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Burn severity and fire history in the northwestern Canadian boreal forest: drivers and ecological outcomes
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- Author / Creator
- Whitman, Ellen
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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 through mechanisms such as serotiny, seed banking, and resprouting from roots and rhizomes of top-killed individuals. Such adaptations confer resilience to boreal forests, and post-fire vegetation communities generally resemble the pre-fire ones, following a stand self-replacement trajectory. 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). Such changes in fire regime appear to be driven by anthropogenic climate change and increasingly severe fire weather. Furthermore, increasing moisture stress is implicated in simultaneous increases in fire activity, and worsening conditions for post-fire establishment of trees. Shifts in fire regime characteristics, such as burn severity and fire-free interval may lead to changes in vegetation composition following fire, thwarting stand self-replacement expectations.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. Inspired by these two fire years, in this dissertation I sought to understand the drivers of burn severity and the ecological outcomes of burn severity (biomass loss) and fire intervals, and how they interact with climate in this fire-adapted ecosystem. Specifically, my objectives were to: 1. characterize drivers and landscape patterns of burn severity, 2. describe the ecological outcomes of burn severity and fire history, and 3. examine the effects of post-fire moisture stress and fire regime on post-fire vegetation communities. These research objectives are contextualized in a methodological and ecological overview in Chapter 1.In Chapter 2 of this thesis I used remotely sensed multispectral imagery and field observations of burn severity to map landscape patterns of burn severity in six large wildfires. Fires were dominated by moderate- and high-severity patches. These patches were extensive, adjacent to similar patches, and had large core areas. Field measurements allowed me to demonstrate that burn severity was primarily a product of pre-fire vegetation structure (fuels) and hydrology (uplands and wetlands), providing a mechanism by which the severity of disturbance experienced is limited by the vegetation communities themselves. Similarly, field measurements of post-fire vegetation communities and shifts in tree species composition (Chapter 3) were largely explained by pre-fire forest structure, hydrology, and climate normals. 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 confer resilience, where fire activity and severity increase there is a possibility for forest change. I found evidence of post-fire shifts in tree species composition (Chapter 3). 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. Where changes in tree species composition and density occurred, they were mediated by burn severity and fire frequency. Shortening of fire-free intervals due to drought is 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 long-interval pair members, due to decreased establishment of conifers (Chapter 4). These differences were persistent and occurred in both uplands and wetlands. Increasing temperatures and aridity are likely to increase fire activity, and to reinforce the changes to forests caused by shifts in fire regimes. Post-fire moisture 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.
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- Graduation date
- Spring 2019
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- Type of Item
- Thesis
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- Degree
- Doctor of Philosophy
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- License
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