Great Slave Lake in southern NWT resulting in vast tracts of disturbed forest accumulating across a 9-year period (Figure 2). As the climate warms and dries, we expect such repeated instances of extreme fire years like we have seen in NWT, but the extent of the impact on mature forest specialists is not yet fully known owing to the relative novelty of these large events. For example, boreal caribou (Rangifer tarandus caribou) are particularly reliant on mature forests for access to arboreal and terrestrial lichens which are their predominant winter food (Schaefer and Pruitt, 1991; Thompson et al., 2015). Reindeer lichens (Cladonia subgenus Cladina) are readily consumed by fire and, due to their extremely slow growth rates (∼4.9 mm/yr; McMullin and Rapai, 2020), can take several decades to recover to pre-fire biomass found in mature forest stands (McMullin et al., 2011; Silva et al., 2019; Greuel et al., 2021). We expect that increasing occurrence of both larger individual fires and higher total area burned will lead to winter habitat loss (Palm et al., 2022) and could lead to boreal caribou congregating at higher densities in small patches of remaining mature forest during winter. While boreal caribou avoid burns most strongly during winter (Konkolics et al., 2021), they may actually select for recent burns during calving (Silva et al., 2020) or summer (DeMars et al. unpublished data) likely due to the greater availability of protein-rich forage provided by early-successional vegetation that helps female caribou meet the nutritional demands of lactation (Denryter et al., 2017). When boreal caribou do use areas within burns, in all seasons they select areas closer to burn perimeters (Palm et al., 2022). Thus, a very large individual fire event may lead to greater habitat loss for caribou than several smaller fires amounting to an equivalent area due to edge effects (i.e., their avoidance of areas deeper within burns).

Similarly, there are many mature forest specialists in the landbird community that are vulnerable to widespread burning (e.g., Canada Warbler [Cardellina canadensis], Brown Creeper [Certhia americana], and Evening Grosbeak [Coccothraustes vespertinus]; Gillihan and Byers, 2020; Poulin et al., 2020; Reitsma et al., 2020; see also text in short-interval reburn section below). Alternatively, landbird species associated with early seral stages (e.g., Alder Flycatcher [Empidonax alnorum] and Dark-eyed Junco [Junco hyemalis]) or habitat generalists (e.g., American Robin [Turdus migratorius]) would thrive or not show a numerical response to larger fire events (Schieck and Song, 2006). Recently, Lane-Shaw et al. (unpublished data) used predictions from species distribution models for 144 landbird species generated from the Boreal Avian Modelling Center (Stralberg, et al., 2025) to quantify potential changes in abundance of these species breeding in mature forests of NWT following the 2023 fire season. Preliminary results suggest that the most negatively impacted species include the mature forest obligates Rusty Blackbird (Euphagus Carolinus), Ruby-crowned Kinglet (Corthylio Calendula), Golden-crowned Kinglet (Regulus Satrapa), and Bay-Breasted Warbler (Setophaga Castanea). For these species, an estimated ∼20% of their populations breeding in mature forest experienced lost or altered breeding habitat in NWT. In total, ∼59,000 breeding males had habitat in mature forests (>80 years) that burned in 2023 across NWT.

Fire affects foraging opportunities for bats. Recently burned habitat often experiences a “pulse” of higher insect productivity (Lacki et al., 2009; Malison and Baxter, 2010). Fire also changes forest structure by reducing understory vegetation, making flying and foraging easier for bat species that are less tolerant of clutter (Blakey et al., 2019); this can increase activity of bat species such as the little brown myotis (Myotis lucifugus) in burned landscapes (Buchalski et al., 2013; Blakey et al., 2019; Low et al., 2024). In contrast, northern myotis (Myotis septentrionalis) is more dependent on forest cover (Henderson et al., 2008) and better adapted to foraging in densely vegetated environments (Norberg and Rayner, 1987; Ratcliffe and Dawson, 2003). As such, northern myotis is expected to be more be negatively impacted by larger burned areas compared to the little brown myotis (Blakey et al., 2019; Snider et al., 2013; Bosso et al., 2018; Kotliar et al., 2002; Azeria et al., 2011; Doty et al., 2023).

