Northern Prairie Wildlife Research Center
The weather of the three States is controlled by three major air masses, the Continental Polar, Maritime Tropical, and the Maritime Polar (Eichenlaub 1979). The Continental Polar air mass, forming over land in the Arctic, brings cold, dry weather in the winter and cool conditions in the summer. The Maritime Tropical, forming over the waters of the Gulf of Mexico to the south, brings warm, moist winter weather and hot, humid summer conditions. The Maritime Polar air mass originates in the northern Pacific Ocean; although it originally carries large amounts of moisture, much of this is lost on the western slope of the Rocky Mountains. The air warms as it descends from the mountains. The Maritime Polar air mass brings mild weather with little precipitation to the Midwest.
The Great Lakes are another major control on climate for Michigan and parts of northern Wisconsin, but much less so for Minnesota. These effects increase the intensity of storms over and adjacent to the lakes during the winter; they also decrease the intensity of storms and increase the stability of air masses over the lakes during the spring and summer. Overall, lacustrine (lake) effects are important regulators of regional and local climate in the three-State area.
Elevation differences and physiographic features in the States are too small to have much influence on the movement of air masses. However, in Michigan, they do influence the degree to which lake effects penetrate the land and the intensity of lake-effect precipitation. Areas where elevation increases rapidly near lakes receive the most lake-effect precipitation.
Climate is responsible for major differences in both soils and vegetation, and climatic differences are the primary basis for separating sections within this study. In Michigan, Wisconsin, and Minnesota, the most distinctive difference in vegetation is between prairie (Sections I and II) and savanna or forest (Sections III through XI). The prairie is partially the product of the dry, cold rain shadow from the Rocky Mountains. This rain shadow becomes weaker to the east, where the impacts of the Maritime Tropical air mass from the Gulf, and to some degree, the arctic Continental Polar air mass begin to outweigh those of the Pacific Maritime Polar air mass.
Latitudinal climatic differences, largely length of growing season and annual input of solar energy, separate Sections II through VI (southern sections with longer growing seasons) from Sections I and VII-XI (northern sections). The far northern parts of Minnesota (Section X) and possibly Wisconsin and Michigan are near the transition to boreal forest, a transition resulting from further reduced growing season and extreme winter temperatures.
Sections I-V, X, and XI, and most of Section IX have more continental climates than Sections VI-VIII, which have climates influenced by the Great Lakes. Areas with continental climates tend to be hotter in the summer and cooler in the winter than areas with a lake-moderate climate. Along the Great Lakes, the air near the coast warms more slowly in the spring and cools more slowly in the fall than in the sections with a continental climate; both native vegetation and human land use reflect this lake-effect climate.
The continental interior of North America, including all of Michigan, Minnesota, and Wisconsin, is known as the Central Stable Region or craton, an area that was relatively stable during the Paleozoic (Dorr and Eschman 1984). The northern portion of the craton, in the northern United States and parts of Canada, where old Precambrian rocks are now exposed at the surface, is called the Canadian Shield. Within the stable craton, there were active areas of uplift where erosion occurred and areas that were sagging down to form local basins (intracratonic basins) in which sediments were accumulating. Much of the Great Lakes Region was an intracratonic basin, now called the Michigan basin.
During the Paleozoic, from Cambrian to Pennsylvanian times, the southern portion of the craton, including Michigan, Minnesota, and Wisconsin, was intermittently submerged beneath shallow seas. Marine and near shore sediments, including limestone, dolomite, evaporites, sandstone, and shale, were deposited over Precambrian bedrock. Thus, the major bedrock distinction in the three States is between Proterozoic (Precambrian) igneous and metamorphic bedrock and younger Paleozoic sedimentary bedrock. Mesozoic sedimentary bedrock is very localized in Minnesota. Soils derived from much of the Precambrian crystalline bedrock are generally acidic, resulting in less productive agricultural lands. The soils derived from marine deposits, including shale and marine limestone, dolomite, and evaporites, are typically more calcareous (less acidic), more nutrient- and moisture-rich loams and clays; they are generally the soils most utilized for agriculture.
All of Michigan, and much of Minnesota and Wisconsin, were covered by ice during the Wisconsinan Glaciation of the Pleistocene Epoch. Modern physiography and soils are the result of postglacial erosion and soil formation processes acting on glacial deposits. Erosion of bedrock and unconsolidated materials occurred beneath the advancing glacier. The advancing ice scoured the bedrock uplands, producing rounded knobs. Rocks and soil materials were carried on top of and in the glacial ice. They were later redeposited and formed diverse features, including moraines, drumlins, eskers, kames, and outwash plains. Lakes and depressions are now common in the glacial landscape. Many lakes formed when large blocks of ice were surrounded by outwash sands as the glacier melted. When these ice blocks melted, deep depressions, kettles, remained as lakes. Lakes also formed in linear depressions that had been scoured out by the glacier. Swamps and marshes occur where vegetation colonized shallow depressions.
The Driftless Area in southwestern Wisconsin and southeastern Minnesota shows no sign of having ever been glaciated and the "Coteau" in southwestern Minnesota was not glaciated during the Wisconsinan Glaciation. Compared to the more recently glaciated parts of the three States, both of these areas are characterized by much more highly dissected topography and more highly weathered sediments, the result of several hundred thousand years of erosion and weathering.
The land surface of the glaciated portions of the three States is composed of several different glacial landforms, each with characteristic slopes, substrate and soils, and drainage conditions, and as a result of these physical factors, different vegetation. The map units described in this study (sections, subsections, and sub-subsections) are an attempt to subdivide the three States into smaller, more uniform areas for ecological study, inventory, and management. The greatest homogeneity in soil, landform, climate, and biota is found at the sub-subsection level of the classification. However, as in any glaciated area, there can be tremendous variability in landform, soils, and drainage class within even a relatively small area.
The present-day vegetation of Michigan, Minnesota, and Wisconsin is a result of the physical environment, post-Pleistocene species migration patterns (Bernabo and Webb 1977, Davis 1981, Delcourt and Delcourt 1988), and human alteration of lands and plant communities. Many publications describe the presettlement and present vegetation. Nature preserves include representative examples of many characteristic ecosystems and have been listed in this publication for that reason.
Disturbances such as logging, agriculture, drainage, fire, and fire exclusion have significantly altered plant cover and composition. Many shade intolerant, early successional species, such as paper birch, bigtooth and trembling aspens, and black cherry, have greatly increased in relative abundance. Because of the complexity of local disturbances, I have not attempted to present the specific occurrences of early successional species.