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Regional Landscape Ecosystems of Michigan, Minnesota, and Wisconsin


SECTION II. SOUTHWESTERN MINNESOTA GRASSLAND


Part of Bailey and Cushwa's (1981) Humid Temperate Domain, Subhumid Prairie Division, Tall-grass Prairie Province; till plains of pre-Illinoian, Illinoian, and Wisconsinan age; tallgrass prairie.
Section II contains two subsections: Upper Minnesota River Country, Coteau des Prairies. The section and subsections are shown on the large three-State map (Plate I).
DISCUSSION: The section, once dominated by tallgrass prairie, is concentrated in western Minnesota, but continues into adjacent South Dakota and Iowa. Like other prairie areas of U.S., this section is characterized by geomorphic features of low relief, which allow the spread of fires over long distances (Rumney 1968, Gleason 1913). The fertile soils of the section have been intensively farmed mostly for grains, potatoes, and sugar beets, almost completely eliminating the original prairie vegetation.

ELEVATION: 750 to 1,995 feet (229 to 608 m).

AREA: 16,278 square miles (42,420 sq km).

STATES: Minnesota.

CLIMATE: Continental, with great differences between winter and summer temperature. The section is strongly influenced by the Maritime Polar (dry, mild) air mass and also influenced by Maritime Tropical (moist, warm) and Continental Polar (dry, cold) air masses. Annual average temperatures range from 44½F in the south to 39½F in the north (Wheeler et al. 1992). Average annual precipitation ranges from approximately 24 inches in the northwest to approximately 30 inches in the southeast (Wendland et al. 1992). Mean annual snowfall is about 40 inches.

Although the total precipitation is not significantly less than that in portions of the forested sections to the east, the timing of precipitation is probably an important influence on the vegetation; only 11 percent of the annual precipitation arrives during November through February (based on Wendland et al. 1992). In contrast, savanna-dominated Section III receives 16 to 23 percent of its precipitation in this same period, while conifer-dominated Section X receives 14 to 29 percent. The combination of low levels of winter precipitation and strong desiccating winds increases the potential for spring fires. Drought conditions also occur relatively often during the growing season (Weaver 1954, Weaver and Albertson 1956). Tree growth is limited under these climatic conditions.

BEDROCK GEOLOGY: Bedrock within most of the section is blanketed with a thick layer of glacial drift that ranges from 100 to 600 feet in depth (Olsen and Mossler 1982). Bedrock, locally exposed here, includes Precambrian granitic bedrock along large stretches of the Minnesota River (Matsch and Wright 1967); Cambrian sandstone and Ordovician dolomite and sandstone along the Minnesota River near Mankato (Sims et al. 1966, Morey 1981); and numerous outcrops of Precambrian quartzite on the Coteau des Prairies and elsewhere (Wright 1972).

The glacial drift is underlain by several types of bedrock. Cretaceous shale, sandstone, and clay are the most common, but there are also Ordovician dolomite, sandstone, and shale; Jurassic shale, dolomite, and gypsum; and Precambrian quartzite, gneiss, amphibolite, undifferentiated granite, and metamorphosed mafic to intermediate volcanic and sedimentary rocks and quartzite (Morey et al. 1982).

LANDFORMS: Gently sloping ground moraines and water-reworked moraines characterize the greatest area, but there are also some areas of glacial lake plain. The Coteau des Prairies has deeply incised, loess-covered ridges underlain by pre-Illinoian tills. End-moraine ridges, including stagnation moraines, occupy only a small percentage of the section's surface. Broad terraces and alluvial deposits are associated with the Minnesota River and its tributaries, as well as numerous narrow outwash channels.

SOILS: Calcareous glacial and lacustrine deposits of late Wisconsinan age are parent material of the Upper Minnesota River Country (Wheeler et al. 1992a). The silt- and clay-rich soils have thick, organic-rich surface horizons typically associated with prairies. The soils are classed primarily as Mollisols (Aquolls, Borolls, Ustolls, and Udolls) by Cummins and Grigal (1981) and the Minnesota Soil Survey Staff (1983).