Despite the potential for widespread mortality of small, ground dwelling mammals during a large fire event, their high reproductive capacity and ability for some individuals to survive the fire in situ, means that they can recolonize burned areas relatively quickly (Hale et al., 2022). In boreal North America, deer mice tend to be the most abundant species immediately post-fire, perhaps owing to their omnivorous diet, followed by red-backed voles (Clethrionomys gapperi) which may repopulate burns within the first 3 years (Olson et al., 2003; Simon et al., 1998 - reviewed in Fisher and Wilkinson, 2005; Zwolak and Foresman, 2007). Meadow voles (Microtus pennsylvanicus) and meadow jumping mice (Zapus hudsonicus) may also be more abundant than red-backed voles in the short-term due to their association with grassy cover that may recover quickly post-fire (Fisher and Wilkinson, 2005). Marten (Martes spp.) and weasels (Mustela spp.), which primarily prey on small mammals, may benefit from edge habitat created by fires that locally increase small mammal populations; however, large homogenous fires may decrease edge habitat and habitat suitability for marten as they avoid areas of low canopy closure and may be reluctant to forage far from the forest edge (Volkman and Hodges, 2024). Red squirrels, conifer seed specialists that are more common in mature forests, may similarly forage within the periphery of recent burns (Fisher, 1999), but are unlikely to recolonize the center of large burns until conifer forests recover.

Reptiles such as red-sided garter snakes (Thamnophis sirtalis parietalis) may experience indirect impacts of large boreal wildfires linked to post-fire changes in predator-prey dynamics and habitat suitability (Rochester et al., 2010). As species that rely on habitat structure to avoid predation, they may experience higher predation rates and lower survival in large, recent burns (Doherty et al., 2022; Howey et al., 2016; Russell et al., 1999; Webb and Shine, 2008; Wilgers and Horne, 2007). Reduced cover (i.e., removal of shrubs, woody debris and the litter layer, and opening of the forest canopy) can make it easier for avian predators to detect them (Doherty et al., 2022; Wilgers and Horne, 2007). Reptiles can benefit from the opening of forest canopies or dense shrub thickets that increase surface temperatures (Bury, 2004; Howey et al., 2016; Russell et al., 1999); however, widespread removal of vegetation, leaf litter and woody material can reduce the thermal heterogeneity that they require (Howey et al., 2016; Smith et al., 2001).

There are many impacts of fire on amphibian species, but those most impacted by large fires will likely be those reliant on cool, moist microhabitats and that use ground cover consumed by fire (e.g., leaf litter) (Rochester et al., 2010; Russell et al., 1999), such as the wood frog (Rana sylvatica) (Bailey et al., 2025; Constible et al., 2001). Reduced habitat structure, lower soil moisture and more variable soil temperatures in their terrestrial habitats present amphibians with increased risk of overheating, desiccation, and predation, reduced availability of habitat refugia and invertebrate prey, and barriers to movement (Browne et al., 2009; Bury, 2004; Cline and Hunter Jr, 2016; Hossack et al., 2013; Rochester et al., 2010; Russell et al., 1999; Schurbon and Fauth, 2003). Amphibians are also affected by changes to their aquatic habitats post-fire and watersheds that are completely combusted in large burns will see the biggest changes. Reductions in dissolved oxygen seem consistently negative for boreal amphibians whereas other chemical changes have been shown to negatively affect wood frog and boreal chorus frog (Pseudacris maculata) abundance but are better tolerated by western toad (Anaxyrus boreas; Jager et al., 2021; Browne et al., 2009). Increased solar radiation and contaminants (e.g., wildfire ash) in their aquatic habitats have negative effects on amphibians (Bancroft et al., 2008; Gomez Isaza et al., 2022; McDonald et al., 2018; Muñoz et al., 2019; Xu et al., 2024). Loss of vegetation surrounding wetlands can reduce protection from temperature extremes and predators, with negative effects on amphibian survival (Beranek et al., 2022) and reproductive success (Muñoz et al., 2019). Wetland habitats also experience changes in productivity, water temperature, and hydroperiods that affect amphibians (dos Anjos et al., 2021; Gomez Isaza et al., 2022; Schurbon and Fauth, 2003). However, certain amphibians are more resilient to large fire extents and might even show positive population responses (reviewed by dos Anjos et al., 2021). For example, western toads exhibit rapid tadpole development and are habitat generalists able to reproduce in almost any water body, which facilitates colonization of burned habitat (dos Anjos et al., 2021) but does not necessarily result in longer-term increases (Hossack et al., 2013).

