When I think about southeastern Minnesota, visions of towering bluffs, valleys filled with spring wildflowers, and winding coldwater streams come to mind. So imagine the disconnect I experienced when I saw a greeting card that showed a cartoonish map with symbols illustrating different parts of the state: All of southern Minnesota was represented with an ear of corn. How could this be?
I am determined to set the record straight. As an ecologist with the Department of Natural Resources, I want to share the beauty, the diversity, and the wonder that I have found during several years of biological surveys in this lovely region.
If geography and ancient history ruled, southeastern Minnesota would probably be a separate state combined with parts of neighboring Wisconsin, Iowa, and Illinois. Together, these 18 million acres are called the Paleozoic Plateau. It is underlain by layers of bedrock that formed over 400 million years ago under a vast sea. After the sea receded, streams—which eventually became the Mississippi River and its tributaries—carved into the bedrock and created a hilly, dissected landscape. Much of the bedrock underlying the landscape is composed of limestone and dolomite. Acidic water, formed when precipitation combines with carbon dioxide, shapes these rocks into caves, sinkholes, and underground drainage, creating a type of landscape known as karst.
The Paleozoic Plateau was largely missed by the last glaciers to move across the state. So instead of being covered by glacial sediments like most of the rest of Minnesota, this rugged, ancient landscape is still visible today.
Also referred to as the blufflands, the driftless area, and coulee country, this is a landscape at once majestic and intimate, ranging from vertical cliffs and wide vistas to narrow valleys, winding creeks, and a myriad of different habitat niches. A walk in the blufflands always offers surprises, and often they are of the faunal or floral variety, because the biological diversity here is higher than in any other part of the state. Minnesota's endangered, threatened, and special concern species list includes 189 plant and animal species that reside here. The region also has the highest number of animal species in greatest conservation need in the state.
Topography and Geography.
Why is this relatively small region of the state so packed with diversity? Part of the answer lies in its varied topography—steep slopes, floodplains, groundwater discharge areas, high ridgetops, and cliffs. These landscape features support native plant communities specially adapted to thrive under those conditions.
Another factor is Minnesota's position at the northwestern edge of North America's Eastern Broadleaf Forest Province. Many of the rare species found here are rare in the state because they are at the western or northern edge of their range. For example, in Minnesota the timber rattlesnake (Crotalus horridus) is found only in the Paleozoic Plateau, but this species ranges more widely throughout eastern North America in appropriate habitats. The blufflands provide the ideal mix of habitats for these snakes—forests and prairies for summer habitat and rock outcrops for winter dens.
Some rare aquatic species are at the northern edge of their range in southeastern Minnesota. One is the crystal darter (Crystallaria asprella), a small, pale-yellow fish of large clear streams with moderate to swift currents. For a handful of rare species, the blufflands are key to survival: Seven rare species of flora and fauna have their primary range here in special habitat conditions. For example, glade mallow (Napaea dioica) is a tall, summer-blooming floodplain plant that is endemic to the region, meaning it is found nowhere else in the world.
Bedrock bluff prairie is also known as goat prairie because its extreme steepness seems best suited for goats to climb. This unique blufflands habitat occurs on south- to west-facing slopes, where hot, dry conditions slow the growth of trees and shrubs. Fires historically helped to keep these areas open and dominated by more than 200 species of native grasses and wildflowers.
Two rare wildflowers exhibit some of the complexity of blufflands habitats. Plains wild indigo (Baptisia bracteata var. glabrescens) is a spring-blooming species adapted to dry conditions. Its pale-yellow flowers attract queen bumblebees, important pollinators on these bluffs. Valerian (Valeriana edulis var. ciliata), a rare wildflower often found in shallow wetlands on calcium-rich soils, also grows in bedrock bluff prairies. The growing conditions on the blufftops are calcium-rich because of the limestone bedrock and moist in some areas because groundwater is close to the surface.
Wooded slopes and ridgetops surround the goat prairies. Red oak, white oak, bur oak, and shagbark hickory are some of the characteristic trees of these fire-dependent forest communities.
Shady and Moist.
On cool north- to east-facing slopes and on narrow valley floors along streams and seeps, a variety of forest plant communities endure. Mesic hardwood forests are dominant, with canopies of sugar maple, basswood, red oak, and often white pine, especially on rocky upper slopes. Here, cool, shady conditions make fires much less frequent. Thick layers of leaf litter break down to build up deep, loamy soils rich in nutrients.
Before tree buds unfurl in spring, a visit rewards the hiker with carpets of spring-ephemeral wildflowers, such as white trout lily (Erythronium albidum) and spring beauty (Claytonia virginica). The diminutive white-flowered squirrel corn (Dicentra canadensis) is a spring ephemeral found nowhere else in the state. Look for it in Forestville/Mystery Cave State Park or Frontenac State Park.
