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Pre-Columbian savannas of North America
Pre-Columbian savannas once existed across North America. These were created and maintained in a fire ecology by Native Americans until the 16th century death of most Native people. Surviving natives continued using fire to clear savanna until European colonists began colonizing the eastern seaboard two hundred years later. Many colonists continued the practice of burning to clear underbrush, reinforced by their similar experience in Europe, but some land reverted to forest.〔
== 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.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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