Wednesday, October 6, 2004

Plant of the Day: Big Bluestem, Andropogon gerardi

Big bluestem once covered a vast area as the dominant plant species of the tall grass prairie. This ecosystem contains some of the most fertile soils in the world; consequently, agriculture has now replaced most of the native prairie, leaving only small, isolated patches of tall grass prairie. Big bluestem was more common in broad valleys and on lower slopes, where soils are more moist—sites also preferred for corn production. Historically fire was an integral part of the tall grass prairie, ignited by lightning strikes before humans settled North America, and later by native peoples who used fire to drive game and stimulate grass growth. The relatively cool surface fires that burned through on a regular basis recycled nutrients held in the leaves and stems that died back at the end of each growing reason, leaving the roots and underground shoots unharmed. Big bluestem is a stately grass, with some plants reaching to a height of 12’ by the end of the summer! Heights from 4-9’ are more common, however. It blooms in late summer, and its unusual flower shape has earned it the name “turkey claw”. Members of the sunflower and pea families accompany big bluestem as important plants of the tall grass prairie.

After we crossed the Missouri River at Omaha, Nebraska, we could see the prominent loess bluffs that run along the western border of Iowa along the river. The bluffs, which reach 200’ above the surrounding floodplain in some places, are composed of silt, or fine sediment the texture of flour. During the Pleistocene Epoch (a.k.a. the Ice Age), glaciers formed in high latitudes and advanced southward to the middle of North America, scraping sediment from the terrain the ice overrode, like a giant bulldozer. There were a number of glacial periods when conditions were colder than today; these were separated by warmer interglacial periods, when conditions were probably similar to those of today. When the last glaciers eventually receded, a tremendous amount of water and sediment were flushed away from the melting ice and deposited in the Missouri River valley. The prevailing westerly winds picked up the silt once it dried, transporting it eastward in large clouds of dust. The silt was deposited east of the river valley. This occurred repeatedly over thousands of years, until impressive bluffs were formed. Erosion has cut into the bluffs, leaving rugged topography that is home to many rare plants and animals.

Iowa lies between the Missouri River on the west and the Mississippi River on the east; and, consequently, lies in the path of migration for many North American bird species. By migrating seasonally, many birds escape the harsh conditions of winter but are able to use rich food supplies that are only available for part of the year. Therefore, many species breed at high latitudes in temperate and arctic areas where insects are abundant in the summer, then fly south to warmer climates for the winter. Most species in North America move in a north-south direction along one of four major flyways: the Pacific, Central, Mississippi, and Atlantic. The valleys of the Mississippi River and its tributaries constitute a convenient migration corridor with plentiful food and many stopping points for rest during the long journey. As we passed through Iowa today, we detected many migrant birds heading south for the winter.

In several places as we biked across the central part of Iowa, we saw small wetlands known as “prairie potholes”. These are remnants of a time, over 12,000 years ago, when massive sheets of ice extended down across the central part of Iowa. As the ice melted back after the last glacial advance, huge blocks of ice became separated from the main part of the glacier. Meltwater from the glacier carried debris picked up by the ice as it bulldozed its way southward; this sediment filled in around the ice blocks, forming depressions when the ice blocks eventually melted. These small potholes, or wetlands, dot the landscape of central Iowa, north through Minnesota, the Dakotas, and across the prairie provinces of Canada. They serve as important wetland habitat for wildlife, particularly waterfowl during spring and fall bird migrations.

We also biked through more hilly terrain in central Iowa as we encountered the end moraines left behind after the last glacial period. Moraines are linear hills that are constructed by a melting glacier. The ice within a glacier is constantly eroding the landscape, pushing forward, and transporting sediment. When the rate of ice meltback matches the internal forward movement of the ice, the ice dumps sediment in one place like a giant conveyor belt, as the terminus remains in one place for a long period. Moraines mark the history of meltback of different lobes of a glacier; the moraines in central Iowa were constructed during recession of the Des Moines lobe.

Just north of McGregor, Iowa, which we passed through west of the Mississippi River, lies Spooks Cave. The cave is hidden below a 90’ limestone bluff. Most caves occur in limestone, because this type of bedrock readily dissolves when water combines with carbon dioxide to form carbonic acid. The landscape where solution is the dominant geomorphic process is known as “karst topography”, after the region in Yugoslavia where it was first studied. Spooks Cave wasn’t discovered until the mid-1900s when Gerald Mielke heard strange noises coming from the base of the bluff. He blasted through the rock, only to find an underground stream that had formed the cave. The mysterious noises disappeared once the entrance to the cave was opened, some say because Mielke had released the spirits, for which the cave was named. Others offer the more scientific explanation that changes in water levels once the cave was opened reduced the roaring noises the water made moving through narrow passages.

At Prairie du Chien, we crossed the Mississippi River into Wisconsin. From there to just east of Spring Green, we rode along the Wisconsin River Valley. The river has carved a magnificent valley through what is known as the “Driftless Area” of Wisconsin. During the winter months, bald eagles congregate in the lower Wisconsin River valley, where there is an abundance of fish, their preferred food. Last winter, biologists counted over 600 eagles in the 180-mile stretch of river extending up from the confluence of the Wisconsin and Mississippi Rivers at Prairie du Chien!

Most of the northern U.S. and Canada were covered by ice during the Pleistocene Epoch, and the landscape we see in those areas today is a result of the ice completely altering the preglacial landscape. The southwestern corner of Wisconsin, however, is a small area that was not covered by ice during the last glacial period. While the ice smoothed out the surrounding area, beveling down areas of rugged relief and filling valleys with sediment transported by the ice, meltwater, and wind, the Driftless Area escaped this dramatic topographic alteration. The modern landscape is much more dissected than adjacent areas, with steep ravines, deeply cut stream valleys, and large areas of exposed bedrock. Caves and sinkholes are common in parts of the Driftless Area underlain by limestone. When Europeans first settled this area, the ridge tops supported tall grass prairie and bur oak savannas, and forests dominated by sugar maple and basswood occurred on moister slopes and in valleys. Because of the topographic diversity, a number of rare plants and animals are found only in the Driftless Area.