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The glacial retreat strongly affected vegetation distribution and composition. Tree species began migrating inland away from encroaching ocean waters, as well as northward due to climatic warming. Mesic species took advantage of the increased moisture in riverine watersheds. Since the Laurentide glacial dome continued to cover eastern and middle Canada, the direction of most of the retreat was from west to east. The remaining large ice dome in the east prolonged the glacial climate along the eastern seaboard. The earlier warming in the western South explains the early development of a spruce and oak assemblage prior to 12,500 years BP in the Ozark Highlands. The prolonged cooler climate in the East maintained the presence of spruce in Virginia as late as 10,000 years BP or later (Wright 1987).
As the glacier waxed and waned between 16,500 to 12,500 years BP, vegetation dynamics were intense. Edwards and Merrill (1977) described this period as "ecologically restive." The assemblage of tree and plant species had no modern analogue (Davis 1983). Boreal vegetation and tundra moved into newly opened land in the northern regions abandoned by the massive ice fields. Boreal forest in the Eastern United States by 12,500 to 11,000 years BP ranged south in a broad band into Virginia and Kentucky (37° latitude) and farther south, narrowing along the Appalachians summit into North Carolina and Tennessee. Here, residual boreal species exist today at high elevation.
Deciduous trees, especially oak, dominated the taxa over much of the South. Mesic hardwoods, such as ironwood, beech, and maples, were important along river systems and wet areas. Watts (1980) research at White Pond in South Carolina indicated the presence of a deciduous forest dominated by oaks at 12,800 years BP. The oaks associated with hickory and beech made up as much as 55 percent of the nearby tree species; some ironwood and hornbeam were present (fig. 24.7). He concludes that the climate was cooler and moister than today, implying that the forest was "mesic", but he urged caution in using this term.
Delcourt and Delcourt (1984) also refer to climate at this period as "cool-temperate" supporting mesic forests. They theorize that abundant moisture was available during the growing season in the mid-latitudes of the Southeast north of the 34° N. latitude. However, there is reliable information to suggest that between 12,500 to 10,000 years BP the climate of the Southeast was cooler but drier, rather than moister than at present. During the late glacial period the ocean level was still 100 to 130 feet lower than at present, and river levels and water tables were correspondingly lower. Lower precipitation, particularly during the summer growing season, would have had profound effects on vegetation (Kutzbach 1987).
Watts (1980) and Delcourt and Delcourt (1984) proposed a mesophytic forest during the late glacial period north of the 34° latitude. However the mesic hickory-beech association composed only 25 percent of the pollen at that time (fig. 24.7). The dominant pollen at White Pond, SC, for the period 12,800 to 10,000 years BP was the shade intolerant, ring-porous, drought resistant oak. Hickory, which comprised 15 percent of the pollen profile, is also ring porous and should be considered part of the oak assemblage. Oak, therefore, was the dominant forest type at the time. It dominated the upland areas. Mesic species, such as beech, ironwood, hornbeam, elm, ash, and maple, were assigned to the waterways or moist areas such as White Pond. Therefore, the pollen evidence from White Pond supports a dry climate rather than a moist climate.
The map by Edwards and Merrill (1977) (fig. 24.10) represents a plausible model for dominant tree taxa at 12,500 years BP for the Atlantic Seaboard, based on a drier climate. Jacobson (1987) also supports a dry climate by indicating that the oak-hickory forest was dominant over the entire Southeast at this time. Oaks and hickories produce leaves rapidly in the spring when soil moisture is highest from winter recharge and transpiration is low, and they have the ability to produce and store energy at leaf flush. If droughty weather occurs later in the growing season, they shut down their systems and wait for sufficient soil moisture. They are shade-intolerant species and need repeated disturbance to maintain quality regeneration on all sites (Brose and Van Lear 1998). Without disturbance in high precipitation regions, oaks and hickories have difficulty competing with diffuse-porous mesic species and lose their dominance in the stand or disappear altogether.
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