Fire spread probabilities for experimental beds composed of mixedwood boreal forest fuels

Although fuel characteristics are assumed to have an important impact on fire regimes through their effects on extinction dynamics, limited capabilities exist for predicting whether a fire will spread in mixedwood boreal forest surface fuels. To improve predictive capabilities, we conducted 347 no-wind, laboratory test burns in surface fuels collected from the mixedwood boreal forest of Saskatchewan. The beds were composed of single fuel types of contrasting characteristics, including feather moss, aspen leaf litter, aspen and alder leaf litter, and twigs. Shredded wood (i.e., excelsior) was included for comparison. An extinction index and logistic model from the literature that balances heat sources and sinks performed well for excelsior, a fuel used to develop the model, but poorly for forest fuels. As a result, we used logistic regression to develop a model for forest fuels finding that fire spread was largely determined by the heat sink, heat of combustion, and fuel bed depth. We found close correspondence between our model and fire spread in an independent sample of beds composed of mixtures of mixedwood fuels (N = 59). Our model can serve as a means of analyzing the relative importance of fuels and weather on extinction dynamics during mixedwood boreal forest fires.