Very large fires can alter post-fire forest composition through their effects on dispersal distances required for establishment. Depending on the pre-fire forest composition and combustion severity of canopy seed sources, soil seedbanks, and below/near-ground budding structures, on-site reproductive sources may be insufficient when distance to an unburned edge is large. This can lead to lagged recovery, post-fire forest compositional changes (e.g., conifer to deciduous), poor recruitment leading to forest structural changes (Girard et al., 2008), or recruitment failure (Johnstone et al., 2010b). For example, white spruce (Picea glauca) is a non-serotinous conifer; white spruce stands within a large burn complex may fail to self-replace owing to long distances to the nearest seed source coupled with relatively low dispersal distances of white spruce seeds (Dobbs, 1976). In contrast, trembling aspen (Populus tremuloides) seeds can travel as much as 10 km from the nearest seed source (Turner et al., 2003) and aspen produces seed prolifically in the summer following a large wildfire year (J. Baltzer, personal observation; Landhäusser et al., 2019). As such, large burn complexes, especially in conifer dominated areas, may be particularly susceptible to forest compositional change (Johnstone et al., 2010b). Although a few large fire complexes may be beneficial in supporting landscape heterogeneity, where many large fire complexes occur in proximity as has occurred in southern NWT, this change can create challenges. Changes in overstory composition alter the ground vegetation community composition (e.g., Day et al., 2020; Greuel et al., 2021) with implications for wildlife forage availability (Jorgensen et al., 2023) and habitat use (Palm et al., 2022) (Figure 3).

For some wildlife species, changes in forest composition from conifer to deciduous dominance will be beneficial. For example, moose (Alces alces) occur throughout boreal North America and consume a variety of forage species including coniferous and deciduous trees and shrubs, graminoids, forbs, and aquatic plants (Joyal and Scherrer, 1978; Jung et al., 2018; Timmermann and McNicol, 1988). However, moose generally prefer deciduous trees and shrubs, notably willow (Salix spp.), birch (Betula spp.), and poplar (especially trembling aspen) which are commonly a dominant component of moose diet throughout the year (Risenhoover, 1989; Shively et al., 2019). For other wildlife species, such changes will be detrimental. For example, landbird species associated with mature conifer forests, such as Olive-sided Flycatcher (Contopus cooperi; Altman and Sallabanks, 2020), Canada Jay (Perisoreus canadensis; Strickland and Ouellet, 2020), Boreal Chickadee (Poecile hudsonicus; Ficken et al., 2020) and others are expected to decline in abundance as forests transition from conifer to deciduous. Conversion to deciduous forests would also likely be detrimental to boreal caribou as they avoid most age classes of deciduous stands relative to other landcover types from late-fall to late-winter (DeMars et al. unpublished data). However, species associated with mature deciduous or mixedwood forests such as the Warbling Vireo (Vireo Gilvus), Magnolia Warbler (Setophaga Magnolia), and Ovenbird (Seiurus Aurocapilla) will likely thrive for some time (see Short-interval reburning) because of such changes in forest composition (Scheick and Song 2006). Knaggs et al. (unpublished data) showed that, in NWT, early seral stage or habitat generalist species tend to be at a higher probability of occurrence between 3 and 10 years post-fire (e.g., Chipping Sparrow, Spizella passerina, White-throated sparrow, Zonotrichia albicollis, American Robin) and 11–30 years (e.g., Alder Flycatcher, Lincoln Sparrow, Melospiza lincolnii, White-crowned Sparrow, Zonotrichia leucophrys), while other species associated more with mature forests started peaking 11–30 (e.g., Hermit Thrush, Catharus guttatus), 31–50 (e.g., Swainson’s Thrush, Catharus ustulatus), or >51 years post-fire (e.g., Yellow-rumped Warbler, Setophaga coronata). Other bird species like the Ruffed Grouse (Bonasa umbellus) and Willow Ptarmigan (Lagopus lagopus) might be more abundant in recent burns due to increase in forage availability (Cringan, 1958; Rusch et al., 2020; Snow, 1996; Weeden, 1963), while Spruce Grouse (Canachites canadensis) would be more abundant in mature conifer forests (Cringan, 1958; Ellison, 1975).

While short-term post-fire changes in small mammals communities are fairly well documented (reviewed in Fisher and Wilkinson, 2005), the longer-term implications of larger fires are less clear. The combined effect of fire size and homogeneity of the burn with respect to fire severity is likely a more important driver of longer-term recovery of small mammal populations than fire size alone. Generally speaking, the abundance of small mammals increases with time after fire (Griffiths and Brook, 2014), and the relative abundance of different small mammal species within the community shifts with forest succession (Fisher and Wilkinson, 2005). For example, red-backed voles begin to replace deer mice as the most common species in the small mammal community once shrubby and herbaceous vegetation replaces grasses (Fisher and Wilkinson, 2005). American marten (Martes americana) used 21-year-old burns in NWT that had abundant standing snags and deadfall (Latour et al., 1994), suggesting they will make use of recent burns provided there is sufficient residual structure to provide subnivean access to prey and/or shelter. Populations of red squirrels and northern flying squirrels will take longer to recover within large burns as both are reliant on habitat features common to mature conifer forests such an abundant supply of conifer seeds (red squirrels), large live and dead trees used for dens, and as fungal and lichen food sources (flying squirrels) (Fisher and Wilkinson, 2005).