PRESETTLEMENT VEGETATION: The original vegetation was almost entirely prairie and prairie wetland (Marschner 1974). Outliers of woodland occurred along the eastern margin, primarily bur oak openings and aspen-oak woodlands (Wheeler et al. 1992a). Woodlands formed narrow borders along major streams; oak openings occurred on dissected bluffs such as those above Big Stone Lake and Lake Traverse in extreme southwestern Minnesota.

Recent studies have further subdivided Marschner's prairies into several prairie types, including tallgrass prairie, wet prairie, brush prairie, and gravel prairie. See subsections or sub-subsections.

The vegetation zone immediately west of Section II's tallgrass prairie (less than 30 miles outside the area of this study to the west)—the short grass prairie or steppe—occupies a long, narrow, north-south-trending zone almost completely within the rainshadow of the Rocky Mountains (Rumney 1968, Küchler 1964). These shortgrass prairies are also called the transitional grassland zone (wheatgrass-bluestem-needlegrass) by Küchler (1964).

NATURAL DISTURBANCE: Fire and drought were once major disturbance factors, but many depressions in the ground moraine and lake plain within the section were also regularly flooded, resulting in periodic tree and shrub mortality. Bison grazing was important for exposing mineral soil and limiting woody vegetation along the prairie/woodland border (Bird 1961). The Native American practice of hunting buffalo with fire over the last 3,000 to 4,000 years, probably contributed to maintaining the prairie. Grazing by other large mammals, insect infestations, and ant activities in the soil all had impacts on vegetation structure and establishment.

PRESENT VEGETATION AND LAND USE: The primary land use is now agriculture, either cultivation of row crops or livestock grazing. Human manipulation of drainage has allowed large areas of poorly drained soils to be farmed. Over most of the section, grassland vegetation is dominated by exotic and native weedy species (Wheeler et al. 1992a). Native prairie vegetation survives primarily on poor agricultural sites, such as steep slopes, poorly drained sites, and on droughty, infertile soils. Small fragments of wet prairies remain, and mesic prairie vegetation persists as even smaller remnants within either wet or dry prairie.

RARE PLANT COMMUNITIES: All prairie types are now rare.

RARE PLANTS: Several plants associated with prairies are now rare. See subsections and sub-subsections.

RARE ANIMALS: Several animals of the prairie are now either rare or extirpated. See subsections and sub-subsections.

NATURAL AREAS: See subsections and sub-subsections.

PUBLIC LAND MANAGERS: See subsections and sub-subsections.

CONSERVATION CONCERNS: Landscape management teams in Minnesota have identified maintenance of river and stream corridors within the section as a high priority; riparian habitat provides continuous cover and travel corridors for many species. Streamside vegetation controls sedimentation and other nonpoint-source pollution of streams. Oxbows are of historic, ecological, and wildlife value. Limiting grazing, road construction, and surface mineral extraction within the riparian zone has been recommended, as has maintenance and restoration of mixed-species forests in riparian areas.

BOUNDARY JUSTIFICATION: The change from dominance by grasses in Section II to dominance by woodland or forest in Sections III to XI is primarily the product of more extreme drought conditions in the prairies of Section II. The prairie-forest ecotone has been related to mid-tropospheric flow patterns during the summer (Harman and Braud 1975, Harrington and Harman 1985).

The actual boundary of the grasslands with woodlands is generally located at or just east of a fire barrier, such as a series of lakes in a stagnation moraine, a series of glacial beach ridges and swales, a large moraine ridge, or a broad flood plain. The present boundary is similar to that drawn by Kratz and Jensen (1983), which corresponds reasonably well with the boundary of continuous prairie vegetation (Marschner 1974) and continuous prairie soils (Cummins and Grigal 1981).

Farther to the east, prairie exists in a broken mosaic, indicating either past conditions that allowed the prairie to expand there, or anthropomorphic expansion of the prairie through burn management. Pollen records indicate such a past expansion of the prairies and savannas approximately 5,000 to 7,000 years ago due to climatic fluctuations (Delacourt and Delacourt 1981). The importance of Indian occupation and management with fire is recognized in many areas where prairie occurred within a mosaic of forest and savanna (Anderson 1990).

The boundary between Section II and Section III is located along the west edge of parts of the Big Stone, Alexandria, and Altamont moraines. The correspondence of continuous prairie to this boundary is only approximate because the characteristics of a given section of boundary, i.e., the size of water bodies or ridges, determine the sharpness of the prairie/woodland boundary.


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