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Holocene Paleoecology of the Boreal Forest and Great Lakes‐St. Lawrence Forest in Northern Ontario

Author:
Liu, Kam-Biu
Source:
Ecological monographs 1990 v.60 no.2 pp. 179-212
ISSN:
0012-9615
Subject:
Abies balsamea, Alnus incana subsp. rugosa, Alnus viridis subsp. crispa, Canadian Shield, Fagus, Holocene epoch, Myrica, Picea glauca, Picea mariana, Pinus banksiana, Populus, Thuja occidentalis, boreal forests, climate, ecotones, glaciation, global cooling, habitats, herbs, highlands, lakes, landscapes, lowlands, paleobotany, paleoecology, palynology, peatlands, plant communities, pollen, sediments, shrubs, soil, swamps, water table, Ontario
Abstract:
This paper presents pollen and macrofossil stratigraphies derived from sediment cores taken near the deepest parts of Nina Lake (46°36' N, 81°30' W), Jack Lake (47°19' N, 81°46' W), and Lake Six (48°24' N, 81°19' W), which are situated along a transect across the ecotone between the boreal forest and the Great Lakes—St. Lawrence forest in northern Ontario. Paleoecological data from this region of steep climatic gradient and varied landforms provide sensitive records of postglacial vegetational and climatic changes. These data are used to evaluate questions concerning the occurrence of late—glacial vegetation communities without modern analog. Hypsithermal ecotonal movements, and individualistic species response to climatic change and soil development. The early postglacial boreal forest that colonized the Canadian Shield upland after ca. 10 000 BP was dominated by white spruce with little or no black spruce, and probably contained more oak, elm, poplar, and heliophytic herbs and shrubs than its modern counterpart. Its lack of modern analog is probably due to the widespread occurrence of fresh, unleached soil in a newly deglaciated landscape. Spruce declined and was replaced by jack pine after ca. 9000 BP as the climate continued to warm. The boreal forest was enriched floristically by the successive immigration of species such as Myrica. Alnus crispa, and A. rugosa. Boreal forest was transformed into Great Lakes—St. Lawrence forest ca. 7400 yr ago when white pine, beech, and hemlock immigrated to Nina Lake. Species of the Great Lakes—St. Lawrence forest responded individualistically to Hypsithermal climatic changes. White pine populations continued to spread northward during ca. 7000—3000 BP, causing the boreal forest/Great Lakes—St. Lawrence forest ecotone to advance ca. 140 km north of its present position, only constrained by the physiographic boundary between the Clay Belt and the Canadian Shield upland. During 6000—4500 BP northern white cedar proliferated in the Clay Belt lowlands due to a warmer and drier climate. The regional water table was lowered, permitting Thuja populations that were otherwise restricted to the margins of swamps and open peatlands to spread to the center. This vegetation response was landform selective, being more pronounced in the Clay Belt than on the Canadian Shield upland. The predominance of calcareous substrates and wetland habitats in the Clay Belt was favorable to the expansion of Thuja under a suitable climate. Neoglacial cooling decimated the populations of northern white cedar in the Clay Belt and those of white pine in the whole region. Spruce, jack pine, and balsam fir increased over the last 4000 yr. The ecotone retreated from near Lake Six after 3000 BP, reaching Jack Lake at ca. 2600 BP, and was stabilized in its modern position during the last millennium.
Agid:
5183357