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

Abstract

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. RÉSUMÉ D'importants changements des climats boréal et subarctique ont lieu depuis quelques décennies, mais l'incertitude demeure quant à la réponse biotique, par exemple si la toundra forestière (la limite des arbres) avancera vers le nord. Des données de végétation prélevées dans la région du lac Mackay aux Territoires du Nord-Ouest ont été utilisées pour chercher des indices de migration de la toundra forestière vers le nord. Près de la limite des arbres, les communautés végétales actuelles sont similaires à celles d'il y a 40-67 ans. Il y a peu ou pas d'indices de régénération récente par graines, d'expansion de peuplements forestiers, ou de colonisation par les arbres de la toundra adjacente. Les épinettes noires nécessitent ~75 ans pour atteindre 2,5 m de hauteur et ainsi être identifiées comme des arbres sur les photographies aériennes ou les images satellitaires. Les branches doivent avoir 20-30 ans pour initier la croissance d'une tige érigée par marcottage. Les tiges érigées des clones d'épinette de la toundra forestière pourraient être beaucoup plus jeunes que leurs branches au sol ou dans le sol. La migration vers le nord de la toundra forestière dépendra des interactions entre les facteurs climatiques, du paysage et du sol, ainsi que de la capacité de migration des espèces, des incendies forestiers, et de la surcompensation de la déforestation par l'afforestation. Dans la toundra forestière, les changements climatiques pourraient ne pas résulter en migrations de biomes, mais plutôt en changements des abondances relatives des espèces déjà présentes.

Arctic sea ice retreat fuels boreal forest advance

“…Near the Barents Sea (17,18,20) and Hudson Bay (2,13,20,21,23), autumn ice was mostly absent throughout the 19th and 20th centuries (44), leaving open water effects complete. Now other factors may limit the ecotones there, particularly water stress, fire (2-5), edaphic conditions, and topography (10)(11)(12) near Hudson Bay, in addition to herbivory (18,21) and land use (2,26) near the Barents Sea.…”

“…However, rising temperatures alone cannot shift forest-tundra boundaries (ecotones) (2)(3)(4)(5)(6). Although some boreal tree species are expanding their geographic ranges at post-Pleistocene rates in Fennoscandia (9) and northwest Alaska (16), many circumarctic ecotones elsewhere are slowmoving or stagnant (2)(3)(4)(5)(6)(8)(9)(10)(11)(12)(13)(14)(15) even where temperatures exceed thresholds thought to determine tree line locations (30). Ecotone inertia appears particularly acute in eastern Siberia (3,8,(27)(28)(29) and northern Canada (2,3,(10)(11)(12)(13)(14)(15)21).…”

Pixel walking along the boreal forest–Arctic tundra ecotone: Large scale ground-truthing of satellite-derived greenness (NDVI)

“…Nevertheless, more ground data are needed to understand how plant characteristics such as LAI, biomass, height, and species composition at the landscape scale (10 2 -10 3 km 2 ) relate to satellite measurements of greenness (Myers-Smith et al, 2020; Berner & Goetz, 2022; Timoney, 2023). Meyers-Smith et al (2020) highlighted two key issues in the spatial mismatch of satellite-derived greenness and field-based observations: poor replication of field observations across landscapes and the challenge in scaling-up field plot samples to image pixel size.…”

Pixel walking along the boreal forest–Arctic tundra ecotone: Large scale ground‐truthing of satellite‐derived greenness (NDVI)

“…With such widespread application of and reliance on remote sensing, more ground data are needed to understand how at the landscape scale (10 2 -10 3 km 2 ) plant characteristics such as LAI, biomass, height, and species composition relate to satellite measurements of greenness (Berner & Goetz, 2022;Myers-Smith et al, 2020;Timoney, 2023). Meyers-Smith et al (2020) highlighted two key issues in the spatial mismatch of satellite-derived greenness and field-based observations: poor replication of field observations across landscapes and the challenge of scaling-up field plot samples to satellite image pixel size.…”