Young jack pine and high severity fire combine to create potentially expansive areas of understocked forest

Abstract

Jack pine is a fire adapted species considered to regenerate well after fires of all severities. The Richardson Fire was an extreme megafire event that burned 576,000ha of jack pine dominated boreal forest in 2011 in northern Alberta, Canada. Initial scouting immediately after the fire identified a great deal of variability in jack pine regeneration seemingly related to fire severity and pre-fire stand composition. We sampled tree regeneration in 56 pure jack pine stands across the range of fire severity and pre-fire stand ages 1year post-fire. We found that jack pine regeneration density was greater in older stands (>60years old) compared to younger stands (<30years old) and in moderate severity burns compared to high severity burns. In young stands with high severity burns, jack pine regeneration averaged only 1164 seedlings per hectare which is well below current stand densities indicating a potentially understocked future forest. At the landscape level, we extrapolated fire severity to the entire fire area using pre and post-fire satellite imagery and found that the area of young jack pine stands which had a high severity burn, and therefore likely low seedling regeneration, was greater than 130,000ha with most of this area occurring in patches greater than 500ha in size. Overall, our results suggest that young jack pine may not be as resilient to high severity fires as previously thought and that a large area of burned boreal forest may be at risk of conversion from a closed canopy forest to a more open canopy woodland ecosystem given the predicted changes in fire regime.