Understanding spring wildfires in Canada's northern forests

The window between snow melt and leaf flush in broadleaf trees defines a critical period of wildfire susceptibility, especially in western boreal forests. Questions remain about how a warming climate might affect those two processes that bookend the spring fire season.

nicely document the relationship of spring wildfire incidence and area burned with the length of the window between snowmelt and greenup across Canada's boreal forests.This phenomenon was particularly relevant in early 2023, as millions of hectares of boreal and hemi-boreal forest were ablaze across Canada with unprecedented synchronicity and apocalypse-like wildfire smoke impacting much of eastern North America.These spring wildfires seem to be on the increase (Hanes et al., 2019) and often defy control.Such a fire destroyed one-third of the town of Slave Lake, Alberta, in 2011.The Horse River Fire in May of 2016 forced the evacuation of more than 80,000 people and burned thousands of buildings in the city of Fort McMurray, making it the costliest natural disaster in Canadian history.
Expanded wildfire seasons have been predicted as a response to the warming climate as early as the IPCC's First Assessment Report in 1990.Empirical evidence demonstrated that spring wildfires have been on the increase in the conifer-dominated forests of western North America, with the wildfire season beginning sooner as snowpacks melt earlier (Westerling, 2016).Across Canada from 1959 to 2015, recent fire seasons have been starting an average of 9 days earlier and ending 7 days later (Hanes et al., 2019).
Spring wildfires are especially prevalent in those forests with large proportions of deciduous trees, particularly trembling aspen (Populus tremuloides), which characterizes the transition from boreal or subalpine forests to the grassland biome in the Canadian prairie provinces and on the eastern slopes of the Rocky Mountains in Alberta and the United States.Those forests often have dense grassy or shrubby understories, which quickly dry in the spring after snowmelt and make up the fine fuels in which fires easily start and spread (Figure 1).Fire seasonality and ignition source (lightning or human) are important parts of the disturbance regime.Human-caused spring fires are characteristic of aspen-dominated and mixedwood boreal forests.Indigenous burning in such forests was traditionally practiced in the spring to reduce fuels and forest ingrowth, and to enhance the growth of food plants and forage for ungulates (Lewis & Ferguson, 1988).
In deciduous stands, the window for spring fires gradually closes as leaves flush in the canopy, shading and cooling the understory with foliage having much higher moisture content than conifer foliage.It is these two bookends of snowmelt and leaf flush that Parisien et al. (2023) have assessed with remote sensing for five boreal ecozones.Analysis indicates an association of human-caused wildfires (collated from the Canadian National Fire Database) with that spring window, especially in the Boreal Plains ecozone.There is evidence that those early-season fires are wind driven more than drought driven (Parisien et al., 2023), which is perhaps not surprising, given that the Build-Up Index of drought (BUI) accrues cumulatively in the absence of rainfall after onset of the fire season.
The shrub and herb understory of aspen-dominated forests dries out rapidly under sunny and windy conditions once the snow is gone in the spring.That understory vegetation can be very rich in species and productive in aboveground biomass.Composition varies throughout the range of aspen-dominated and mixedwood forests, but can be expected to have high cover of grasses such as rough fescue (Festuca spp.) or bluejoint reedgrass (Calamagrostis canadensis), forbs such as fireweed (Chamaenerion angustifolium), and deciduous shrubs such as bush cranberry (Viburnum edule), snowberry (Symphoricarpos occidentalis), and wild rose (Rosa acicularis).Twigs of trees and shrubs can remain low in moisture if the soil is not yet thawed and sap has not yet risen from the roots, while dead grass culms prevail for many weeks before new green leaves sprout from their base.
