No evidence of a northward biome shift of treeline in the Mackay Lake region, north-central Canada

Major changes in boreal and subarctic climates have occurred in recent decades, but uncertainty exists as to how biota are responding, e.g., whether the subarctic forest-tundra (‘treeline’) will advance northward. Ground-based vegetation data from the Mackay Lake region of the Northwest Territories were used to determine if there is evidence of a northward biome shift. Near the treeline, current plant communities are similar to those documented 40–67 years ago. There was little or no evidence of recent tree regeneration by seed, expansion of tree stands, or colonization by trees of adjacent tundra. Black spruce stems require ~75 years to reach 2.5 m in height and be discernible as trees on photography and satellite imagery. Branches 20–30 years old are the preferred age for initiation of an upright stem from layering. Upright stems of clonal spruce within the forest-tundra may be far younger than their near-ground or belowground stems. Whether northward biome migration will occur will depend on the interplay between climatic, landscape, and soil factors, species migration capacities, wildfires, and whether afforestation outpaces deforestation. In the forest-tundra, climatic change may not result in significant biome shifts, but rather in changes in the relative abundance of species already present.

近几十年来，寒温带与亚寒带气候已发生显著变化，但学界对生物群的响应模式仍存在诸多不确定性，例如亚寒带森林苔原的林线（treeline）是否会向北推进。本研究借助加拿大西北地区马更些湖区域的地面植被调查数据，旨在验证是否存在生物群系向北迁移的相关迹象。在林线附近，当前的植物群落与40至67年前记录的群落特征高度相似。几乎未发现近期通过种子实现树木更新、林分扩张，或是树木向邻近苔原拓殖的相关证据。黑云杉（Black spruce）的树干需约75年方可长至2.5米高度，且此时方可在航拍照片与卫星影像中被识别为独立树木。通过压条萌生直立茎的最优枝条年龄为20至30年。森林苔原带内无性系云杉的直立茎，其表观树龄可能远低于其近地面或地下茎的实际树龄。生物群系是否会发生北向迁移，取决于气候、景观与土壤因子间的相互作用、物种迁移能力、野火干扰情况，以及造林速率是否超过采伐速率。在森林苔原带中，气候变化或许不会引发显著的生物群系更替，而是会改变现有物种的相对丰度。