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Regional Landscape Ecosystems of Michigan, Minnesota, and Wisconsin


SECTION IV. DRIFTLESS AREA


(Paleozoic Plateau or Blufflands); part of Bailey and Cushwa's (1981) Humid Temperate Domain, Humid Hot-Summer Continental Division, Eastern Deciduous Forest Province; highly dissected, loess-capped unglaciated landscape, including some glacial features of pre-Illinioan age; oak savanna, tallgrass prairie, midgrass prairie, sugar maple-basswood forest.
Section IV contains three subsections: Prairie du Chien, Eau Claire, Maple-Basswood Forested River Ravines. The section and subsection are shown on the large three-State map (Plate I).
DISCUSSION: Section IV is a highly eroded, unglaciated landscape, with some till deposits of pre-Wisconsinan age near the margins of the section. It has been called the Driftless Area in Wisconsin (Curtis 1959) and Blufflands (Kratz and Jensen 1983) or Paleozoic Plateau (Hargrave 1992) in Minnesota; the section continues south into Iowa and Illinois.

ELEVATION: 603 to 1,450 feet (184 to 442 m).

AREA: 16,203 square miles (41,986 sq km).

STATES: Minnesota and Wisconsin.

CLIMATE: Continental. Annual average precipitation ranges from 29 inches in the west to 34 inches in the southeast (Hargrave 1992, Wendland et al. 1992). Annual average snowfall ranges from 32 inches in the south to approximately 50 inches in the north (Wendland et al. 1992). Growing season precipitation ranges roughly from 11 to 16 inches, and growing season length ranges from 129 to 170 days. Extreme minimum temperature ranges from -30½F in the south to below -40½F in the north (Wendland et al. 1992).

BEDROCK GEOLOGY: Large exposures of bedrock occur in the steep ravines. These exposures are primarily Ordovician dolomite, limestone, and sandstone in Minnesota, with Cambrian sandstone, shale, and dolomite exposed along the valley walls of the Mississippi River (Morey 1981, Sims et al. 1966). Devonian dolomite and limestone are more locally exposed along the western edge, in Minnesota. In Wisconsin, Ordovician dolomites have the greatest exposures (Ostrom 1981).

LANDFORMS: A loess-capped plateau, deeply dissected by river valleys. The greatest amount of relief—as much as 600 feet—occurs along the Mississippi River. In Wisconsin, much of the section was never glaciated. Parts of the section, as mapped, are considered to have been blanketed with pre-Illinoian-age till, most of which was probably removed before the area was covered with Wisconsinan-age loess. In eastern Minnesota, loess lies directly upon bedrock. Paleozoic sedimentary rocks crop out in the valley walls, but are generally mantled with colluvium or loess.

Along the western edge of the section, where glacial drift is several feet thick, topography is controlled by the underlying glacial till; further east, where glacial drift is thin, topography is largely bedrock controlled (University of Minnesota et al. 1973). In most of the section, there is no sign of past glaciation. In these areas, beneath the surface deposit of loess is a layer of limestone residuum, underlain by bedrock.

LAKES AND STREAMS: Few natural lakes except for some in the northern part of Sub-subsection IV.2, all on old glacial drift. Water tables are commonly deeper than 20 feet (University of Minnesota et al. 1973). Sinkholes are common in the southwest. Several major rivers flow through the section, forming steep ravines and, on some of the streams, broad alluvial plains. These rivers include the Mississippi, Wisconsin, Kickapoo, Chippewa, Black, Root, Whitewater, Zumbro, and Canon.

SOILS: Loess thickness is quite variable; in Minnesota, loess deposits range from 20 feet thick on broad ridgetops to less than a foot thick on valley walls (Hargrave 1992). In Wisconsin, loess deposits range from 16 feet thick along the Mississippi River to 2 to 4 feet thick and locally discontinuous at the eastern edge of the section (Hole 1976). The predominant soils are Udalfs, with localized Aquents along the flood plains of major rivers (Cummins and Grigal 1981). Cambrian siltstones, sandstones, and shales influence the soil properties.

PRESETTLEMENT VEGETATION: Major vegetation types were tallgrass prairie and bur oak savanna on ridge tops and dry upper slopes, sugar maple-basswood-oak forest on moister slopes, sugar maple-basswood forests in protected valleys and on north-facing slopes, wet prairies along the rivers, and some mesic prairie on the flood plain further back from the river (Finley 1976, Lange 1990). There were probably also oak forests that contained no sugar maple. Marsh and flood-plain forests were also common on river flood plains.

