The Effect of Broadleaf-Tree Greenup on Springtime Wildfire Occurrence in Boreal Canada

: The broadleaf tree species of the boreal biome of Canada have low flammability compared to conifers, which is in large part due to the high moisture content of their foliage. However, there is a period following snowmelt and prior to leaf budding (i.e., greenup), termed the ‘spring window’ by fire managers, when these forests are more conducive to fire ignition and spread. The goal of this study was to evaluate the length and variability of the spring window from year to year across ecological regions of boreal Canada and to determine whether it is associated with an increased number of human-caused wildfires. We used remotely sensed descriptions of snow cover and greenup to describe the annual spring window for nine ecological regions from 2001 to 2018. Then, we statistically compared the timing of the windows and associated the temporal patterns of fire-conducive weather to human-caused wildfire occurrence. The results show a positive association between the number of human-caused wildfires and the timing of the spring window in only two of the eight regions (the Boreal Plain and Taiga Plain, both in western Canada); however, these are two of the most fire-active areas in the country. A specific set of factors must coincide for regional fire regimes to exhibit a fire-prone spring window: (i) a relatively high (>20%) proportion of broadleaf forest cover, (ii) a high load of human ignitions (because lighting is rare in the spring), and (ii) frequent windy and dry weather conditions. Although the fire regimes that are active in the springtime are mostly confined to parts of western Canada at present, other areas of boreal Canada may see an increase in spring wildfires if projected climatic changes are borne out and if a growing number of people settle into boreal wildlands.