Pollen evidence of Pleistocene and Holocene Vegetation on the Allegheny Plateau, Maryland
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AbstractWhen the Wisconsin ice sheet stood at its maximum position, tandra vegetation bordered the ice sheet. In the eastern United States tundra vegetation extended at least 300 km due south of the ice border at 2700 feet (800 m) on the Allegheny plateau. Spruce and jack (and/or red) pine forest grew at lower elevations in Virginia. On the coastal plain, and further south, in the piedmont of northern Georgia, jack pine dominated the forest vegetation over a large region. As the ice sheet receeded, the vegetation went through a series of changes. Coniferous forest was replaced by deciduous forest, beginning in 13,600 B.P. in Georgia. The frequency of white pine began to increase in Virginia about the same time, and the frequencies of deciduous trees, about 1000 years later. On the Allegheny plateau no change took place in the tundra vegetation until 12,700 B.P., when the tundra was replaced by open, spruce woodland. Jack and/or red pine grew mixed with, or nearby the spruce. Pollen from deciduous trees (mainly oak, ash and hornbeam) reached the site in greater quantity than before. Possibly the increase indicates a change in wind direction. On the Allegheny plateau, 10,500 years ago, the boreal woodland was replaced by a mixed coniferous/deciduous forest which included white pine. At about the same time (or perhaps a thousand years later), a similar change occurred in Connecticut. At lower elevations in the Shenandoah valley, spruce forests including white pine were replaced by oak and other hardwoods. In the early Holocene, at a time we were unfortunately unable to pinpoint by radiocarbon dating, deciduous forest began to grow on the Allegheny plateau. Later there was a series of changes in the composition of the forest. High frequencies of oak pollen occur throughout the sequence, with successive maxima of hemlock, beech, and finally hickory. High percentages of chestnut pollen occur with a maximum approximately coincident with the maximum of beech. These changes are probably significant both from stratigraphical and paleoecological points of view, and should be studied at sites where radiocarbon dating will be possible. The early maximum of chestnut pollen is a major difference between the pollen sequence in the Alleghenies and southern and central New England, suggesting that this species was very slow to move northward, arriving in New England just 2,000 B.P. as the result of migration, not climate change. AccessDownload (3.35 Mb) |