Variation in flammability of jack pine/black spruce forests with time since fire in the Northwest Territories, Canada

The main objective of the study was to describe the variation in flammability with time since fire of jack pine (Pinus banksiana Lamb.) stands having a black spruce (Picea mariana (Mill.) B.S.P.) understory at maturity. The study area was located in the Hay River district, Northwest Territories (Canada). Three very different methods (experimental fires, expert opinion survey, and fire behavior models) provided results that converged towards a sigmoidal model of flammability as a function of time since fire. The model shows a low flammability immediately after a stand-replacing crown fire, followed by a sharp increase during the period between approximately 5 to 25 years after the disturbance, and a plateau thereafter where the flammability remains relatively constant as a function of time since fire for given conditions of wind speed and fire danger conditions. The flammability at the level of that plateau was further shown to vary with both wind speed and fire danger conditions.; In the development of the model, we described the dynamics of the fuels through a chronosequence of 1 to 108 years since fire. The detailed sampling, performed by fuel strata in each stand, showed a different progression for the ground, surface, ladder and crown fuels as a function of time since fire. We also looked at the fuel structure through the use of a vertical distribution (1-m sections) of the load and bulk density of the fine fuels and total biomass (excluding roots and) of each stand. This analysis clearly showed the important contribution of the black spruce trees growing in the understory of our stands to the ladder fuel loads and that bark flakes on the stems of jack pine trees may be an underestimated source of ladder fuels. Finally, our work towards the development of the flammability model also led us to describe the fire weather, fire danger, and fire potential climatology of our study area.