$title="Introduction - Prairies in Illinois"; include "/local/php/ismsite/midewin/header.php"; ?>
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|Although to many casual observers Illinois appears to be a monotonous landscape of agricultural fields and urban areas, the state has considerable biological and geological diversity. A common classification scheme used in Illinois recognizes 14 Natural Divisions which are geographic regions having similar topography, soils, bedrock, plants, and animals (Schwegman et al. 1973). Divisions contain different sections sharing common geological and ecological features. Sections contain a variety of natural communties, each with distinctive biota.|
|Midewin is located in the Grand Prairie Division, the largest of the natural divisions. The topography and soils of the Grand Prairie are the result of its geological history. The topography is flat to gently rolling with low ridges formed by glacial deposits called moraines The soils are developed in glacial debris (till), wind-blown silt (loess), and sediments deposited by glacial streams (outwash) and lakes that formed in front of glaciers. The fine-textured soils with high clay content are poorly drained in many places, resulting in extensive wet prairies and marshes. The vegetation in tallgrass prairie is dominated by big bluestem (Andropogon gerardii) and Indian grass (Sorhgastrum nutans). Trees are present along streams and in scattered groves, savanna, and upland forest on moraines and other glacial hills.||
|Most of the
Midewin National Tallgrass Prairie lies within the Grand Prairie
Section of the Grand Prairie Division and shares the characteristic
features described above. The southwestern corner of Midewin is in
the Kankakee Sand Area Section which is covered by outwash sand
deposited by pulses of glacial meltwater like the Kankakee Torrent.
Sandy soils support different vegetation and wildlife. They drain
rapidly and support vegetation better adapted to dry conditions.
The probability of fire may be greater because the vegetation dries
more rapidly, but on the other hand there is less fuel to feed a
The Natural Divisions classification system recognizes six prairie community classes (Schwegman, et al, 1973):
1. Prairie. Also called black soil prairie. This type is the most widespread in Illinois. It has deep, fine-textured soils formed in silt and clay, ranging from dry to wet. The kinds of plants vary based on the amount of moisture.
2. Sand Prairie. Soils are coarse-textured sands or sandy loam and range from dry to wet. Wet sand prairie is similar to wet prairie in the kinds of plants that are present. Little bluestem and prickly pear cactus (Opuntia compressa) occur on the drier sand prairies, whereas pale coneflower (Echinacea pallida), gray-headed coneflower (Ratibida pinnata), and compass plant (Silphium lancinatum) occur on more mesic sites.
3. Gravel Prairie. Soils are gravelly and rich in calcium and range from dry to mesic (moist, but not wet). Some characteristic plants are little bluestem and prairie dropseed.
4. Dolomite Prairie. This type is the most restricted in Illinois. It occurs where dolomite, a type of sedimentary rock, is less than 1.5 m from the ground surface. At Drummond Dolomite Prairie at Midewin, Dolomite pavement (exposed dolomite bedrock) supporting only lichens and mosses occurs. These soils are subject to seasonal variations in soil moisture because the bedrock impedes drainage, but the shallow soils have little water holding capacity. Common plants are little bluestem , side-oats grama, and hairy beard-tongue (Penstemom hirsutus). In moister areas, prairie dropseed, and leafy prairie clover (Dalea foliosum), blue-joint, and Ridells goldenrod (Solidago riddellii) are present.
5. Hill Prairie. Hill prairies typically occur on steep, south to southwest-facing slopes. The substrate is frequently loess (wind-blown silt and clay). Because of direct exposure to sunlight, these prairies are usually dry. Typical plants include little bluestem, side-oats grama, blue-eyed grass (Sisyrhinchium campestre), green milkweed (Asclepias viridiflora), and gray scurf-pea (Psoralidium tennuiflorum).
6. Shrub Prairie. The soils are sandy, commonly acid. Shrubs and prairie grasses dominate.
The different prairie types normally do not occur as discreet, sharply delineated communities on the landscape but merge into one another. The typical prairie landscape in Illinois is a level to gently rolling mosaic of grasses and forbs with occasional timber in groves or along streams. It is a complex and diverse ecosystem responding especially to subtle variations in soil moisture and texture.
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