Introduction:
North American Grasslands

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The Midewin 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.
Terrain within the prairies is generally level to rolling. The glaciated northeastern sector is generally more level than the unglaciated western part. Extensive glacial till plains and glacial lake beds are almost dead flat. Broken topography in the tallgrass prairie is generally forested. Extensive badlands (very broken, eroded topography) occur in parts of the western grasslands. Trees, especially junipers and ponderosa pine, occur in some of the areas of more broken topography. They are common in the North Dakota badlands, and on bedrock, especially sandstone, escarpments throughout the Great Plains. Extensive dunes occur throughout the western prairies, most notably the Nebraska Sandhills which cover much of the State, but smaller dune fields are widespread. The dunes fields are mostly stabilized by grassland today, but many have been active in the recent past.

 

Grand Prairie in Illinois
The Grand Prairie in Illinois
Midewin NTP in
Red


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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http://exhibits.museum.state.il.us/exhibits/midewin/nagrasslands.html, Last modified September 1st 2011, 03:13PM.