The extent of burns can affect patterns of amphibian occupancy in the longer term, through increased distance to source populations (Bailey et al., 2025; Hossack et al., 2013) and reduced habitat connectivity that impedes movement (Beranek et al., 2022; Cline and Hunter Jr, 2016). These effects would be influenced by heterogeneity and the existence of habitat refugia within large burn perimeters (Mahony et al., 2022; Schurbon and Fauth, 2003). For wood frogs, post-fire breeding persistence in wetlands can be high if unburned habitat is available nearby, but probability of persistence decreases when a greater proportion of the surrounding terrestrial habitat is burned, and it could be several years or decades before wood frogs return to breed in these areas (Bailey et al., 2025). A similar relationship was found for Columbia spotted frogs (Rana luteiventris) with effects on occupancy 7–21 years after wildfire (Hossack et al., 2013). Large, intense wildfires can also negatively affect persistence of amphibian species and communities across a broad geographic area and periods of time, and that even species with large populations and/or ranges can be affected (Beranek et al., 2023).

For snakes, habitat refugia (i.e., unburned areas) are important for persistence after fire and re-establishment in the longer term (Robinson et al., 2013; Beaupre and Douglas, 2012). Larger fires could reduce the number and proximity of habitat refugia on the landscape in boreal systems (McKenzie et al., 2004; Mackey et al., 2021), potentially limiting the ability of red-sided garter snakes to recolonize after fire (Santos et al., 2022), especially if large fires simultaneously affect many individuals and/or den sites, a situation that was observed in 2023 (J. Wilson, pers. obs.). Nevertheless, snake populations can persist in situ and be resilient to fire (Halstead et al., 2019; Maloney, 2024; Russell et al., 1999; Smith et al., 2001), even large fires (Santos et al., 2021).

In summary, responses to large burn areas or fire sizes vary considerably across taxa. Species reliant on large tracts of mature forest or nearby forest edges will fare poorly in the short- and longer-term as area burned increases and access to these habitats decreases. Many taxa are ambivalent to burned condition per se but have forest structural and compositional requirements for protective or thermal cover or other habitat attributes that will determine their response to the fire. Taxa preferring more open habitats or early post-fire vegetation types will fare well in the face of large burns. Longer-term land cover changes associated with large burns (i.e., conifer to broadleaf deciduous) will favour some species and disadvantage others, effectively altering the structure of terrestrial wildlife communities.

More complete belowground combustion (i.e., severe burning of the soil organic layer and tree roots) can result in marked structural changes to the forest with implications for wildlife habitat conditions (Figure 6). Specifically, more severe belowground combustion should accelerate post-fire tree fall because the combustion of tree roots and the organic soil supporting them compromises the structural integrity of the trees. Dead fallen vs dead standing trees alter post-fire regeneration conditions (e.g., via shading), fuel structure, decomposition, and wildlife use (Palm et al., 2022; Parker et al., 1984; Parro et al., 2015). For example, increased deadfall in burns, either immediately post-fire or as standing dead trees that fall down over time, is thought to impede movements of boreal caribou (caribou avoid burns from ∼11–30 years post-fire; J. Hodson personal communication) and may be one of the mechanisms contributing to their avoidance of burns (Schaefer and Pruitt, 1991; Palm et al., 2022). More severe belowground combustion at the time of fire will accelerate tree fall and may therefore limit access to or movement through burns for a longer period of time and limit access to the protein-rich summer forage that caribou access in new burns. Combustion of roots and soil in 2023 was noticeably greater than in the next largest fire year in NWT (2014) and immediate post-fire tree fall was widespread, which was much less common in the 2014 fire season (J. Baltzer, personal observation).