Prairie was restricted primarily to the broader ridge tops, which were unfavorable sites for trees due to thin soils, rapid drainage, and desiccating winds; all these conditions were also good for carrying fires across the landscape (Finley 1976). Prairies also occurred on steep slopes with south or southwest aspect.

Large areas dominated by maple-basswood forest are treated as Sub-subsections IV.3.1, IV.3.2, and IV.3.3. (See figure 4.)

NATURAL DISTURBANCE: Fire was important on the upland prairie and oak-dominated ecosystems; it also occurred in some lowlands, such as oak-dominated flood-plain forests and wet meadows (Finley 1976). Windthrows were recorded in the original GLO survey notes for Sauk County, including areas of 40 and 1,500 acres (Lange 1990). Recent records of tornados and ice storms indicate their local impact on forest vegetation.

PRESENT VEGETATION AND LAND USE: In Minnesota, Wheeler et al. (1994) found abundant species characteristic of oak openings and barrens (based on herbarium collections), especially in the west, where a greater portion of the topography is relatively flat upland. Prairie species are also relatively common in the west, but rare elsewhere, where the land is in agriculture. In eastern Minnesota, where the most dissected topography with a greater percentage of lowland is present, both oak opening/barren and prairie flora are much less common.

Cliffs and associated flora and fauna are common within this section. Maple-basswood forest and mesic oak forests are present on east and north slopes; oak forest and woodland occur on south to west slopes. Relict pine forests are also common, especially at the southern edge of the section in Wisconsin. Hemlock is locally common in the southern half of the section, especially on northern and eastern sandstone exposures along the drainages of the Kickapoo and Baraboo Rivers. In Wisconsin, yellow birch, along with black ash, is characteristic of seepages on the lower, steep slopes. Hemlock, yellow birch, and black ash are much rarer in Minnesota than in Wisconsin.

In Minnesota, the western part of the section is heavily farmed, with approximately 90 percent in crops and pasture and the remainder in woodland. Farther east in Minnesota and Wisconsin, along the steep coulees, bluffs, and ridges above the Mississippi River, Wisconsin River, and other major streams, 50 to 60 percent is in crops and pasture and the remainder is in woodland (University of Minnesota et al. 1973, Hole 1976, Raile 1985).

Cottam (1949) found that prairie and savanna had been replaced by oak forest after fire suppression. Hix (1988) found steepness of slope and aspect, as well as soil depth and texture, to be important factors determining the forest composition along the Kickapoo River. Many of the remaining areas of prairie in Minnesota and Wisconsin are dry-mesic prairie on droughty sites, often steep slopes.

A recent study of Sauk County and portions of Columbia County, Wisconsin (Lange 1990), documents land use since European settlement within part of the section. Most prairies and savannas were farmed. Forests dominated by oak and sugar maple were used for fueling lime kilns and creating charcoal for smelting; oak was used for barrels and railroad ties. Fires were common, even after European settlement, and were documented as occurring frequently both in uplands and wetlands.

RARE PLANT COMMUNITIES: A rare natural community, alJPGic talus slope, is found only along the steep bluffs of tributaries of the Mississippi River within this section. Prairies and savannas, once common on the flat ridge tops and slopes, are now rare, but examples of bluff prairie, dry oak savanna, dry prairie (bedrock bluff subtype) persist locally. High-quality examples of calcareous seepage fen, dry cliff, flood-plain forest, maple-basswood forest, moist cliff, oak forest, and white pine-hardwood forest also occur in the section.

RARE PLANTS: Rare plants (and animals) reflect several different but characteristic landscapes of the section. Many are associated with the prairies and savannas on ridge tops and steep slopes, others with forests with a relict northern flora, and still others with bedrock cliffs. See subsections.

RARE ANIMALS: See comments for rare plants. The fauna also reflects the isolated, relict watersheds of the section, which have rich fish and mussel faunas, as well as dragonflies and mayflies with restricted distribution. The southern half of the section contains many significant bat hibernacula. See subsections.

NATURAL AREAS: This section has more natural areas than any other in the three States, reflecting the large number of rare plants and animals found here, as well as the unique topography. Natural Areas are listed under subsections.

PUBLIC LAND MANAGERS: See subsections.

CONSERVATION CONCERNS: Grazing is common in woodlots in both Minnesota and Wisconsin. Logging pressure on oak saw logs is high due to the depressed farm economy and strong timber market, and there is a concern about future logging on federally managed flood-plain forests. Development pressure is increasing on the blufflands, especially on the Mississippi and lower Wisconsin Rivers. Mesic prairie has been almost eliminated. In Wisconsin, bluff prairies are being lost due to woody species encroachment and lack of fire.

BOUNDARIES: Section IV is bounded on all sides by more rolling topography.


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