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National Tallgrass Prairie is part of the North American Grasslands that extends
from Texas to Saskatchewan and from the Rocky Mountains to the Prairie Peninsula in the Midwest.
Within the North American Grasslands, rainfall decreases from the
east to west, and the prairie vegetation consequently changes. The
most visually obvious response to reduced precipitation is that the
grasses and forbs become shorter, which leads to a somewhat
oversimplified classification scheme of tallgrass, mid- or
mixed-grass, and short-grass prairie. Tallgrass prairie occurs in
the eastern part, reaching its ultimate development in the Prairie
Peninsula. Short-grass prairie occurs on the high plains east of
the Rocky Mountains, and mid-grass prairie occurs in between. The
height of the vegetation not only becomes shorter westward, but the
species composition also changes. For example, big bluestem (Andropogon gerardii) and Indian
grass (Sorghastrum nutans) are
dominant in the tallgrass prairie, whereas buffalo grass
(Buchloë dactyloides) and blue grama (Bouteloua
gracilis) are prevalent in the short-grass prairie.
A strong east to west decrease in precipitation exists across the North American grasslands, with most of the grassland region having a moisture deficit in which evaporation exceeds precipitation. The Prairie Peninsula, however, is distinctive in having a moisture surplus on average, with precipitation exceeding evaporation. Thus, a puzzle to many observers has been why this wedge-shaped extension of prairie even exists.
The grasslands experience a "monsoonal" climate characterized by low winter precipitation and high summertime precipitation. Dry western and arctic air masses predominate in the west, but to the east, the westerly air stream is pinched between two air masses containing more moisture. The Alberta storm track, to the north, is responsible for heavier snowfall in Minnesota and northeastern Iowa. To the southeast, moist, tropical air from the Gulf of Mexico brings abundant precipitation. During summer, continental heating causes air to rise and creates low pressure in the mid-continent, drawing in moist air from the Gulf. Areas nearer to the Gulf, including the Prairie Peninsula, receive proportionately more precipitation.
During non-drought years in the eastern grasslands, summer precipitation is as great as in adjacent forested areas. During drought years, however, the Prairie Peninsula suffers more extreme drought because a wedge of dry, westerly air penetrates farther east than usual, blocking moist air from the Gulf (Borchert, 1950, Wright, 1968). The greater severity of drought in the Prairie Peninsula is one of the important factors favoring the persistence of prairie in a climatic regime otherwise favorable for forest.
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