Severe burning directly impacts wildlife. For example, Knaggs et al. (2020) tested for the effects of fire severity (the first 2 years post-fire) on a community of 42 landbird species nesting in the large 2014 Birch Lake burn complex and surrounding unburned areas in southern NWT. Functional diversity, based on life history traits (foraging and migration strategy and foraging, breeding, and nesting substrate), decreased with increasing fire severity. The same pattern emerged with species richness, but only in peatlands. Unburned peatlands had higher species richness than unburned uplands, but species richness became more similar across habitat types as burn severity increased (Knaggs et al., 2020). Density or occupancy models were generated for 20 species and 86% of them showed significant burn severity effects (positive or negative), sometimes depending on prior vegetation (i.e., uplands vs peatlands). For example, the Olive-sided Flycatcher showed higher densities in recent burns irrespective of fire severity, while the Common Yellowthroat (Geothlypis trichas) showed the opposite response. Other species, such as the Black-backed Woodpecker (Picoides arcticus) and Three-toed Woodpecker (P. dorsalis), benefit from the emergence of nesting substrate and food resources because of large numbers of standing dead trees the first few years post-fire (Knaggs et al., 2020). Clearly there are differences in species responses, and we have insufficient knowledge of the responses of most wildlife to severe burning. Moose may respond negatively in the short-term to burn severity. Post-fire forage availability and burn severity were not strongly related across different soil moisture classes in a <5-year-old fire in north-central British Columbia, and moose used areas of low and medium burn severity more than areas of severe burning (McNay et al., 2021).

There are few published studies evaluating effects of fire severity on boreal small mammals, but studies from temperate forests found that, in the short-term, generalist species like deer mice were more abundant in areas of high burn severity (Culhane et al., 2022; Fontaine and Kennedy, 2012), whereas species dependent on the litter layer such as shrews were most negatively affected (Culhane et al., 2022). Fires that severely burn vegetation cover and seed or berry crops (in tree crowns, seed banks, and live shrubs) had negative impacts on foraging behaviour of specialist small mammals (e.g., voles [Microtus sp.] and Albert’s squirrel [Sciurus aberti]) but less so for generalist species (e.g., mice [Peromyscus sp.]) in a study in the southern United States (Morandini et al., 2023). In some cases, small mammal abundance does not differ across fire severity categories, but diversity and hence community structure are reduced after fires (Olson et al., 2003), especially after the most severe fires (Culhane et al., 2022). Terrestrial predators of small mammals like weasels select severe burns, which likely reflects greater availability of their preferred prey (deer mouse and meadow vole) in these areas (Volkman and Hodges, 2024). In contrast, Pacific marten (Martes caurina) favor lower severity burns which may reflect their need for areas with more abundant snags and downed woody debris for foraging and higher abundance of primary prey species like red-backed vole and red squirrel (Volkman and Hodges, 2024).

Because high severity burning generally results in greater loss of above- and below-ground biomass (Certini et al., 2021), severely burned areas are likely less suitable for amphibians and snakes owing to reduced cover and drier soil with more variable temperatures (Jager et al., 2021; Constible et al., 2001; Smith et al., 2001). Although lack of cover would typically increase predation risk to snakes, amphibians, and small mammals, it is possible that the risk from avian predators could be lower in severe burns with few or no remaining standing trees (i.e., perches for raptors such as American kestrel [Falco sparverius]; Widén, 1994) and substantial downed wood that can serve as protective cover. Changes to aquatic habitat for amphibians (e.g., oxygen levels; water temperature; concentrations of ash, sediment and nutrients; loss of vegetation around wetlands) are also exacerbated with severe fire (Jager et al., 2021; Gomez Isaza et al., 2022; Santos et al., 2022; Beranek et al., 2023).

Although fire can help to create roosting habitat for boreal bats by damaging or killing large trees (e.g., Johnson et al., 2009), these trees must remain standing after fire. Accelerated post-fire tree fall due to severe burning, combined with less residual vegetation (i.e., fewer green islands) in severe burns, would likely result in fewer roosts available on the landscape. Little brown myotis may be less impacted than northern myotis if they have access to alternate roosts (e.g., buildings). While reduction in vegetative clutter in forests can improve foraging opportunities for some bats (e.g., little brown myotis, big brown bat [Eptesicus fuscus]; Blakey et al., 2019; Low et al., 2024), severe burning can result in poor foraging habitat that is too open and well-lit for bats to fly (Jung, 2020). The short-term pulses in insect productivity that typically occurs after fire (Lacki et al., 2009; Malison and Baxter, 2010) are less likely to occur, or at least at a lower magnitude, in high severity burns with extensive combustion of above- and below-ground biomass; in fact, insect abundance may actually be lower post-fire (Dole et al., 2023). Few studies on the response of bats to fire have been conducted in the boreal forest where the availability of roost trees on the landscape is limited and bats’ foraging opportunities are already constrained by high latitude. However, a large, severe wildfire in Yukon created large patches of homogeneously burned areas that had negative effects on little brown myotis (Jung, 2020). Most importantly, the fire resulted in the nearly complete loss of suitable roost trees: although dead trees (snags) remained in burned uplands, none of them were sufficiently large (≥30 cm diameter at breast height) to meet the roosting requirements of this species (Crampton and Barclay, 1998; Jung et al., 2004). Sites where at least some large standing trees remain are likely very important for the persistence of bats post-fire (Jung, 2020).

