An ecosystem frozen in time

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Iowa's "Driftless Area" (also known as the "Paleozoic Plateau")

An algific talus slope (also known as a cold air slope) is a rare and almost unknown ecosystem. The entire world's supply consists of a few hundred tiny patches in the Driftless Area of Iowa, Wisconsin, Minnesota and Illinois-and many of those sites are under threat. These slopes' unusual geology keeps them cool on the hottest summer days, so they host many species found nowhere else in Iowa-and, in some cases, nowhere else in the world.

By Bill Witt

Blizzards and sub-zero temperatures may be unpopular with many Iowans, but they're the perfect forecast for some of the world's rarest plants and invertebrates. Weather that piles snow atop steep, wooded, solidly frozen, north-facing slopes in the driftless area of Iowa, Minnesota, Wisconsin and Illinois is key to recharging one of the most remarkable phenomena in nature: the hydrogeologic air-conditioning systems known as cold air slopes or algific (cold air) talus (loose rock) slopes.

Algific talus slopes are a unique legacy of the Midwest's geologic history. If Iowa's most recent glaciers had bulldozed the part of the Upper Mississippi Valley now known as the Paleozoic Plateau and filled its limestone-walled valleys with the clay and till that leveled most of our landscape, we would know species such as Discus mcclintockii (a tiny snail) and Chrysoplenium iowensi (a small saxifrage-like plant) only from the fossil record, or in some cases (balsam fir or yellow birch, for example) from sites hundreds of miles farther north.

But the ebb and flood of glacial tides left the region, also called the Driftless Area, a chilled, green island amid waves of ice. For perhaps hundreds of thousands of years, it offered refuge to cold-adapted plants and animals, while all around the ice pulverized their habitats. And then, when the present interglacial warming period began some 10,000 to 15,000 years ago and Iowa's glaciers receded, Nature continued to supply enough ice for relict species to survive in scattered, tiny living museums.


What are they?

The existence of a cold air slope depends on the combination of several factors:

Topography: a steep, north-facing slope to minimize radiant warming of the sun.

Vegetation: mature forests to provide cool, moist shade on hot summer days.

Geology: thin surface soils atop deeply fractured limestone bedrock with sinkholes and fissures, overlying an impermeable layer of shale or slate

A cold air slope's hydrogeologic system works like this:

Winter: Sub-zero air descends through the fissures and sinkholes, supercooling the bedrock to depths of 30 or 40 feet or more.

Spring: Snowmelt and rainwater percolate through the surface soils and down into the bedrock. There the water freezes in massive veins and blocks.

Summer: The bedrock warms and with it the subterranean ice, which produces chilled water vapor and meltwater. Here's where the slates and shales come into play: if they were absent, most of the meltwater and cold, moist air would simply sink deeper and deeper through the fractured bedrock as they reached strata below the frozen zone. But the shales force a change in direction, and water and air must flow downward and outward at angles, as though over a huge, irregular underground washboard. Where large, open fissures intercept the subsurface flow, the results can be startling, as attested by August dog-days hikers who have stood, shivering, in 36-degree air blasting out of the mouth of an "ice cave."

Even when the cooling effects don't attract human notice, a difference of 6 or 8 degrees at the root zone and 3 or 4 degrees at the surface, combined with less solar gain, can make all the difference for plants such as the federally listed Northern monkshood (Aconitum novaborescence), the globally listed and federally endangered Discus macclintockii (along with its half-dozen, miniscule mollusk cousins) and several score other species, many of which live nowhere else on Earth.


Where are they?

Cold air slopes are themselves exceptionally rare. Studies of topographic maps and aerial photos in the 1980s and early '90s identified about 600 sites in the four-state region that met the basic criteria-and that's the whole world's supply. Closer investigation showed that about one-third of these had been destroyed or severely degraded by logging, grazing and livestock confinement, quarrying, home building and so on. Of the remainder, well over half of the high-quality sites occur in northeast Iowa: in Winneshiek, Allamakee, Clayton, Dubuque, and Fayette counties. In addition to their scarcity, cold air slopes are usually small-ranging from about a quarter-acre up to a few acres.

Recognition of cold air slopes as fragile refuges for unique relict species led the world's scientific community to declare them a globally threatened and endangered habitat. The current threats to these sites are primarily grazing, sinkhole filling and invasive garlic mustard.

In response, the U.S. Fish and Wildlife Service-aided by willing landowners, the Iowa DNR, county conservation boards, and organizations like INHF and The Nature Conservancy-established the Driftless Area National Wildlife Refuge. Its holdings, currently 775 acres scattered in remote valleys throughout northeastern Iowa, comprise what is likely the least-visited wildlife refuge in the Lower 48. That is by design, since the Refuge's primary mission is the protection and preservation of the extremely rare and vulnerable species that live on them. Refuge units with sufficient buffer area around the algific slopes are open to public uses like hunting and wildlife observation.

A handful of cold air slopes are accessible to the public, however. The best-known is the "ice cave" at Bixby State Preserve, north of Edgewood in Clayton County. Another small slope, with good signage, occurs next to the Riverside Trail below the bluffs of Phelps Park in Decorah. Finally, the UNI Museum in Cedar Falls features a large, cutaway diorama of a cold air slope and its vegetation, complete with a geologic cross section and a vent that blows cold air at the press of a button. The UNI greenhouse also has a model slope with real plants, excluding endangered ones.

Bill Witt is a frequent contributor of feature articles and photographs in Iowa and regional magazines. A former five-term State Representative, he works in the Business and Community Services division at the University of Northern Iowa in Cedar Falls.