By summer, the ground is covered by many of the 40 species of ferns that occur in the blufflands and later-blooming wildflowers such as zigzag goldenrod (Solidago flexicaulis), wild geranium (Geranium maculatum), and red baneberry (Actaea rubra). The floor of mesic hardwood forests features blooming plants from early spring to late fall.
Cool Crevices in Summer.
Higher up the slopes in the coolest, rockiest valleys, habitat remnants from the last ice age persist. Here, ice from winter lingers in crevices in the limestone bedrock well into summer. Air moving through these fissures in the rocks is cooled and slowly melts the ice. Where cool air and icy water emerge, thick moss carpets and other northern plants grow. Balsam fir, Canada yew, and the endangered Iowa golden saxifrage (Chrysosplenium iowense) occur in these places. These northern plants are remnants from 10,000 years ago when the climate was colder and these species were much more common.
Tiny Pleistocene-aged snails, dependent on the continuous presence of cold air, reside on many of these slopes. One federally threatened plant species, Leedy's roseroot (Rhodiola integrifolia ssp. leedyi), is known from only seven places in the world—four of them on these persistently cold cliffs in Olmsted and Fillmore counties.
These cliffs and slopes are extremely fragile places—one misstep could send rocks and snails and plants tumbling downhill.
Another mosaic of native plant community types resides along the region's big rivers, including the Mississippi and major tributaries such as the Root, Zumbro, Whitewater, Cannon, and Vermillion rivers. Their floodplains hold marshes and sedge meadows, as well as floodplain forests dominated by silver maple, green ash, cottonwood, river birch, and swamp white oak.
More than 150 bird species breed in or migrate through the floodplain habitats. Nearly half of North America's songbirds and 40 percent of its waterfowl spend at least part of their lives in the Mississippi flyway. Bright-blue cerulean warblers (Setophaga cerulean), brilliant yellow-orange prothonotary warblers (Protonotaria citrea), and soaring red-shouldered hawks (Buteo lineatus) are some of the birds at the western edge of their range here.
The habitat types that once dominated southeastern Minnesota and connected all of the bluffs has today virtually disappeared. A once-continuous mosaic of savanna, forest, and prairie thrived here. Oak savanna and tallgrass prairie covered the uplands above the steep slopes. Deep, rich soils, created by prairie grasses and wildflowers over thousands of years, have nearly all been turned into croplands, leaving fragmented islands of native habitat.
Before European settlement, blufftop fires burned across oak savanna and prairie, often reaching woodlands below. This natural process rejuvenated native grasses and forbs and created openings for oak regeneration. Another impact to bluffs in recent decades came from grazing cattle, sheep, and goats. Now, with fires suppressed, grazing continuing, and row crops prominent, eroding topsoil tumbles down steep slopes, often leaving deep ruts and many feet of sediment on valley floors.
Many floodplains have also been converted to croplands. Perhaps this altered landscape is the reason for the greeting-card artist's perception that the area is all corn. Visitors today must travel deep into the valleys and onto the steep slopes to find the tremendous diversity in this place.
The diverse, intact native plant communities that still exist in the blufflands are threatened in various ways. Oak forests and woodlands, bluff prairies, and savannas all require fire to thrive and support the plants and animals native to these habitats. Controlled burning on many public and some private lands has helped restore this vital process. However, because most land ownership in this region is private, there are many places without fire. In the absence of fire, invasive species such as nonnative buckthorn and honeysuckle displace native species, and much of the native diversity is lost.
Threats to the region's mesic hardwood forests include nonnative earthworms and buckthorn. Garlic mustard is a relatively new invasive species that is spreading quickly.
Floodplain forests, emergent marshes, and sedge meadows have been greatly changed by the locks and dams that have removed most of the natural flooding regime in many areas. The latest threat to remaining floodplain forests: Reed canary grass forms dense thickets that seem to prohibit trees from regenerating.
Rivers and streams have been impaired by siltation, pollution, and increased flooding caused by land use changes.
Despite all the pressures and threats, this ancient landscape continues on. Myriad pockets of diverse habitats support many of the plants and animals that evolved over thousands of years to be perfectly adapted to this place.
On a cloudy day this past winter, I traveled along Highway 74, which runs through the middle of the expansive Whitewater Wildlife Management Area in Winona and Wabasha counties. Light snow covered the bluffs and the floodplain. There wasn't another car in sight or, for that matter, any sign of other humans. I marveled at the expanse of bluffs visible from the valley. Oak forests and bluff prairies, many of them burned in the past year, stretched across successive rows of hills.
As I came to a bend in the road, the sun suddenly came out, warming my face. I felt grateful to be in this beautiful valley that still possesses such natural richness.