Forest Type and Wildfire in the Alberta Boreal Mixedwood: What Do Fires Burn?

Two determinants of fire behavior are fire weather and spatial variation in fuels. Their relative importance in boreal forests has been unclear. I evaluated the effect of fuels on a ∼74 000-km2 landscape in the boreal mixedwood region of western Canada. My data were the compositions, or the proportional areas of different forest types, of 48 mapped lightning fires and of their immediate surroundings. I measured areal compositions from forest inventory maps, using a five-way classification representing deciduous forest, three types of coniferous forest, and wetlands. The fires burned between 1980 and 1993. Fire sizes ranged from 70 ha to 70 000 ha. By multivariate linear regression, fire surroundings explain 57% of the variation in forest types within mapped fires. Fire compositions are not representative of the study area as a whole, or of a fire's surroundings, and are unrelated to fire size and location within the study area. Using the model, I predicted the areas of the five types burned within all other lightning fires >200 ha in the study area during 1961–1996 and estimated type-specific mean annual burn rates. These rates vary by an order of magnitude. Deciduous stands burn at the lowest rate, and black spruce stands burn at the highest rate. Fires exhibit significant preferences between forest types at both local and regional scales. Preference orderings are similar at both scales and are generally consistent with the rank order of estimated burn rates. Preferential burning may result from between-class differences in vertical canopy structure and foliage characteristics. The statistical model and the postulated variations in fire behavior between classes indicate that landscape-scale fuels management may be feasible in this system. The rank ordering of burning frequencies and preferences is the inverse of the planned disturbance rates under forest management.