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Central Prairies and Plains

The great grasslands of the Midwest and Great Plains are defined, both ecologically and aesthetically, by plants that are mostly herbaceous. The mid-continent is a grassland landscape because climatic factors and the influence of fire make the growth of woody plants difficult. Due to the combination of excessive summer heat, persistent winds, and low precipitation, evaporation often equals precipitation. Trees have shallow, widespread root systems and a huge, persistent biomass to support. Where rainfall is sporadic or strongly seasonal, trees are limited to areas where moisture is more predictable, such as along water courses and coulees, moist valleys in the rolling glacial moraine country, where trees grow sheltered from the harsh environment around them. Wildfire also is essential to the survival of the prairie, keeping it free of trees (see "The Role of Fire".

Prairie Classification

Ecologists classify prairies by moisture regime, or the dominant seasonal patterns of rainfall and drought, combined with the soil's ability to hold moisture: xeric, or dry; mesic, or moist; and hydric, or wet. These basic terms relating to moisture are used to describe other plant communities as well, such as forests. Moisture regimes are determined by topography and soil type. In general, dry prairies are found at the highest elevations on gravely, sandy, or sandy loam soils. Mesic prairies are seldom inundated by water and are on loamy soils with varying degrees of silt and clay. Hydric prairies are often flooded during winter and spring. The underlying soils are heavy clay or highly organic peat. In general, the tallgrass prairie region is composed of hydric and mesic prairies, the mixed-grass prairies are mesic or xeric, and the shortgrass prairie is xeric. The classification is not just a regional one, however. Dry gravel ridges within the tallgrass region may support xeric prairie, and wetlands in the shortgrass prairie region may support hydric plants.

The Role of Fire

Fire is an important force of change in the prairie landscape. Historically, fires of varying intensities and durations burned vast tracts of land. During drought years, these raging fires drastically altered the landscape, turning forests to savannahs and savannahs to prairies. Herbaceous plants are resistant to fire because their growing points are below the soil surface, where they are protected from the fire's most intense heat. Woody plants have little ability to withstand a hot fire. Most trees have thin bark, with the actively growing cells, called the cambium, close to the surface. The heat of a prairie or woodland fire is intense enough to kill the cambium of most trees. The fire-tolerant bur oak is able to survive because the cambium is protected by a thick layer of corky bark that insulates the living cells from the intense heat. Because of its fire tolerance, bur oak is the dominant tree of the oak savannah.

Grasses have extensive underground networks of fibrous roots that probe deeply and thoroughly for moisture. In fact, most of the biomass of both grasses and prairie forbs, or native wildflowers, is below the ground. These plants produce foliage and flowers in response to available moisture. In times of plenty, growth is lush and flowers are abundant. During drought cycles, plants may remain in a semi-dormant condition during the growing season to conserve resources. Soil compaction and grazing by bison also helped shape prairie vegetation. Bison once roamed the Midwest, but the herds were decimated by hunters a century ago, and those that remain are all corralled. Deer, fox, songbirds, and insects are plentiful.

Gardening in the prairie states can be a challenge. The intense winter cold, summer heat, and periodic drought produce stressful conditions for plants. However, winter snows protect them from the worst of the cold, so perennials can survive. In fact, many are adapted to the mid-continental climate; a large number of beautiful perennials such as blazing stars (Liatris species), coneflowers (Echinacea species), sunflowers (Helianthus species), and asters are native to the prairie province. Both perennials and vegetables thrive, especially in the eastern reaches of the prairie region, where there is more precipitation and the soil is richer.

The three broad associations in the prairie province are tallgrass, mixed-grass, and shortgrass prairie. Savannah, an open mixture of trees and prairie plants, is a common ecotone where the forest meets the tallgrass prairie.

Oak Savannah

Where the prairie meets the forest are open, park-like areas called savannahs. In savannahs, small expanses of prairie are interspersed with open groves of trees that shelter shrubs and woodland wildflowers. Grasses, sedges (Carex species), and sun-loving wildflowers mingle with woodland plants such as starry Solomon's-seal (Smilacina stellata) and bellwort (Uvularia grandiflora) in unique combinations. The savannah is a fire-dependent community. Extended drought cycles favor prairie plants, as fire burns away trees not adapted to surviving the blistering heat. The woodland retreats and the prairie makes inroads. During wet cycles, the trees are favored, and in time reseed and advance into the prairie. With them come fire-sensitive shrubs and woodland wildflowers. These drought and wet cycles may last 100 years or more.

Tallgrass Prairie

The easternmost portion of the prairie province has deep, rich soils and ample rainfall for luxuriant growth. This is the tallgrass prairie association. Bluestems, switchgrass, asters, goldenrods, and milkweeds (Asclepias species) are a few of the common plants. In tallgrass prairie there is, on average, less than one tree per acre. Shrubs occur in scattered patches, especially where the soil is moist.

Mixed-Grass Prairie

Farther west, the mixed-grass prairies have rich soils but more moderate rainfall. With less than 20 inches of rain per year, trees find it very hard to grow. Grasses and perennial forbs dominate. Extreme summer drought and bitterly cold winter temperatures are the rule, not the exception. If conditions become too rigorous during the growing season, plants go dormant to conserve their resources. When the rains return, plants resume growth with few ill effects. This region is peppered with wetlands, especially in the north. These wetlands, called prairie potholes, are critical habitat for nesting waterfowl, shorebirds, waders, and songbirds.

Shortgrass Prairie

In the rain shadow of the Rocky Mountains, the shortgrass prairie is the driest mid-continental grassland. On the high plains, the annual rainfall of 15 inches is scarcely more than that in a desert. The plants are adapted to extreme drought and cold. The vegetation is short in stature, and comparatively drought-tolerant grasses outnumber the forbs. There are more annuals in the shortgrass prairie because the open soil between the grasses provides a seedbed for germination. Antelope, badgers, prairie dogs, and shorebirds abound.