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 The transition between the forests of eastern North America and the tall grass prairie of the midwest and central plains rarely forms a distinct line in the Chicago region. The boundary is especially indistinct in areas with sandy soils. The sand prairie and black oak savanna region is really a mosaic of plant communities. These groupings of associated plants reflect natural variables such as soil types, moisture availability, exposure to sunshine, and temperature conditions. The existence of these biological communities and their distribution is also a testament to the effects human activity, management and exploitation in historic and prehistoric times.
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