The strength of fuel self-regulation in Alaska’s wildfire regimes (1984-2020)

Fuel depletion following wildfires in the North American boreal forest can temporarily reduce the probability of a subsequent fire. Landscape-wide shifts to younger, less flammable forests may act as an important negative feedback to a warming-driven increase in biomass burning, but only if regenerating-fuel limitations remain strong during extreme fire weather. Here, we describe the climate sensitivity of regenerating-fuel limitations within Alaska’s historical fire regime by analyzing 568 examples where fires either spread into or stopped at the edge of a previously mapped burn between 1984 and 2020. We classified the binary outcome (reburn or limited incursion) for every 30-m (meter) pixel located along the contact perimeters of interacting fires and then used logistic regression to assess the number of years since last fire (YSLF), contact perimeter length, and fire weather indices as predictor variables. During the first 30 YSLF, contact perimeters became ~2.5x more likely to not prevent active fires to spread into recently burned areas, and up to 2x more likely when fire weather conditions were more extreme. Rates of reburning by area were one to three orders of magnitude lower than rates of burning in greater than 30-year-old fuels. Using these observations to parameterize a statistical fire model, we estimate that the rates of historical burning would have been ~3x greater without the negative feedback created by fuel limitations. We conclude that fuel limitation has been an important, self-regulating aspect of Alaska’s wildfire regimes and this negative feedback will continue to significantly limit fire spread.