Regional Landscape Ecosystems of Michigan, Minnesota, and Wisconsin
SUB-SUBSECTION III.1.1. Leaf Hills
DISCUSSION: The Leaf Hills, treated as a natural division by Kratz and Jensen (1983), is topographically diverse. Sugar maple-basswood is present on ice-stagnation topography and ground moraine; it also occupies areas of outwash with many small lakes. The boundary of sugar maple-basswood with prairie is occasionally sharp at the boundary of the subsection, where the ice-stagnation topography consists largely of lakes. Where the ice-stagnation topography has few or only small lakes, oak forest or savanna dominates.
Correlation of soils, as described in Cummins and Grigal (1981), to original forest vegetation is relatively poor within the sub-subsection. This is not a comment on the quality of the soil survey, but rather on the transitional and dynamic nature of the climate, soils, and vegetation of this unit. With present climatic conditions, the lakes and irregular moraines at the western boundary are adequate to allow oak savanna and sugar maple-basswood to establish and persist; warmer, drier conditions would result in an expansion of prairie to the east. According to Grimm (1984), Wright's (1976) radiocarbon-dated pollen diagrams from within the Big Woods showed prairie covering most of the area 7,200 years ago, followed by an expansion of oak woodland between 5,000 and 2,400 years ago. About 300 years ago, small populations of mesic hardwood species expanded dramatically, apparently in response to a climatic change.
ELEVATION: 1,100 to 1,600 feet (335 to 488 m).
AREA: 6,662 square miles (17,260 sq km).
STATES: Minnesota.
CLIMATE: See subsection.
BEDROCK GEOLOGY: See subsection.
LANDFORMS: Irregular stagnation moraine, ground moraines (with drumlins), end moraines, and outwash.
LAKES AND STREAMS: Many lakes larger than 160 acres; many smaller lakes and wetlands, primarily in kettles on the stagnation moraine and pitted outwash. Few large streams are found in the sub-subsection, but many small streams originate within the rough terrain.
SOILS: Soils are derived from loamy and sandy till and sandy outwash; loamy soils are the most common. Mollisols formed under prairie vegetation, and Alfisols formed under forest vegetation. The most common Mollisols are Udolls, Borolls, and Aquolls; the common Alfisols are Boralfs, Udalfs, and Aqualfs (Cummins and Grigal 1981).
PRESETTLEMENT VEGETATION: Oak woodland and sugar maple-basswood were the most common vegetation. At the west edge, tallgrass prairie, brush prairie, and aspen-oak woodlands were present; the sharpness of the transition to oak woodland or sugar maple-basswood forest depended upon the steepness of the moraines and the number of lakes and wetlands. Steep topography and/or many lakes resulted in an abrupt transition from prairie to oak woodland and maple-basswood.
NATURAL DISTURBANCE: Fire was the most prevalent form of disturbance in the (oak) woodlands and prairies at the western edge. Wind-throw was probably more prevalent in the maple-basswood forest.
PRESENT VEGETATION AND LAND USE: Wheeler et al. (1992b) found flora associated with oak openings and barrens to be relatively common in the northern portion of what was treated as the Leaf Hills.
In the western part of the Alexandria moraine, where topography is steep, there are remnant dry-mesic prairies and potholes rimmed by wet prairie and marsh. Scattered aspen and oak openings remain. Maple-basswood forest occurs in pockets at the fire-protected east ends of many lakes.
RARE PLANTS: Trillium nivale (snow trillium), Cirsium hillii (Hill's thistle), Cypripedium candidum (white lady's-slipper).
RARE ANIMALS: Buteo lineatus (red-shouldered hawk), Clemmys insculpta (wood turtle).
NATURAL AREAS: The Nature Conservancy Preserves: Seven Sisters Prairie, Ordway Prairie, Staffanson Prairie, Moe Woods.
PUBLIC LAND MANAGERS: See subsection.
CONSERVATION CONCERNS: Overgrazing of remnant prairies of the Alexander moraine system; non-sustainable forest management on private lands.
BOUNDARIES: Rough ice-stagnation topography, kettle lakes, and creek or river channels form the western border with the prairie. This is similar to what Grimm (1984) found in the Big Woods, where he interprets both the occurrence of water bodies and irregular ice-stagnation topography as important factors determining the vegetation; the water bodies and irregular topography provided a barrier to fire, thus allowing the forest to survive.
Although both the Leaf Hills and Big Woods are part of the Des Moines lobe (Wisconsin Glaciation), the tills of the Leaf Hills are generally considered to be sandier than those of the Big Woods. The ridges of the Leaf Hills are also larger. Outwash, stagnation topography, and ground moraine are present in both the Leaf Hills and the Big Woods subsection (Hobbs and Goebel 1982).
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