Aspects of the Early Postglacial Forest Succession in the Great Lakes Region

Abstract

Pollen diagrams of lake sediments imply that during fluctuating retreat of the Wisconsin ice sheet the Great Lakes region was dominated by a spruce forest that may have contained some thermophilous deciduous trees like ash, elm, and oak but contained no pine and little birch, the two genera that today are such common associates of spruce in the Boreal Forest of Canada. Following relatively rapid destruction of this spruce forest about 10,500 years ago, presumably as a consequence of the climatic warming, birch and pine apparently invaded the area and were followed in rapid succession by elm, oak, and other deciduous trees. Although forest composition cannot be described in detail, the almost certain absence of any type of pine from the late—glacial forest requires special explanation. The hypothesis here presented proposes that during Wisconsin glaciation jack pine was unable to follow spruce in a general southward migration across the Great Lakes region to the central United States but took refuge instead in the Appalachian Highlands, and that during late—glacial time the versatile spruce was such an adaptable pioneer on deglaciated ground that it soon formed a closed forest. Birch and especially pine, far removed in their glacial refuges, were slower migrants and did not reach the western Great Lakes area in quantity until the spruce forest deteriorated. The subsequent succession of elm and oak may also reflect relative rates of migration from glacial refuges.