The formation of the North American Prairies started with the
upwelling of the Rocky Mountains. The mountains created a rain shadow that
killed most of the trees.The parent material of most prairie soil was deposited
during the last glacial advance that began about 110,000 years ago. The
glaciers expanding southward scraped the landscape, picking up geologic
material and leveling the terrain. As the glaciers retreated about 10,000 years
ago, it deposited this material in the form of till. Wind based loess deposits
also form an important parent material for prairie soils.
Tall grass Prairie evolved over tens of thousands of years
with the disturbances of grazing and fire. Native ungulates such as bison, elk,
and white-tailed deer, roamed the expansive, diverse, plentiful grassland
before European colonization of the Americas. For 10,000-20,000 years native
people used fire annually as a tool to assist in hunting, transportation and
safety. Evidence of ignition sources of fire in the tallgrass prairie are
overwhelmingly human as opposed to lightning. Humans, and grazing animals, were
active participants in the process of prairie formation and the establishment
of the diversity of graminoid and forbs species.
Fire has the effect on prairies of removing trees, clearing
dead plant matter, and changing the availability of certain nutrients in the
soil from the ash produced. Fire kills the vascular tissue of trees, but not
prairie, as up to 75% (depending on the species) of the total plant biomass is
below the soil surface and will re-grow from its deep (up to 6 feet) roots.
Without disturbance, trees will encroach on a grassland, cast shade, which
suppresses the understory. Prairie and widely spaced oak trees evolved to
coexist in the oak savanna ecosystem.