More severe burning increases the likelihood of post-fire forest compositional change (typically conifer to deciduous) and even complete recruitment failure (i.e., forest to non-forest; Johnstone et al., 2016; Whitman E. et al., 2019; Baltzer et al., 2021). Severe canopy combustion can lead to the death or combustion of seeds in aerial cone banks, reducing rates of establishment post-fire (Reid et al., 2023; Splawinski et al., 2019). Likewise, more complete combustion of the soil organic layer alters seedbed conditions by exposing underlying mineral soil, favoring faster growing, broadleaf deciduous taxa over the historically dominant spruce species (Johnstone et al., 2016; Whitman E. et al., 2019; Baltzer et al., 2021). Indeed, modelled projections of these processes demonstrate strong likelihood of landscape level forest compositional changes at high latitudes from conifer to broadleaf deciduous in response to climate warming and increasing fire (Mekonnen et al., 2019; Micheletti et al., 2021). Forest compositional changes or loss attributable to severe combustion can be further compounded by fire in permafrost environments such as those of NWT where fire accelerates thaw. Specifically, burning, but particularly severe burning where there is more complete combustion of the soil organic layer that protects permafrost (i.e., ground perennially at or below 0 °C) drives rapid permafrost thaw (Talucci et al., 2024; Holloway et al., 2020). This can lead to a range of outcomes, but in thaw sensitive landscapes (e.g., permafrost peatlands), both forest compositional changes and net forest loss are possible in response to thaw (e.g., Lara et al., 2016; Baltzer et al., 2014). As described above, a transition from conifer to deciduous may favor moose and lead to habitat losses for mature conifer forest specialists like boreal caribou (Stewart et al., 2023) and squirrels. Boreal bats and other tree-roosting species are associated with large and/or dead trees, therefore recruitment failure because of severe burning that turns forest into non-forest (Johnstone et al., 2016; Whitman E.et al., 2019; Baltzer et al., 2021) would reduce the amount of available habitat. Similar responses are anticipated for the woodpecker community and all other species associated with mature forests. Deep burning also modifies the post-fire soil microbial community with implications for biogeochemical cycling and plant community composition and dynamics, adding complexity to projections of post-fire successional dynamics (Day et al., 2019; Whitman T. et al., 2019; Eckdahl et al., 2023; Eckdahl et al., 2024).

Boreal caribou is a species at risk for which we have a better understanding of long-term impacts of severe burning. During winter, boreal caribou avoid areas of higher burn severity for up to 30 years, whereas in summer they exhibit weak avoidance of high and low severity burn areas (Palm et al., 2022), or even similar selection to unburned forest (A. Kelly, personal communication). Areas selected within burns corresponded to areas with higher percent cover of lichens, and remaining lichen cover within burns was negatively related to burn severity (Palm et al., 2022; Pinno and Errington, 2016), highlighting the importance of variable burn severity and associated post-fire residual vegetation. Negative relationships between winter movement speeds of barren-ground caribou and burn severity have also been found which may indicate greater foraging activity within areas of lower burn severity. However, movement speeds in all burns were still higher than in unburned forests suggesting recent burns were primarily used for movement rather than foraging (Rickbeil et al., 2018). On the other hand, moose may benefit from high severity fires, though this appears to depend on time after fire and time of year. Moose forage biomass was up to three times greater in high severity burns than low severity burns in a 14-year-old fire in Alaska, and moose consumed more forage biomass and a higher proportion of available forage in these high severity burns (Lord and Kielland, 2015). However, areas of lower burn severity may provide a combination of both forage and cover that favors use by moose. For example, in a 20-year old fire in Alaska, moose selected areas of low burn severity in winter, and high burn severity in summer (Brown et al., 2018).

Lower severity burning leads to greater likelihood of self-replacement in conifer stands (Baltzer et al., 2021). Notably, spruce establishes well on thicker residual soil organic layers, which is not the case for faster growing broadleaf deciduous trees or pine (Johnstone et al., 2010a; Baltzer et al., 2021). Lower severity fire also corresponds with a greater frequency of residual vegetation (i.e., “green islands”; Perera et al., 2009), which may enhance landscape connectivity of burned areas, helping to accelerate regeneration, maintain structural diversity, and promote use by species known to avoid wildfires (reviewed in Sommers and Flannigan, 2022). For example, boreal caribou have been shown to use residual patches within burns for calving demonstrating their importance in reducing impacts of fire on this species (Skatter et al., 2017).

