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Late Glacial Landscapes—Mesic Or Xeric?

Soil types, drainage, and aspect strongly influence vegetation composition and distribution. In the Southeast, the Piedmont, Sandhills, and Highland Rim consist of rolling uplands, ridges, and hills. Sandy Coastal Plain soils are well drained. These landscapes were dissected by riverine drainages. Even with current levels of precipitation, upland and deep sandy soils are well drained or excessively drained and droughty.

Since it has been established that the climate at 12,500 to 9,500 years BP was more arid than today, the landscape would have supported fewer and smaller trees. The lack of precipitation during the summer growing season (Kutzbach 1987) would drastically reduce tree growth. Only the hardiest trees would survive, and the droughty conditions would enhance the frequency of fire. On droughty sites, grasses would be favored because they are more drought and fire tolerant. The harsh conditions resulted in widely spaced trees in park-like savannas, enhancing both browsing and grazing potential for wildlife. In many areas, only scrubby forms of oak and pine could survive extended drought.

In Florida during the late glacial period, the water table was 50 feet lower than today (Watts 1971). Throughout the Southeast, forests along the rivers were probably open, with rich herbaceous plant communities on river terraces. This condition may explain the increased presence of ironwood and hornbeam pollens in core samples during this period. Further support for open forests was reported by Jacobson and others (1987), who said, "the widespread appearance of ironwood, in particular, supports the notion that a broad woodland of open-grown vegetation existed south of the ice sheet; this tree flowers and produces abundant pollen only when growing in well lighted conditions." Davis (1983) indicated that the abundance of ironwood pollen in early Holocene sites in New England is compatible with a drier climate and a higher fire frequency. Ironwood today is characteristic of woodlands in Minnesota, growing along the prairie margin where fires are frequent. Delcourt and others (1999) also postulated a climate that promoted frequent fires favored taxa such as ironwood and hornbeam. They also noted that disturbances would have created patchy sunlit spots for the weedy growth of ironwood and hornbeam in shrubby thickets near mesic sites.

Ironwood and hornbeam made up less than 10 percent of the pollen profiles at mesic sites north of the 34 N. latitude where the cool temperate mesic forest was proposed (Davis 1983, Delcourt and Delcourt 1984, Watts 1980). Considering all the evidence, it appears that ironwood and hornbeam were growing in open conditions rather than in a closed canopy forest.

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