Regional Landscape Ecosystems of Michigan, Minnesota, and Wisconsin
SUBSECTION III.4. Southern Oak Plains
DISCUSSION: Much of the subsection is a rolling plain of loess-mantled ridges over sandstone and carbonate bedrock and till. At the southwestern edge of the subsection are moraine ridges that are a continuation of those found in the Big Woods, but they are smaller. As a result, fires from the surrounding prairies to the south, west, and east burnt frequently enough to maintain oak opening rather than forest. At the northern edge, Kratz and Jensen (1983) include St. Croix moraines; these moraines are sandy and stony, with some inclusions of calcareous drift. Oak openings or woodland is also the dominant vegetation on these moraines.
The subsection has traditionally been called the Southern Oak Barrens in Minnesota (Kratz and Jensen 1983), but this name implies a lack of moisture or fertility as being responsible for the open savanna dominated by bur oak. The bur oak savannas were actually located on mesic sites, and the open structure was caused by fire frequency, not low fertility.
SUB-SUBSECTIONS: None.
ELEVATION: 650 to 1,400 feet (198 to 426 m).
AREA: 3,593 square miles (9,306 sq km).
STATES: Minnesota and Wisconsin.
CLIMATE: Annual normal precipitation ranges from 28 inches in the north to 31 inches in the south, and growing season precipitation ranges from 12.5 to 13 inches (Hargrave 1992). Annual average snowfall ranges from less than 40 inches in the southwest to 52 inches in the northeast (Wendland et al. 1992). Average growing season ranges from 146 to 156 days in Minnesota and has been estimated at 150 days for Wisconsin (Hole and Germain 1994). Extreme minimum temperature ranges from -35½F to -40½F (Reinke et al. 1993).
BEDROCK GEOLOGY: Glacial drift is generally less than 100 feet thick, and maximum drift thickness is about 200 feet (Olsen and Mossler 1982). Ordovician and Devonian dolomite (some limestone, sandstone, and shale) is locally exposed, especially in the dissected stream valleys at the eastern edge (Morey 1976, Olsen and Mossler 1982). The gently sloping topography of the subsection is largely the result of resistant, underlying dolomite of the Prairie du Chien Group, which has produced a broad cuesta (Martin 1965, Finley 1976).
LANDFORMS: Much of the subsection is a loess plain over bedrock or till. Also included are late Wisconsinan end moraines, stagnation moraine, and outwash. Topography is generally gently rolling, but there are more dissected ravines at the eastern edge. The stagnation moraine in the southwest is not large, but slopes are often steep.
LAKES AND STREAMS: The few large lakes are restricted to the end moraines to the north. The southern two-thirds of this subsection has a fairly well developed drainage network, resulting from the loess cap over glacial till; the end moraines in the north have an undeveloped drainage network (Hargrave 1993). Wisconsin's prairie pothole region located in northwestern St. Croix and southwestern Polk Counties, has numerous small lakes, ponds, and wet depressions. This is one of Wisconsin's most significant grassland landscapes, where restoration efforts may focus on grassland birds. Two major rivers flow through the subsection, the Mississippi and the St. Croix.
SOILS: The subsection is a mosaic of Alfisols and Mollisols, with Alfisols correlated with savanna vegetation and steeper slopes, and Mollisols correlated with either upland prairie on relatively flat ridgetops or wetland prairies in broad depressions. Common soils include Aquolls, Udolls, Udalfs, and Aqualfs (Cummins and Grigal 1981). Prairie soils cover only a minor portion of the landscape at the northern edge of the subsection in Wisconsin (Hole 1976), even though the vegetation was mapped as tallgrass prairie (Finley 1976). The eroded surface under the mantle of loess is sandy or gravelly (Cummins and Grigal 1981). Twenty to forty feet of local relief characterizes most of the subsection. Soils within the subsection are locally derived from the underlying residuum of dolomite bedrock (Cummins and Grigal 1981, Hole and Germain 1994).
PRESETTLEMENT VEGETATION: The presettlement vegetation was primarily bur oak savanna, but also included tallgrass prairie and maple-basswood forest. The tallgrass prairie was concentrated on the least dissected portions of the landscape, in the center of the subsection and at the northern edge in Wisconsin. The Wisconsin prairie, one of the largest in the State, is unusual in having forest soils rather than prairie soils. Oak brushlands were common along the edges of the prairie in Wisconsin. Bur oak savanna was concentrated on the rolling moraine ridges at the western edge of the subsection and in the dissected ravines at the eastern edge. Maple-basswood was restricted to minor portions of the landscape with the greatest fire protection, either in steep, dissected ravines or where stream orientation reduced fire frequency or severity.
NATURAL DISTURBANCE: Fire is the most important disturbance within the subsection. The GLO surveyors commented on the prevalence of fires in the Wisconsin part of this subsection (Finley 1976). Native American land management with fire may have been partially responsible for maintaining the prairies. The Upper Mississippian Orr culture occupied the Mississippi and St. Croix river corridors as early as 1400 to 1600 A.D. (Tanner 1986). The Dakota tribe had villages along both the east and west side of the Mississippi River in the 1760's; there were still Dakota villages west of the Mississippi in 1810.
PRESENT VEGETATION AND LAND USE: Wheeler et al. (1992b) found species associated with oak openings and barrens to be abundant in the western portion of the Southern Oak Plains subsection. Prairie species were more common in the east, where the topography was lessdissected, than on the moraines of the southwest. Much of the area has been farmed.