Similarly to the short-term effects, many of the longer-term effects of fire on ground-dwelling, less mobile species described earlier (see “Area burned and fire size”) will be more pronounced with higher severity burning. Widespread combustion of features that these species use as refugia, like logs, hollow trees or burrows, could slow or prevent the recovery of local populations (Robinson et al., 2013) and potentially reduce their resilience to future fires. Species dependent on cool, moist microhabitats, like wood frogs, may show reduced persistence and time-lagged declines in occupancy, especially where a large proportion of their terrestrial habitat is severely burned (Bailey et al., 2025; Hossack et al., 2013; but see Hunter, 2022). Changes to amphibian breeding wetlands (e.g., productivity, hydroperiod, contaminants) are more dramatic when burning is severe (dos Anjos et al., 2021; Gomez Isaza et al., 2022).

In summary, critical impacts of severe burning in the short term relate to post-fire forest structure that affects landscape permeability, availability of large snags for roosting and foraging, and protective cover from predators and thermal variation. Longer term land cover changes associated with severe burning (i.e., conifer to broadleaf deciduous) will favor some species and disadvantage others, altering the structure of terrestrial wildlife communities in similar ways as large fires but likely more consistently reinforcing forest compositional change than is the case for large fires.

Short-term consequences of short-interval reburning may mirror those of high severity burning for several taxa. These include more complete combustion of material legacies including the soil organic layer (Hoy et al., 2016; Walker et al., 2018; 2019) and standing and downed woody debris (Turner et al., 2019), as well as delayed regrowth of vegetation (Day et al., 2020; Hollingsworth et al., 2013). These changes will create challenging conditions for wildlife species requiring soil moisture, standing trees, vegetation structure, and cover (e.g., amphibians, red-sided garter snake, bats, woodpeckers; Figure 9), with potential impacts on foraging, survival, reproductive success and dispersal (Bailey et al., 2025; Beranek et al., 2023; Bury, 2004; Doherty et al., 2022; Hossack et al., 2013; Howey et al., 2016; Mahony et al., 2022; Muñoz et al., 2019; Rochester et al., 2010; Russell et al., 1999; Schurbon and Fauth, 2003; Webb and Shine, 2008; Wilgers and Horne, 2007; Thomas and Jung, 2019; Jung, 2020; Grindal and Brigham, 1999; Patriquin and Barclay 2003; Henderson and Broders, 2008). Where short-interval reburning is low severity, we expect more woody biomass to remain on site, including live trees in many cases and the soil organic layer should be more intact. As such, the short-term consequences of short interval fire will depend strongly on the severity of that fire.

In the longer term, short-interval reburning has a variety of consequences for regeneration. When fires are too close together in time, reproductive structures (cones, rhizomes) are damaged or destroyed causing forest compositional change, commonly conifer to broadleaf deciduous forest, or with multiple short-interval fires, failed regeneration leading to shrub thickets or grassland (Hayes and Buma, 2021; Whitman E. et al., 2019). Short-interval reburning results in more complete combustion of the soil organic layer (Hoy et al., 2016; Walker et al., 2018; 2019), which negatively impacts rhizomatous species and can lead to a shift toward reproduction from off-site seed sources (Day et al., 2020; Hollingsworth et al., 2013), leading to longer vegetation recovery times. These changes may not be unprecedented in the history of this area. Indeed, there is evidence from paleoecological studies that more widespread burning has reduced forest cover dramatically in the past (e.g., Girardin et al., 2024). Whether the type of short interval burning we are seeing today will move the system back toward this state is uncertain but initial reconnaissance in the southern NWT suggests that transitions from forest to prairie or aspen parkland ecosystems may be common in response to short interval reburning in southern NWT (Figures 10a,b). Where short-interval reburning is low severity, there may be many live trees remaining (e.g., Figure 10c) and recovery of ground vegetation may be much more rapid owing to the maintenance of on-site seed sources and intact belowground structures. However, very few data are available for either short or long-term impacts of short interval burning, the variability in its outcomes, or the vegetation conditions that are most likely to support this fire behaviour. This is a major gap in our understanding of changes in fire behaviour.

