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Full Glacial Landscape (18,000 Years BP)

Pollen core samples provide clues to tree distribution and composition of the full glacial landscape, but they must be interpreted with caution when making broad statements about the complex characteristics of forest types. Because all pollen cores are taken from mesic sites (wet areas), like bogs, natural ponds, pocosins, etc., mesic species may be overrepresented.


Delcourt and Delcourt (1984) developed a vegetation map for Eastern North America for 18,000 years BP, based on a number of pollen studies scattered throughout North America (fig. 24.1). This map represents the potential distribution of vegetation types in regions of Eastern North America. Delcourts' map shows boreal forest in the Southeast to 34 N. latitude. Jack pine was the dominant species followed by spruce, and oak formed a minor component near the boreal/deciduous interface near 34 N. latitude (Watts 1980). Below 34 N. latitude, oaks were the dominant tree species with hickory as an associate. However, the excessively dry climate of the time affects vegetation assemblages throughout the Southeast (Barry 1983, Delcourt and Delcourt 1979). A potential misinterpretation of the oak-hickory assemblage, as depicted, is that it may give the impression of a completely forested landscape.


It is evident from the massive dimensions of the glacial systems and their influence on worldwide precipitation that what is termed forest at 18,000 years BP did not have a closed canopy. Rather, trees were scattered over a dry landscape, occupying sites where moisture and growing conditions were favorable for tree survival and growth (fig. 24.5). And what about the size of individual trees? Under these droughty conditions, many soil types in the South may have supported only scrub trees, or no trees at all. Extensive areas were probably dominated by prairie or sagebrush-dominated areas (Watts 1980) (fig. 24.5). Delcourt and Delcourt (1979) have estimated that current mean annual precipitation may have been reduced by more than half during full glaciation. Reducing present mean annual precipitation for the Southeast by more than 50 percent produces climatic conditions similar to the arid areas of the West today (U.S. Department of Agriculture 1965) (fig. 24.6).


Significant herbaceous pollen (grasses, sagebrush, and smartweed) appears in the profile from White Pond, SC (fig. 24.7). This quantity of herbaceous pollen would not be possible under a closed canopy forest (Watts 1980). Away from this mesic site, on xeric (dry) uplands, drought-tolerant grasses and sagebrush probably dominated the landscape.


Due to the extremely arid climate, vegetation consisted of trees clumped in favorable locations or scattered over the landscape in open park-like savannas, in association with prairies and scrublands. Organization of the vegetation mosaic was controlled by the moisture gradient from the well-drained uplands to the moist areas of the bottomlands.


Precipitation was also greatly reduced in the now super-humid Appalachian Summit. The arid climate produced a mosaic of grasslands, park-like savannas, and tundra at higher elevations. Dominant tree species were firs and spruces on moister sites with jack pine occupying drier sites (Delcourt and Delcourt 1984, Watts 1983).


South of 34 N. latitude, the deciduous tree assemblage dominated by oaks and hickory was able to survive. The arid climate and variations in soil types also produced a mosaic of park-like savannas with extensive prairies, sagebrush, and scrublands. Large grazing mammals existed throughout the South and required large amounts of herbage to live. Their existence supports the idea of extensive rangeland in the South because that habitat would have been necessary for their survival (Graham and Mead 1987, Guilday 1982, Kurten 1988, Lundelius and others 1983).


As a result of the arid climate and lower ocean levels, rivers and water tables were considerably lower (Edwards and Merrill 1977). Riparian areas, seeps, and springs provided a refuge for moisture-loving trees such as beech. Delcourt and Delcourt (1979) theorize that the eastern escarpment of the Mississippi River and the eroded gorges where streams entered the river provided refuge for many tree species during full and late glaciation. They also suggest these landscape features provided important migratory routes for tree species during changing climates.


Beaver may have been important during this period for creating inundated wetlands that supported shrubs and mesic herbaceous plants. These wet habitats probably attracted many species of wildlife, as well as waterfowl displaced by the glaciers.


Ocean levels are believed to have been 400 feet lower than at present. The exposed Continental Shelf extended from 60 to 90 miles beyond the present shoreline (Edwards and Merrill 1977, Jacobson and others 1987) and is believed to have harbored northern hardwoods in the vicinity of the Carolinas. This exposed and relatively flat land would have contained bogs and swamps interrupted with scrublands dominated by deciduous species along with southern pine.


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content: Wayne D. Carroll
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created: 4-OCT-2002
modified: 08-Dec-2013