Although Parisien et al. (2023) did not report a widening spring wildfire window over the 21 years of data they examined, they nonetheless note that spring greenup dates were relatively stable within ecozones while snowmelt dates were more variable.If indeed there BURTON is a widespread advance in snowmelt timing, this suggests sort of a phenological mismatch if the composition and genetics of understory species have not adjusted to take full advantage of the growing season.Phenological maladjustments to a warming climate are widespread (Kharouba et al., 2018), including between plant flowering and the emergence of their pollinators, and disrupted feeding and breeding success of birds.The distribution of many temperate and boreal tree species may eventually be compromised by insufficient winter chilling and accelerated budbreak, which makes trees susceptible to growing season frosts (Burton & Cumming, 1995).
The Parisien et al. (2023) study hints at, but does not explore, the potential for greater boreal disturbance through spring wildfires as the climate warms.Such a mechanism is much like the greater synchrony of spruce budworm (Choristoneura fumiferana) development and black spruce (Picea mariana) budbreak that is now facilitating outbreaks of that defoliating insect in those forests (Pureswaran et al., 2015).If we are experiencing more spring wildfires, however, the biotic and abiotic shifts in seasonality will not only affect local ecological dynamics and timber supplies, but threaten human safety and infrastructure too.
Like all good research, the Parisien et al. (2023) paper prompts as many questions as it answers.For example, is there a relationship of wildfire spread (final size) or severity to the proportion of deciduous content?One might hypothesize that some intermediate ratio of aspen and spruce or pine is most susceptible, as more conifers can be expected to facilitate hotter and faster moving crown fires.We might also ask whether spring wildfires are increasing in the same regions where forests are experiencing a greater transition to broadleaf species in recent decades (Whitman et al., 2019).2023) point out that fire-conducive weather is also required for spring wildfires, which occurs more reliably in the western boreal forest than in eastern boreal ecozones.The timing of such suitable fire weather is important, as earlier fire weather also accelerates snowmelt (widening the spring window), but later fire weather accelerates bud flush, thereby narrowing the spring wildfire window.This is but one of several interesting positive and negative feedback implications posed by the tripartite system of a warming climate, disturbance regime, and vegetation response.
Another unknown is whether the temperature threshold required for the accumulation of heat sums and eventual leaf flush in the dominant boreal broadleaf species (Populus tremuloides, P. balsamifera, Betula papyrifera) might be greater than the 0°C used here.Although that 0°C base for calculating heat sums has been used in other studies of leaf flush, it also appears that tree species and provenances differ in the base temperature from which heat sums (degree-days) are accumulated.For example, Burton and Cumming (1995)  | 5985 BURTON known that over-winter chilling and photoperiod also influence the timing of leaf flush (Pletsers et al., 2015).More research and analysis is clearly needed to understand the ecophysiology of bud break and greenup not only in trees, but in shrubs and herbs as well.
Finally, it should be noted that observations of reduced burn severity in boreal forests dominated by broadleaf species are prompting many commentators to promote a greater abundance of broadleaf trees in managed forests (e.g., Astrup et al., 2018).While a greater abundance of boreal broadleaf tree species may reduce the incidence, spread, and severity of mid-summer fires, the Parisien et al. (2023) paper is a good reminder that such a policy comes with the risk of more spring wildfires too, at least in some regions of western Canada.

CO N FLI C T O F I NTER E S T S TATEM ENT
The author declares no competing interests.
Is the earlier disappearance of snow due to reduced precipitation in winter in general, more falling as rain instead of snow, or earlier and hotter spring thaw days?Conversely, what is constraining earlier spring greenup?Is soil thaw and sap mobilization under an insulating blanket of thatched grass not accelerating in pace with snowmelt?Parisien et al. ( used 3.5, 2.1, and 3.7°C as base temperatures for the above three species, respectively, although those values were inferred from very coarse species range distributions.Other studies continue to use the standard definition of "growing degree-days" with a 5°C base temperature for the accumulation of heat sums as a predictor of leaf flush (e.g., Zohner & Renner, 2014).It is further F I G U R E 1 A spring fire (April 2018) in broadleaf-dominated woodland in Prince Alberta National Park, Saskatchewan.Note the open canopy and broad connectivity of cured grass and shrubs in the understory.Photo: © Parks Canada (used with permission).