Forest compositional change, longer recovery time, or failed regeneration due to short-interval reburning would have direct consequences for the wildlife taxa that rely on mature conifer forests. Reduced conifer forest recovery on the landscape would decrease habitat availability for species reliant on that forest type for all or part of their resource requirements. For example, as lichen biomass in NWT forests may take more than 70 years to recover to half that found in late-successional forests (Greuel et al., 2021), decreasing fire return intervals will for the most part lead to a loss of winter habitat for boreal caribou. In contrast, short-interval reburns leading to conversion from conifer to broadleaf deciduous stands may benefit species such as moose that predominantly forage on deciduous browse during winter (Fisher and Wilkinson, 2005; Maier et al., 2005); however, burns in peatlands may not lead to an increase in food availability for moose (DeMars et al., 2019). Similarly, conversion to grasslands associated with recruitment failure may benefit wood bison (Bison bison athabascae) that prefer to forage on grasses and sedges (Larter and Gates, 1991; Redburn et al., 2008) and meadow voles whose abundance and survival is positively correlated with grass cover (Adler and Wilson, 1989). Black bears (Ursus americanus), which are a generalist omnivore species, may also benefit from conversion to grasslands or deciduous vegetation as they tend to consume grasses, forbs and deciduous plants during spring and summer (Mosnier et al., 2008; Romain et al., 2013; Lesmerises et al., 2015; Jorgensen, 2021). Black bears tend to shift towards using younger conifer-dominated stands later in the summer and fall (Tomchuk, 2019), likely reflecting greater availability of berries in these habitats (Nelson et al., 1983; Tomchuk, 2019), and bears may thus benefit from shorter-interval reburns if they increase the prevalence of these stands on the landscape. When short-interval reburning is not stand replacing, fire will remove ground fuels and can leave an open canopy of mature trees, a fire behaviour that commonly occurs in mature jack pine stands (Figure 10c; Stocks, 1989). Anecdotally, many of these stands support high biomass of caribou lichen (J. Baltzer pers. obs.; M. Parisien pers. comm.), which would be beneficial for caribou winter habitat.

Repeat burning can also reduce the quality of habitat conditions in fresh burns by consuming material legacies from the previous fire. Notably, short-interval reburning can lead to the complete combustion of standing and downed woody debris, which reduces protective cover and habitat for a variety of wildlife and modifies the regeneration microenvironment dramatically for plants (Figure 8). For example, although snowshoe hare favour deciduous browse during winter, their need for protective cover (usually provided by conifer forest) might preclude them from benefiting from widespread conversion to deciduous-dominated stands or shrublands (Hutchen and Hodges, 2019). Similarly, lynx (Lynx canadensis) avoid travelling through open areas in recent burns and rely on the presence of preferred prey (snowshoe hare) meaning likely avoidance of short-interval reburns (Vanbianchi et al., 2017). Predators that rely on downed woody debris for subnivean access to prey and shelter, such as American marten (Viau et al., 2024), would also likely be negatively impacted.

Landbirds associated with mature forests such as Canada Warbler (Cardellina canadensis) (Reitsma et al., 2020) and Evening Grosbeak (Coccothraustes vespertinus) (Gillihan and Byers, 2020) are expected to experience important nesting habitat loss in response to shorter fire return intervals. The Olive-sided Flycatcher (Contopus cooperi) is an aerial insectivore often considered a “fire specialist”, but it also thrives at mature coniferous/recent burn ecotones and open mature conifer stands (Altman and Sallabanks, 2020). Thus, the anticipated loss of these edges and adjacent mature stands owing to very large fires and short fire return intervals will likely result in important breeding habitat loss for this species and other species associated with mature forests. For example, the entire woodpecker community will likely also experience important declines owing to loss of standing snags critical for feeding and nesting. Similar responses are anticipated for other species (e.g., bats and northern flying squirrels) using large, mature and dead trees (Senior et al., 2021; Loeb and Blakey, 2021). Similarly, woody debris used by amphibians and reptiles for habitat and refugia (Robinson et al., 2013) may not be available if wildfire occurs too soon after the last fire (Hossack and Pilliod, 2011), and deep accumulations of leaf litter used by amphibians can take years to recover from fire (Schurbon and Fauth, 2003). If more frequent large fires lead to a shift in the age-class distribution of the landscape towards higher prevalence of younger stands, we could see a shift in the boreal small mammal community towards dominance of early-successional or non-forest species like deer mice and meadow voles and lower populations of red squirrels.

In summary, most of the documented impacts of short-interval burning are those of loss of aboveground biomass and conversion of forests from conifer to broadleaf deciduous dominated or to a non-forested state. These changes will drive marked shifts in the wildlife community occupying areas of short-interval fire. However, the impacts, both short and longer term, are mediated by the severity of the fire (see Figure 11) such that low severity, short interval fire may have relatively modest or even beneficial effects in some cases.