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.边缘效应（edge effects）会引发非生物与生物因子的改变，其对早期演替物种有利，却会排挤依赖更为稳定、未受干扰环境的森林内部特化物种。本研究聚焦于人为营造的林缘，尤以加拿大阿尔伯塔省油砂矿区的高地北方森林为例，探讨其如何影响森林结构与组成。由于地下沥青的勘探与开采，该区域已出现严重的森林破碎化，形成了规模庞大的扰动足迹网络，部分区域的扰动足迹局部密度可达40千米/平方千米。本研究分析了林缘距离与林缘朝向的交互作用，结合油砂扰动形成的不同间隙规模，如何影响林木的大小依赖性死亡率、更新定植与株密度。研究团队于2023至2024年间在46处林缘样地采集了野外数据，结果显示扰动足迹规模越大，边缘效应越显著。西向林缘的大树死亡率更高，紧邻井场的区域尤为明显。林木更新定植与林缘距离呈负相关，但局部响应随林缘朝向而异。例如，西向林缘的更新定植率高于南向林缘。整体而言，林缘周边（0-20米范围内）的林木株密度最高，这印证了两类扰动林缘均存在边缘填塞效应。本研究结果表明，林缘朝向是影响油砂线性与非线性扰动林缘的边缘效应强度与范围的关键因素。鉴于该区域林缘网络规模庞大，即便微弱的边缘效应也可累积，进而对区域尺度的森林结构与组成产生显著影响。