Initial succession after wildfire in dry boreal forests of northwestern North America

Abstract

Wildfires in the boreal forest of North America are generally stand renewing, with the initial phase of recovery often governing the vegetation trajectory for decades. Here, we investigated post-fire vegetation changes in dry boreal forests of the Northwest Territories, Canada, during the first 5 years following the unusually severe 2014 wildfire season. We sampled post-fire tree regeneration and the understory plant community at 1, 3, and 5 years post-fire across different stand types within fires that burned in 2014. Post-fire trajectories of tree recruitment, understory cover by plant functional types, and plant diversity varied widely among sampled stands, as well as among years post-fire. Tree seedling density reached relative equilibrium by 3-years post-fire, whereas trends in understory plant cover and understory species assemblages suggested an ongoing change that will extend beyond 5 years of observation. In almost half of sampled stands, the composition of recruited trees differed from that of the pre-fire stand, suggesting a change in tree species dominance. An analysis of regional climate revealed a significant, albeit spatially variable, warming and drying trend that will further accelerate forest stand transformation through both climate drivers of plant community composition and indirectly through increasing fire activity. While the 2014 wildfires enhanced the structural and compositional heterogeneity of the region, they also triggered vegetation changes that are likely to be persistent. As such, this study exemplifies the speed and variability that characterizes post-fire stand development in a strongly moisture-limited part of North America.