Biodiversity and Forest structure one year following the 2016 Fort McMurray (Alberta) fire event

Exploration practices for oil sands developments in the boreal forest of western Canada create a network of thousands of kilometers of linear features, particularly seismic lines that dissect these forests posing significant environmental challenges. As wildfire is one of the prevalent stand-replacing natural disturbances in the Canadian boreal forest, it is an important driver of environmental change and stand development that may contribute to the mitigation of such linear industrial footprint. Here, we evaluate the short-term cumulative (also known as combined) effects of seismic lines and wildfire on biodiversity and site conditions. One year after the Horse River (Fort McMurray, Alberta) fire event in the spring of 2016, we compared dissected and undisturbed forests in burned and unburned boreal peatlands, assessing changes in overall stand structure and the responses of a variety of organisms. Soil moisture was significantly higher on seismic lines than in the adjacent forest, suggesting why most of the study sites within the fire perimeter showed little evidence of burning at the line in relation to the adjacent forest. Low fire severity on seismic lines seemed an important driver of local species diversity for ants, beetles, spiders, and plants in disturbed peatlands, resulting in similar species composition on seismic lines both within and outside the burned area, but different assemblages in burned and unburned adjacent forests. Our results suggest that fire did not erase seismic lines; rather, wildfire might increase the influence of this footprint on the recovering adjacent forest. Longer term monitoring will be necessary to understand how boreal treed peatlands respond to the cumulative effect of wildfire and linear disturbances.

Methods
Data were collected south of Fort McMurray (Alberta, Canada) along the 2016 Horse River wildfire southwest perimeter (56°31'33" N, 111°18'10" W) at 15 sites in treed peatlands dominated by black spruce (Picea mariana), with nine sites within the fire and six sites outside the burned area. All sites were dissected by conventional seismic lines, were at least 200 m from roads, and were at least 2.4 km from each other to guarantee independent samples. At each site, three 50 m transects were run parallel to each other within each of three habitat types: one transect along the center of the seismic line (“Line”), one transect along the forest edge (10 m from the line edge; “Edge”), and one transect in the adjacent forest (50 m from the line edge; “Forest”). Both Edge and Forest transects were located on the same side of the seismic line, with side selected randomly, except for sites in the burned area where only one side was available. A sampling point was placed every 10 m along each transect, for a total of 15 sampling points per site (overall, 225 sampling points). Ground dwelling arthropods were collected using one pitfall trap at each sampling point (five traps/transect and 15 traps/site). Propylene glycol was used as preservative and a square roof of corrugated plastic was suspended on a wire over the trap to reduce rain and debris falling into the container. Traps were serviced every three weeks from May 20 to September 15 2017. Vegetation data were collected using a 1 m x 1 m quadrat placed adjacent to each pitfall trap. Percent cover of all vascular plant species was visually estimated to the nearest 0.1%, from the ground to the overstory level. We used 1 m height as the threshold to distinguish understory (≤ 1 m) and overstory (> 1 m). Additional variables, related to site conditions, were collected at each sampling point, including soil moisture and temperature, stem basal area and density from tree stems ≥ 1.0 cm in diameter at the base, tree regeneration (< 1.0 cm in diameter at the base), shrub densities (including assessment of density ofbog birch (Betula pumila), Labrador tea (Rhododendron groenlandicum) and willows (Salix spp.)), downed woody material volume and density, percent canopy cover, and relative burn severity.