RARE PLANT COMMUNITIES: Both tallgrass prairie and oak savanna, once the characteristic vegetation, are now rare due to fire suppression or conversion to agriculture. High-quality examples of several prairie and savanna types persist, including bluff prairie, dry prairie, mesic prairie, dry oak savanna, and dry oak forest. Mesic oak forest, rich fen, and tamarack swamp (seepage subtype) also occur.
RARE PLANTS: Many of the rare plants are restricted to prairies. Minnesota only: Asclepias sullivantii (Sullivant's milkweed), Desmodium illinoense (Illinois tick-trefoil), Eryngium yuccifolium (rattlesnake-master), Erythronium propullans (dwarf trout lily), Orobanche uniflora (one-flowered broom-rape), Parthenium integrifolium (wild quinine), Polanisia Jamesii (James' polanisia), Valeriana edulis (valerian). Wisconsin only: Anemone caroliniana (Carolina anemone), Astragalus crassicarpus (prairie plum). Minnesota and Wisconsin: Besseya bullii (kitten-tails), Lespedeza leptostachya (prairie bush clover).
RARE ANIMALS: Minnesota only: Buteo lineatus (red-shouldered hawk), Falco peregrinus (Peregrine falcon), Lanius ludovicianus (loggerhead shrike), Seiurus motacilla (Louisiana waterthrush), Clemmys insculpta (wood turtle), Emydoidea blandingii (Blanding's turtle). Minnesota and Wisconsin: Crotalus horridus (timber rattlesnake).
NATURAL AREAS: Minnesota: State Natural Areas: Falls Creek, Hastings, Hythecker Prairie, Iron Horse Prairie, Lost Valley Prairie, Pigs Eye Island Heron Rookery, Prairie Creek Woods, Shooting Star Prairie, St. Croix Savanna, Wild Indigo; Nature Centers: Carpenter, Lee and Rose Warner, Maplewood, River Bend; Others: Bailey Woods, Belwin Outdoor Education Center, Hwy 56 Wildflower Route, Jim's Prairie, McKnight Prairie, Nerstrand Woods, Poplar Lake Open Space, Ritter Farm City Park, Wilder Forest. Wisconsin: State Natural Areas: Kinnickinnic River Gorge and Delta, Apple River Canyon.
PUBLIC LAND MANAGERS: Minnesota: Wildlife Management Areas: Aurora, Beaver Creek, Cannon River, Cartney, Gores Pool #3, Nelson Fen, Oak Glen, Paul Hugo Farm, Tutstrum, Upper Iowa River; State Parks: Afton, Fort Snelling, Helmer Myre, Lake Louise, Nerstrand Woods, R.J. Dorer, Rice Lake; County Parks: Beaver Lake, Spring Lake, Square Lake; Regional Parks: Battle Creek, Cottage Grove Ravine, Grass-Vadnais (Snail Lake), Hidden Falls-Crosby, Lake Elmo, Lebanon Hills, Lilydale-Harriet Island; Park Reserves: Miesville Ravine, Murphy-Hanrehan, Spring Lake; Nature Preserves:
Grace, Tamarack, Tanglewood; Other: Mississippi National River and Recreation Area (and Wisconsin), Old Mill Park, Twin Cities Army Ammunition Plant, Rice County Wilderness Area. Wisconsin: State Parks: Kinnickinnic, Willow River; National Scenic Riverways: Lower St. Croix; Wildlife Areas: St. Croix Islands.
CONSERVATION CONCERNS: Most surviving prairie/savanna remnants are associated with steep gravelly-sandy bluffs, usually along river valleys. A few deep soil prairies persist in rights-of-way and cemeteries, but agricultural use totally dominates the landscape. Good wetland communities and significant aquatic habitats persist, especially along the St. Croix River and its major tributaries. The watersheds of the St. Croix, Mississippi, Whitewater, and Root Rivers have all been identified as critical landscapes for biodiversity protection in Minnesota.
BOUNDARIES: In my treatment, the Minnesota part of the subsection is smaller than that delineated by Kratz and Jensen (1983). I have combined the heavily eroded, steep bluffs of the eastern portion of their "Southern Oak Barrens Section" into the Blufflands (Paleozoic Plateau (Hallberg et al. 1983) or Driftless) of Section IV, which continues into Wisconsin. The central, less dissected portion of the subsection, which supported upland tallgrass prairie and bur oak savanna, is retained in this subsection.
The subsection continues into west-central Wisconsin, where upland prairie and bur oak openings grow on silt loams over acidic, sandy loam glacial drift on undulating to rolling topography (Hole 1968, 1976; Hole and Germain 1994).
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| Figure 14.—Subsection III.4: St. Croix Savanna Scientific and Natural Area, Washington County, Minnesota. Broad, dry ridges of sand and gravel support open savannas of bur oak, surrounded by tallgrass and midgrass prairie. Most of the tallgrass prairie and oak savanna has been converted to agriculture. Minnesota Department of Natural Resources photo by J.C. Almendinger. |
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| Figure 15.—Subsection III.4: A small remnant of wet prairie persists in a moist depression in Ramsey County, Minnesota. Within the subsection, wet prairies originally occurred in swales and shallow depressions within tallgrass prairies and oak savannas, and also along the margins of marshes and wet meadows. Minnesota Department of Natural Resources photo by D.S. Wovcha. |
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