Where forests meet: the Black Hills are a crossroads of vegetation

Natural History, June, 2002 by Robert H. Mohlenbrock

Home to such well-known landmarks as Mount Rushmore, Devils Tower, and the Crazy Horse Memorial, the Black Hills straddle the border between South Dakota and Wyoming. The modern name for these mountains reflects one bestowed by the native Lakota, for whom this is sacred land: Paha Sapa (hills of black). The range does look dark from a distance, because 95 percent of its tree cover consists of ponderosa pine, which has deep green needles.

A hundred and fifty million years ago, when dinosaurs roamed the earth, this region was relatively flat and the climate was tropical or subtropical. Instead of ponderosa pines, the dominant plants were cycads, which resemble tree ferns. Between 150 and 100 million years ago, the cycads were joined by figs, sassafras, oaks, and willows, as well as such evergreen plants as sequoias and palms. Then, about 60 million years ago, the terrain began to be uplifted, blocking the eastward flow of warm air from the Pacific. As the region became cooler and drier, temperate species gradually began to replace the tropical plants, and eventually some of the evergreens were replaced by deciduous trees (most of them migrants from the East).

As the Cascade Range and the Rocky Mountains rose up in the West, beginning about 30 million years ago, the Black Hills region became cold and arid. Coniferous forests developed, and grasslands appeared in the driest areas along their periphery. Following the last ice age, which ended about 12,000 years ago, northern forest species such as white spruce and paper birch migrated south into the region. Because several habitats--Rocky Mountain, Great Plains, northeastern deciduous forest, and northern forest--seem to meet here, the Black Hills boast an unusual collection of trees, ferns, and wildflowers. According to Wyoming botanist Robert Dorn, 30 percent of the 1,260 species now found in the Black Hills originated in the Great Plains, 25 percent (including ponderosa pine) in the Rockies, 5 percent in deciduous forests, and 1 percent in northern forests. (The remaining species generally have widespread distribution.)

The range's uninhabited areas fall mainly within Black Hills National Forest, most of which lies in South Dakota. One of the best introductions to the region is to drive U.S. Highway Alt 14, a National Scenic Byway that passes through Spearfish Canyon. Exposed on the canyon walls are various types of shale and limestone, while the streams in the canyon bottom are lined with lush vegetation. The byway includes a number of "interpretive stops" where visitors can park and learn about particular habitats. For hikers, a good way to sample a cross section of vegetation is to follow the trail that begins at the Spearfish Canyon Resort and leads to Roughlock Falls.

HABITATS

Ponderosa pine forest. Beneath the pines are several distinct communities of understory plants. On exposed, rocky, south-facing slopes, particularly in the southern Black Hills, the major species are little bluestem, yucca, sagebrush, sand lily, and various gramas and needlegrasses. Juniper, Oregon grape, buffalo berry, and blue wild rye dominate at about 7,000 feet on a centrally located limestone plateau. In the northern part of the region, in relatively moist areas between 4,000 and 5,000 feet, the pines are joined by bur oak, and the understory contains chokecherry, Oregon grape, and melic grass. Plants with a western flavor--mountain mahogany, skunkbush, and American black currant--appear at the western edge of the Black Hills.

Rocky Mountain juniper forest. On some dry, rocky exposures in the southern Black Hills, the principal tree is Rocky Mountain juniper. The understory includes several prairie grasses as well as the shrubby skunkbush.

Deciduous forest. In waterside habitats in the eastern mountains, box elder and green ash grow with American elm, eastern cottonwood, red-osier dogwood, and Bebb willow. Wildflowers here include water parsnip, fringed loosestrife, and hedge nettle. In the northeastern foothills, on relatively dry slopes, bur oak dominates above an understory of hop hornbeam, smooth sumac, coralberry, and poison ivy. Among the wildflowers are a red columbine, aster, figwort, wild sarsaparilla, fleabane, and avens. In the northwest, particularly in the wake of natural forest fires or controlled burns, quaking aspen and paper birch take over. Common in the understory are chokecherry, beaked hazelnut, a wild rose, red baneberry, thimbleberry, and bracken.

Meadows and grasslands. The foothills harbor several treeless habitats. The wetter areas support meadows containing Missouri goldenrod, false toadflax, golden-glow, Indian paintbrush, Mariposa lily, death camas, and prairie smoke. Grasslands are found in drier zones. At the southern edge of the hills is a bunchgrass community of little bluestem, while elsewhere, particularly along the western edge, grow short prairie grasses--the most prominent being various bluegrasses, buffalo grass, and wheat grass--as well as prickly pears. Rabbitbrush and three kinds of sagebrushes give the lowlands to the west and south a distinctly Wild West appearance.