Effects of stand composition on fire hazard in mixed-wood Canadian boreal forest

Surface fuels were examined in 48 stands of the Canadian mixed-wood boreal forest. Tree canopy was characterized with the point-centred quadrant method and stands were characterized as deciduous, mixed-deciduous, mixed-coniferous or coniferous according to the percentage of conifer basal area. Woody debris loadings were measured with the line intersect method and the Litter, duff, shrub loads and depths or heights were sampled with various quadrats. No significant difference was found among stand types for total woody debris load, large basal diameter shrub loads and load or depth of litter and duff. However, conifer stands had significantly heavier loads of small diameter elements (twigs and shrubs) and conifer pieces were more numerous within these stands than in deciduous stands. The BEHAVE prediction system was used to evaluate the impact of these differences on the potential of fire ignition in situations where topography and weather were constant. The qualitative and quantitative changes in fuels, resulting from species replacement and fast decay rates, influence fire hazard. Simulations of fire behaviour showed that in the mixed-wood boreal forest fires were less intense and spread more slowly in deciduous stands than in mixed or coniferous stands. Moreover, spring fires were more intense than summer fires, and differences between seasons increased with the increase of deciduous basal area.