Payette S, Eronen M, Jasinski J J P
Centre d'études nordiques, Université Laval, Quebec, Qc, Canada G1K 7P4.
Ambio. 2002 Aug;Spec No 12:15-22.
Creating a global perspective on past treeline changes is problematic due to the varying methods and definitions used. A general lack of a detailed description of the modern treeline position and vegetation complicates any comparative analysis of the magnitude of the most important changes. However, one seemingly common factor in most regions was an extremely rapid dispersal of trees when climate warmed drastically from full glacial conditions. Most Arctic treelines reached their northernmost positions in the early Holocene and receded to present positions starting at about 5.8 ka. The early occupation of the northernmost sites in ice-free and early deglaciated areas was possible because of the close proximity of invading trees in nearby glacial refugia, particularly in Fennoscandia and northern Russia. In Canada, the Northwest Territories and Quebec-Labrador were out of phase with this general trend due to their late deglaciation. However, even here colonization was rapid, indicating that the tree species were present adjacent to the glaciers. Following this trend and based on the present evidence, we propose a scenario of a continuous but modest occupation of eastern Beringia by spruce during the late-Pleistocene instead of an exceptionally rapid spread of conifers from the glacial refugium south of the Laurentide ice sheet (2000 to 3000 km in about 200 years), which typically has been assumed. Macrofossil evidence of scattered occurrences of "exotic species" (for instance Siberian larch in central Sweden) far from their natural range limits in the early Holocene highlight the disparity between pollen and macrofossil analyses. It questions the validity of assigned pollen percentages to indicate the presence of a species within a region as these species were not observed in the pollen record. Thus, it is likely that trees were present at any given site well before the rise in pollen abundance. There is still a large potential to improve our knowledge about the environmental history of the circumboreal treeline areas. In particular, future research should concentrate not only on patterns of species displacement, but on finding the factors, apart from climate, which cause treeline shifts.
由于所使用的方法和定义各不相同，要建立一个关于过去树线变化的全球视角存在问题。普遍缺乏对现代树线位置和植被的详细描述，这使得对最重要变化幅度的任何比较分析都变得复杂。然而，在大多数地区，一个看似共同的因素是，当气候从全冰期条件急剧变暖时，树木迅速扩散。大多数北极树线在全新世早期到达其最北位置，并从大约5800年前开始退缩到当前位置。在无冰和早期冰消的地区，最北部的地点能够被早期占据，是因为附近冰川避难所中入侵树木的距离很近，特别是在斯堪的纳维亚半岛和俄罗斯北部。在加拿大，西北地区和魁北克 - 拉布拉多地区由于冰消作用较晚，与这一总体趋势不同步。然而，即使在这里，树木的定殖也很快，这表明冰川附近存在这些树种。遵循这一趋势并基于现有证据，我们提出了一个晚更新世期间云杉持续但适度占据白令陆桥东部的情景，而不是通常所认为的针叶树从劳伦泰德冰盖以南的冰川避难所异常迅速地扩散（约200年内扩散2000至3000公里）。全新世早期，远离其自然分布范围界限的“外来物种”（例如瑞典中部的西伯利亚落叶松）零星出现的宏观化石证据凸显了花粉分析和宏观化石分析之间的差异。这质疑了用指定的花粉百分比来表明一个地区内某一物种存在的有效性，因为在花粉记录中并未观察到这些物种。因此，很可能在花粉丰度上升之前，任何给定地点就已经有树木存在。在改善我们对环北极树线地区环境历史的认识方面仍有很大潜力。特别是，未来的研究不仅应关注物种替代模式，还应关注除气候之外导致树线移动的因素。
