Dynamics in tree structure and composition along boreal forest edges: a case study in Alberta's in situ oil sands

Edge effects, which result in changes to abiotic and biotic factors, benefit early seral species while displacing interior forest specialists that rely on more stable, undisturbed conditions. This study investigated how anthropogenically-created forest edges, particularly in upland boreal forests of Alberta’s oil sands region, influence forest structure and composition. Due to underground bitumen exploration and extraction, the area has experienced significant forest fragmentation, creating an extensive network of footprints, with some footprints having local densities reaching up to 40 km/km2. We examined how the interaction of distance from forest edge and edge orientation across different gap sizes from oil sands disturbances influences size-dependent mortality, recruitment, and stem density of trees. Field data were collected in 2023 and 2024 along 46 forest edge sites, with edge effects found to be more pronounced with larger disturbance footprints. Large-tree mortality was higher along west-facing edges, particularly against wellpads. Recruitment of trees was inversely related to edge distance, although local responses varied by orientation of the edge. For example, recruitment was higher along west-facing edges when compared to south-facing edges. Stem densities overall were highest near edges (0–20 m), supporting an edge sealing effect along edges of both disturbances. Our results emphasize edge orientation as a key factor influencing the magnitude and extent of edge effects in forest structure along linear and non-linear oil sands footprints. Given the extensive network of edges in the region, even small edge effects can accumulate, leading to substantial, landscape-scale influence on forest structure and composition.