Burn severity and fire history in the northwestern Canadian boreal forest: drivers and ecological outcomes
A short-interval reburn (9 years between high-severity fires) in Wood Buffalo National Park. A dead jack pine sapling with a non-serotinous cone is visible in the foreground, representing the trees that were regenerating after the first fire. After two fires, the site has changed from a jack pine forest to an open aspen woodland with a grass understory.
Wildfire is the dominant stand-renewing disturbance in the northwestern Canadian boreal forest. Fires burn extensive areas in Canada, disturbing an average of 1.96 Mha yr−1, primarily in the boreal zone. Fires generally occur every ~30 – > 200 years in this region, due in part to a lack of fuel that allows young stands to resist reburning. Boreal understory plants and trees are adapted to stand-renewing wildfire. Such adaptations allow boreal forests to recover successfully after fire, and post-fire vegetation communities are generally similar to the pre-fire ones, following a burned forest to self-replace. Recently, the area burned, average fire size, and fire season length in northwestern Canada have increased. Severe fire weather has enabled reburning of young forests at very short intervals (sometimes ≤ 10 years between fires). Shifts in fire regime characteristics, such as burn severity and fire-free interval may lead to unexpected changes in vegetation composition and forest structure following fire.
The 2014 and 2015 fire seasons in the Northwest Territories (NWT) and Northern Alberta (AB) were severe, with fires burning approximately 4 Mha, including instances of short-interval reburning (fewer than 30 years between repeat fires). Inspired by these two fire years, in this dissertation I sought to understand the drivers of burn severity (biomass loss) and the ecological outcomes of burn severity and fire intervals, and how they interact with climate in this fire-adapted ecosystem.
Pre-fire vegetation structure and composition (fuels) and hydrology (uplands and wetlands) largely explained the resulting burn severity. In this way, the vegetation communities themselves partially controlled the severity of the disturbance by fire that they experienced. Similarly, field measurements of post-fire vegetation communities and shifts in tree species composition were due to pre-fire forest structure, hydrology, and site climate conditions. The landscape of the NWT and AB was altered by the extensive area burned in 2014 and 2015, however, burn severity is highly variable. Post-fire ecological outcomes from these years are likely to be variable, as well.
Although both site- and stand-level controls on burn severity and post-fire vegetation make boreal forests resilient to disturbance, where fire activity and severity increase there is a possibility for forest change. I found evidence of unexpected shifts in tree species composition after fire. Black (Picea mariana) and white (P. glauca) spruce dominance declined in uplands following fire, whereas shifts in dominance of jack pine (Pinus banksiana) were variable, and trembling aspen (Populus tremuloides) dominance generally increased following fire. Burn severity and fire frequency drove these changes in tree species composition and density where they occurred.
Shortening fire-free intervals due to drought are likely to accelerate climate-driven shifts from conifer-dominated boreal forests to open woodlands and grasslands. Paired sites that reburned at short fire-free intervals had significantly lower post-fire recruitment of trees than at long-interval sites, due to decreased establishment of conifer seedlings. Post-fire drought stress in both short- and long-interval sites interacted with burn severity, further reducing tree seedling density. This dissertation suggests that increasing wildfire activity and severity may alter the composition and structure of northwestern Canadian boreal forests, accelerating expected ecosystem changes as northern climates warm and dry.
Variation in burn severity within a fire in Wood Buffalo National Park. The severity ranges from unburned residual islands (green), moderate severity scorched areas with dead vegetation (orange), and severely charred trees (black).