Broadleaf tree phenology and springtime wildfire occurrence in boreal Canada

Abstract

Although broadleaf tree species of the boreal biome have a lower flammability compared to conifers, there is a period following snow melt and prior to leaf flush (i.e., greenup), termed the “spring window” by fire managers, when these forests are relatively conducive to wildfire ignition and spread. The goal of this study was to characterize the duration, timing, and fire proneness of the spring window across boreal Canada and assess the link between these phenological variables and the incidence of springtime wildfires. We used remotely sensed snow cover and greenup data to identify the annual spring window for five boreal ecozones from 2001 to 2021 and then compared seasonality of wildfire starts (by cause) and fire‐conducive weather in relation to this window, averaged over the 21‐year period. We conducted a path analysis to concomitantly evaluate the influence of the spring window's duration, the timing of greenup, and fire‐conducive weather on the annual number and the seasonality of spring wildfires. Results show that the characteristics of spring windows vary substantially from year to year and among geographic zones, with the interior west of Canada having the longest and most fire‐conducive spread window and, accordingly, the greatest springtime wildfire activity. We also provide support for the belief that springtime weather generally promotes wind‐driven, rather than drought‐driven wildfires. The path analyses show idiosyncratic behavior among ecozones, but, in general, the seasonality of the wildfire season is mainly driven by the timing of the greenup, whereas the number of spring wildfires mostly responds to the duration of the spring window and the frequency of fire‐conducive weather. The results of this study allows us to better understand and anticipate the biome‐wide changes projected for the northern forests of North America.

Understanding spring wildfires in Canada's northern forests

“…Parisien et al (2023) 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.…”

“…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 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).…”

“…Is soil thaw and sap mobilization under an insulating blanket of thatched grass not accelerating in pace with snowmelt? Parisien et al (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.…”

“…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.…”