Prairie grasslands description
Description | Landscape areas
Imagine a kaleidoscope of color that shifts with the seasons: the smoky haze of pasque flowers in spring, the purple of blazing star and deep orange of butterfly weed in hot midsummer, the red-gold of raspy grasses in fall, the white sparkle of frost on straw-colored grasses in winter.
This is the prairie palette that marked the seasons for native peoples of long ago, when vast grasslands spread from the northwestern to the southeastern tips of the state. These grasslands ranged from sparsely vegetated sand dunes to vast fields of big bluestem up to eight feet tall, from low, wet sedge meadows to short-grass prairies high on the bluffs of the Mississippi River. Bison and elk roamed the plains, and prairie birds such as the upland sandpiper and sandhill crane were plentiful.
With the advent of European settlement, much of the flat and fertile prairie land fell to the settler's plow. Now, just a century and a half later, only one percent (about 150,000 acres) of the original 18 million acres of prairie remains. Urban sprawl, agricultural expansion, and gravel mining continue to threaten this rich resource. As natural prairie habitats dwindle, so too, do the species of prairie mammals, birds, and insects. At one time, prairie birds--marbled godwits, upland sandpipers, sprague's pipits, chestnut collared longspurs, bobolinks, meadowlarks, kingbirds--were numerous. Waterfowl covered the marshes; bison roamed the western areas, and elk and deer were common. Protection is critical if this complex ecosystem, with its shifting array of color and form, is to survive into the future.
Prairie communities are as varied as the landforms on which they occur?beach ridges formed by ancient glacial lakes; gravelly hills left by retreating glaciers; sandy plains deposited by ancient rivers; steep bluffs; and rolling plains. Moisture levels vary throughout, determining the types of plant communities that develop. Since fire tends to eliminate trees and shrubs while enhancing the growth of prairie grasses, wildfire frequency also affects the types and persistence of prairie communities.
At the lowest end of the moisture spectrum, dry prairies are dominated by short- to mid-height (up to 2 feet tall) grasses and forbs (flowering plants) adapted to dry conditions. Little bluestem, side-oats grama, and porcupine grasses typically dominate; dotted blazing star, pasque flower, and puccoons are characteristic forbs. Biologists identify four subtypes of dry prairie.
- The sparsely vegetated barrens subtype occurs on deep deposits of sand left primarily by glacial meltwater rivers. Winds have often reworked these deposits into dunes during subsequent periods of severe drought.
- The sand-gravel subtype occurs on nearly level to steeply sloping gravel-rich deposits left by melting glaciers or deposited along the shores of large glacial lakes.
- The hill subtype, richest in species, climbs steep slopes of loamy till also deposited by glaciers.
- Along steep Mississippi River bluffs in southeastern Minnesota, the bedrock bluff subtype, often called goat prairie, sprouts from a thin layer of soil over bedrock.
In all four cases, frequent fires as well as low moisture levels help to prevent the encroachment of trees.
Dry Oak Savannas
Where the frequency and intensity of fire in the dry prairie is lower, species such as bur oak and northern pin oak may invade successfully to create a community called dry oak savanna. Here small to large, single to clumped, gnarled and spreading bur oaks dot typical dry prairie vegetation. Dry oak savannas were once much more common, occurring in the transition zone between prairie and forest on the same landforms as dry prairies (except bedrock). Today they are very rare; most have been plowed under for cropland or, with the suppression of fire, have succeeded to woodlands.
In mesic prairies, moderate soil moisture levels support grasses up to six feet tall. Big bluestem, Indian grass, and prairie dropseed generally dominate; heart-leaved alexander, maximilian sunflower, and wood lily are typical forbs. These rich prairies occur mainly in southern and western Minnesota on level or gently rolling land. Their black, nutrient-rich soils range from sandy to silty and support more than 300 plant species.
Where mesic prairies occur in cooler climates, as in the Aspen Parklands of northwestern Minnesota, less frequent burning allows the invasion of stunted trees such as young aspen and scrub oak, and shrub species such as slender willow, bog birch, and hazel. These communities, in which brush comprises 30 to 50 percent of the total vegetation cover, are called mesic brush prairies.
In flat or low-lying areas with poorly-draining, mineral (inorganic) soils, snow melt or rainwater may accumulate and stand for short periods, or the water table may remain close to the surface for much of the growing season. Here grasses such as prairie cordgrass, blue-joint, bog reed-grass, big bluestem, and mat muhly dominate the wet prairie, along with several sedge species. Wet prairies occur throughout the prairie zone, but are especially common on the broad, poorly drained flats of the Glacial Lake Agassiz Interbeach Area in Minnesota's far northwestern corner.
Where fire is either less frequent or less intense, wet prairies may succeed into wet brush prairies, in which willow species and other shrubs cover 30 to 50 percent of the ground. The shrubs form thickets that dot a continuous cover of typical wet prairie grasses and forbs. Web brush prairies occur primarily in the northwestern part of the state.
Calcareous Seepage Fens
A rare and different kind of prairie wetland, the calcareous seepage fen, is dominated by sedges such as prairie sedge, sterile sedge, and tussock sedge adapted to soils continuously saturated with cold groundwater. Typically, groundwater that has been percolating through a permeable layer of porous material (overlying a less permeable layer) surfaces where the porous layer is exposed at a slope. Where the cold, oxygen-poor groundwater emerges, organic matter decomposes very slowly, and eventually a layer of peat accumulates, typically more than a half meter deep. Certain very rare plant species, such as hair-like beak rush and beaked spike rush, specifically adapted to the water's cold temperature, high pH, and uniquely high calcium and magnesium bicarbonate content, occur only in calcareous fens.
The prairie subtype of the calcareous seepage fen contains prairie grasses and forbs intermixed with sedges and rushes. These unique fens occur sporadically throughout much of the prairie zone.
One of the last of the great glacial movements in Minnesota, the Des Moines Lobe advanced across present-day North Dakota into Minnesota and on into Wisconsin and Iowa. The level to gently rolling lake plains and ground moraines of the Prairie Grasslands Biome are its legacy. The northern portion was primarily influenced by Glacial Lake Agassiz and its drainage. From there, the Red River flows north, and the Minnesota River, south. The southern portion features a high plateau of quartzite bedrock topped with till that defines the character of the area.
The Ecological Classification System classified this biome on the basis of its bedrock, glacial deposits, topography, climate, and plant communities. It formed four subsections, or landscape areas, rich in its diversity of prairie communities.