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Regional Landscape Ecosystems of Michigan, Minnesota, and Wisconsin


SECTION III. SOUTHEASTERN MINNESOTA AND WEST-CENTRAL WISCONSIN SAVANNA


Part of Bailey and Cushwa's (1981) Humid Temperate Domain, Humid Hot-Summer Continental Division, Eastern Deciduous Forest Province; Des Moines lobe (late Wisconsinan age) glacial features and pre-Illinoian glacial features; savanna prevalent, also tallgrass prairie, sugar maple-basswood forest.
Section III contains four subsections: Hardwood Hills, Big Woods, Anoka Sand Plain, Southern Oak Plains. The section and subsections are shown on the large three-State map (Plate 1).
DISCUSSION: A large part of the section is characterized by gently sloping ground moraine and end and lateral moraine ridges, with calcareous soils. The relatively long growing season, coupled with fertile soils, has resulted in heavy agricultural use of the land.

ELEVATION: 650 to 1,600 feet (198 to 488 m).

AREA: 16,208 square miles (41,989 sq km).

STATES: Minnesota, Wisconsin.

CLIMATE: Continental, with great differences between summer and winter temperatures. The section is influenced by three air masses: the Maritime Polar (dry, mild), Maritime Tropical (moist, warm), and Continental Polar (dry, cold). Total annual precipitation ranges from 24 to 32 inches across the section, and precipitation increases from west to east (Wendland et al. 1992). Annual snowfall averages 44 to 48 inches. This relatively light winter precipitation may partially account for stress on many tree species and may increase the potential for spring fires, which are important for maintaining prairie and savanna conditions. The section receives more precipitation in the form of snow than Section VII to the north. This snow comes in the form of moisture-carrying storms from the southwest. In contrast, Section VII receives dry, cold air from the northwest (Wisconsin Statistical Reporting Service 1967).

BEDROCK GEOLOGY: Glacial drift thickness is quite variable, ranging from less than 100 feet of drift in the east, with local exposures of bedrock along the St. Croix River, to 500 feet of glacial drift in parts of the western portion of the section (Olsen and Mossler 1982). Bedrock underlying the section is diverse; in the southwest are Cretaceous shale, sandstone, and clay; lower Precambrian granite; metasedimentary and metaigneous gneiss, schist, and migmatite; and amphibolite and granulite (Morey 1976). In the northwest are metasedimentary rocks; iron formation; greenstone; and metavolcanic rocks, including basalt, andesite, pillow lava, tuff, and ultramafic and rhyolitic rocks. Farther east, the underlying bedrocks are Ordovician and Cambrian sandstone, shale, and dolomite to the south and Cretaceous shale, sandstone, and clay to the north. At the extreme eastern edge of the section, Ordovician and Devonian dolomite, with some limestone, sandstone, and shale, are locally exposed, especially in the dissected stream valleys.

LANDFORMS: Section III has surface sediments and landforms from the most recent Wisconsin Glaciation. End moraine, ground moraine, and outwash are the most common landforms; a loess cap (wind-deposited silt) covers much of the section. The Anoka Sand Plain (III.3) has been historically interpreted as an outwash plain, but is now considered to be partially a lacustrine feature (Keen and Shane 1990, Lehr 1992, Meyer et al. 1993, Meyer 1993, Meyer and Hobbs 1993).

SOILS: Soils are generally fine textured, either derived from glacial till or aeolian silt (loess) deposits (Cummins and Grigal 1981). The till often contains abundant clasts of sedimentary bedrock. Loess deposits are up to 16 feet deep. The soils are primarily Alfisols (Udalfs), Mollisols (including Udolls and Aquolls), and some Psamments (USDA Soil Conservation Service 1967, Cummins and Grigal 1981). The soils of the Anoka Sand Plain are sands.

PRESETTLEMENT VEGETATION: Bur oak savannas (openings) and oak forest were the characteristic vegetation of the section. Tallgrass prairie occupied the least dissected, rolling parts of the section; prairie grasses dominated the groundcover of the open savannas, which often occupied slightly more irregular, rolling to moderately hilly topography. Sugar maple-basswood forest occupied the steepest, most fire-protected sites.

NATURAL DISTURBANCE: Fire occurred regularly in the savannas and prairies. Fire frequency was largely controlled by climatic fluctuations (Kline and Cottam 1979, Grimm 1984). Windthrow was not commonly referenced by GLO surveyors, but was probably important in the forests.

PRESENT VEGETATION AND LAND USE: Most of the level to hilly topography is farmed, except for parts of the Anoka Sand Plain and major river corridors; primarily the steepest and most dissected topography remains in natural vegetation.

RARE PLANT COMMUNITIES: Both tallgrass prairie and oak savanna, originally the dominant vegetation, have become rare because of fire exclusion and agricultural development.

RARE PLANTS: See subsections and sub-subsections.

RARE ANIMALS: See subsections and sub-subsections.

NATURAL AREAS: See subsections and sub-subsections.

PUBLIC LAND MANAGERS: See subsections and sub-subsections.

CONSERVATION CONCERNS: Minnesota is attempting to maintain the oak component in the section's forests, as well as the native prairies, prairie chicken habitat, and sedge-dominated wetlands. Many of the native prairies in the section are in danger of being mined for gravel. Forests need to be managed as larger tracts, with increased amounts of old-growth forest.

BOUNDARY JUSTIFICATION: The boundary between Sections II and III is based on the predominance of savanna in Section III and prairie in Section II. As already discussed in Section II, savanna becomes more common than prairie as intensity of fire decreases and the interval between fires lengthens; these conditions typically result from increased topographic relief. Prairie was scattered throughout Section III, primarily on relatively flat topography and on some alluvial soils. The boundary between Sections III and Section IV is based on a major physiographic boundary: Section III consists of rolling to hilly ground moraine and end moraine; Section IV consists of the highly dissected, steep topography of the Driftless Area, also called the Paleozoic Plateau or Blufflands in Minnesota.


Previous Section -- Section II. Southwestern Minnesota Grassland
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Next Section -- Section IV. Driftless Area

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