The Roles of Pine and Spruce in the Forest History of Minnesota and Adjacent Areas

Compilation of the percentages of pine and spruce pollen from throughout northeastern and north—central North America shows that at the present time pine exceeds 10% in the tundra, forest/tundra transition, and boreal forest, and that spruce percentages are generally higher than pine in these areas except in Manitoba. The conifer—hardwood forest has much more pine pollen, especially to the west, and it has very little spruce. In the deciduous forest, pine pollen is generally less than 10%, and in the prairie only a few percent. The late—glacial spruce zone of New England may record a vegetation similar to the boreal forest of today, which contains Pinus banksiana, but the evidence indicates that in the Great Lakes region pine trees were absent. As a result of the climatic change that set off the rapid retreat of the ice sheet from the Great Lakes region, starting about 12,000 years ago, the peripheral spruce forest rapidly deteriorated. The spruce was replaced in south—central Minnesota by birch and alder, and these by elm, oak, and other deciduous trees about 9,500 years ago. In east—central Minnesota, however, the wave of spruce destruction did not arrive until about 10,500 years ago, and by this time pine had invaded from the east, perhaps by way of the northern end of the Great Lakes when the ice retreated into Canada. Here the spruce forest was replaced by pine in a very few hundred years. In northeastern Minnesota spruce remained important until about 9,000 years ago. The pine involved in these forest transformations was either jack or red pine (or both), according to studies of pollen morphology. White pine arrived in eastern Minnesota about 7,000 years ago, also from the east. Just before this time, the prairie, which had developed in western Minnesota by 8,000 years ago, expanded to the east, as did the oak savanna that bordered it on the side of the forest. Effects of the prairie expansion can be detected in the pollen sequence even in northeastern Minnesota. At the end of the period of prairie expansion, about 4,000 years ago, white pine expanded slowly to the west, reaching northwestern Minnesota about 2,700 years ago. Jack and red pine arrived in this region only about 1,000 years ago.