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Pre-Columbian savannas of North America
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Postglacial events
During the Last Glacial Maximum about 18,000 years ago, the influence of Arctic air masses and boreal vegetation extended to about 33° N. latitude, the approximate latitude of Birmingham and Atlanta. Southeastern forests of the glacial period were dominated by various spruce species and jack pine; fir was abundant in some locations. With the exception of the absence of certain prairie elements, the understories of these forests were generally typical of modern spruce-fir forests within and near Canada.
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Postglacial events
During the Last Glacial Maximum about 18,000 years ago, the influence of Arctic air masses and boreal vegetation extended to about 33° N. latitude, the approximate latitude of Birmingham and Atlanta. Southeastern forests of the glacial period were dominated by various spruce species and jack pine; fir was abundant in some locations. With the exception of the absence of certain prairie elements, the understories of these forests were generally typical of modern spruce-fir forests within and near Canada. Humans spread across the continent as five thousand years passed following the retreat of the glaciers, while deciduous forests expanded northward. In the east, pockets of boreal elements remained only at high elevations in the Appalachian Mountains and in a few other refuges.
Warming and drying during the Holocene climatic optimum began about 9,000 years ago and affected the vegetation of the southeast. Extensive expansions of prairies and savannas occurred throughout the southeast, and xeric oak and oak-hickory forest types proliferated. Cooler-climate species migrated northward and upward in elevation. This retreat caused a proportional increase in pine-dominated forests in the Appalachians. The grasslands and savannas of the time expanded and were also linked to the great interior plains grasslands to the west of the region. As a result, elements of the prairie flora became established throughout the region, first by simple migration, but then also by invading disjunct openings (including glades and barrens) that were forming in the canopy of more mesic forests.
At about 4,000 years BP, the Archaic Indian cultures began practicing agriculture. Technology had advanced to the point that pottery was becoming common, and the small-scale felling of trees became feasible. Concurrently, the Archaic Indians began using fire in a widespread manner. Intentional burning of vegetation was taken up to mimic the effects of natural fires that tended to clear forest understories, thereby making travel easier and facilitating the growth of herbs and berry-producing plants that were important for both food and medicines. The result in many regions was "the conversion of forest to grassland, savanna, scrub, open woodland, and forest with grassy openings". After the death of 90% of the native population around 500 years ago, grasslands, savanna, and woodlands succeeded to closed forest.
Historic or remaining savanna areas
Savanna surrounded much of the continent's central tallgrass prairie and shortgrass prairie. Fire also swept the Rocky Mountains aspen as frequently as every ten years, creating large areas of parkland. In the far southwest was California oak woodland, while further north was the Oregon White Oak savanna. The Central Hardwood Region covers a wide belt from northern Minnesota and Wisconsin, down through Iowa, Illinois, northern and central Missouri, eastern Kansas, and central Oklahoma to north-central Texas, with isolated pockets further east around the Great Lakes. The Eastern savannas of the United States extended further east to the Atlantic seaboard.
In the southeast, longleaf pine dominated the savanna and open-floored forests which once covered from Virginia to Texas. These covered 36% of the region's land and 52% of the upland areas. Of this, less than 1% of the unaltered forest still stands.
Destruction of the savannas
Industrialized sawmills in the early 20th Century cleared many tall savanna old-growth trees, while fire suppression methods adopted in the 1930s and 1940s stopped much of the regular burning which the savanna required. By the latter half of the 20th Century many researchers had rediscovered both the prehistoric use of fire and methods to practice burning, but by then almost all prairie and savanna lands had been converted to agriculture or succeeded to full-canopy forest. Modern conservation of savanna includes controlled burning, and at present about a year are burned.
See also
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