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Vegetation Changes in the Southeast (The Early Holocene 9,500 to 7,500 Years BP)

Oak species already dominated the forest of the middle and lower Southeastern United States by 9,500 years BP; they have continued to dominate until present times. Hickories also were important. Southern pines were increasing and would later become a major component of the southeastern forest (Watts 1983). As boreal tree species migrated north, oaks and hickories became dominant in the upper South, extending into Virginia and Kentucky (Delcourt and others 1999, Edwards and Merrill 1977, Watts 1980) (fig. 24.12).

Southeastern coastal plant communities were probably unstable due to changing sea levels. Sea levels were constantly advancing on the land as the glacier melted. Pollen data indicate increasing presence of southern pine on the Coastal Plain. Disturbance, due to climatic instability, created open areas favorable for pine regeneration (Edwards and Merrill 1977, Spurr and Barnes 1973, Watts 1980).

The Southern Appalachians were also undergoing environmental changes. Increasing temperature shifted boreal spruce and fir to higher elevations, while lower elevations were occupied by mixed hardwoods species, with oaks as the dominant component (Delcourt and Delcourt 1985, Watts 1983). Mountainsides were eroding due to dying vegetation, resulting from unstable climatic conditions. Sediment from this erosive period was deposited in our modern floodplains (Chapman 1985).

The land area of Florida shrunk as the sea level rose. Fresh water was limited because water tables were still very low (Watts 1971). The southeastern climate was becoming warmer but remained dry until about 8,500 years BP when precipitation increased. Oaks were dominant as they had been during the Ice Age. Vegetation was composed of scrub oak, with increasing incidence of southern pines (Davis 1983, Delcourt and Delcourt 1984, Edwards and Merrill 1977, Milanovich and Fairbanks 1980, Watts 1983). As water tables stabilized by 5,000 years BP, forests assumed modern characteristics.

Unstable plant communities characterized the period of warming and deglaciation. Tree species were migrating from refuges occupied during the Ice Age. Some species moved fairly rapidly, while others migrated much more slowly (Davis 1983). Along with the changing vegetation was an increase in the frequency of fire, which is demonstrated by increased amounts of charcoal in pollen profiles (Delcourt 1985, Delcourt and others 1999, Watts 1980) (fig. 24.7). Fires were both natural and human-caused, but our ancestors were probably the predominant source of ignition that resulted in the increased fire frequency during selected seasons (Delcourt 1985, Delcourt and others 1999, Van Lear and Waldrop 1989). The combination of the migration of tree species, high erosion due to dying vegetation, and droughty growing seasons would not have favored a closed-canopy forest.

Oak, pine, and hickory are all relatively shade-intolerant, disturbance species that need openings and sunlight for regeneration. Increasing mean annual precipitation and closed-canopy forest would not have allowed these species to dominate the landscape for thousands of years, as indicated by pollen analyses (Delcourt and Delcourt 1985, Watts 1980). Mesic shade-tolerant species, such as beech and maple, would have dominated forests under a continuous closed-canopy forest. Therefore, the dominance of oak, pine, and hickory in the Southeast was due to frequent disturbance, which created open landscapes favorable for regeneration of shade-intolerant species (Fralish and others 1991). Increased fire frequency and climatic instability would have provided natural settings conducive to the dominance of oak, hickory, and southern pines (Abrams 1992, Brose and Van Lear 1997, Myers and Van Lear 1998).

The annual fires of prehistoric humans established and maintained the open forests, savannas, and prairies observed nearly 10,000 years later by the first European immigrants.

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