Stone Ground Flowers

Natural History, June, 2000 by Robert H. Mohlenbrock

Born of molten rock, isolated mountains offer fragile niches to plants.

THIS LAND: GEORGIA

Rising above the flattened Piedmont terrain east and southeast of Atlanta are several large rock masses known as monadnocks. Based on the name of a mountain in New Hampshire, this term refers to an isolated rocky hill or mountain that stands out in a region otherwise worn down to a plain. (New Hampshire's Mount Monadnock derives its name from the Abenaki for "island mountain.") Ten miles east of Atlanta is the best known of these Georgia landmarks, Stone Mountain, but the vegetation there is far from pristine, having been decimated by the hiking traffic. From a naturalist's point of view, better sites to visit are Panola Mountain and Arabia Mountain, which are located about twelve miles south and southeast of Stone Mountain.

Arabia Mountain, which geologists date to about 375 million years ago, consists of granitic gneiss, while Stone Mountain and Panola Mountain, formed between 350 million and 275 million years ago, are made of a type of granite. All three mountains had a similar origin. Magma, or molten rock, surged upward through existing rock, cracking it, spreading it apart, heating it, and incorporating much of it. The pressure was not great enough to cause a volcanic eruption, however, so instead the magma slowly cooled into a hard mass. Eventually the surrounding layers of older rock eroded away, leaving this mass exposed as a monadnock. Continued weathering has carved pits and cracks in the surface, where particles of soil accumulate, providing suitable niches for the growth of plants.

Panola Mountain was the first place in Georgia to be designated a State Conservation Park; it also ranks as a National Natural Landmark. Two excellent trails are available, one that goes through a moist woodland near the base of the mountain and another, higher up, that winds through dry woodland. Above the dry forest the granite is relatively bare and flat; some plant communities here are quite fragile, and visitors may reach them only under the guidance of a park ranger. At Arabia Mountain, however, visitors can wander across the exposed rock that begins just a few feet above the small parking area. The trail is marked by stone cairns.

Although they seem barren, the flat rock outcrops, known as pavement rock, support some lichens and mosses. In addition, a limited assortment of flowering plants survive in cracks and small depressions, called solution pits, where the rock has been dissolved away. Some of the solution pits contain soil a foot deep and have a rocky rim that holds in water following a rain. Usually the water slowly evaporates and the soil dries out as the summer progresses, but in a few of the deeper ones, a shallow pool persists for most of the year. At the other extreme are pits that have lost part of their rocky rims through erosion, so water drains away rapidly after it rains; these often dry up by late spring.

Robert H. Mohlenbrock, professor emeritus of plant biology at Southern Illinois University, Carbondale, explores the biological and geological highlights of U.S. national forests and other parklands.

For visitor information, write: Park Superintendent Panola Mountain State Conservation Park 2600 Highway 155 S.W. Stockbridge, Georgia 30281 (770) 38%7801 www.mindspring.com/~panolamt

HABITATS

Moist woodland on the lower mountain slopes and extending down to streams that meander in the valleys contains tulip poplar, sweet gum, and umbrella tree (a magnolia). Smaller trees and shrubs include red maple, Carolina allspice, and the fragrant spicebush. Among the wildflowers, most of which bloom in April and May, are jack-in-the-pulpit, several kinds of violets, and Asiatic dayflower. Near the streams is an abundance of lady fern, netted chain fern, royal fern, cinnamon fern, and New York fern, often growing beneath a layer of mountain laurel.

Dry woodland has a diversity of species but is dominated by loblolly pine, rock chestnut oak, and a shrubby layer of azaleas and New Jersey tea. Beginning in late spring and continuing through the summer and into autumn comes a steady procession of wildflowers, including bird's-foot violet, fire pink, Indian pink, cinquefoil, blue-eyed grass, southern beardtongue, Carolina phlox, spotted wintergreen, and blazing star.

Pavement rock depressions that retain shallow water for all or most of the year contain two aquatic or semiaquatic plants, both confined to these granitic habitats in Georgia and surrounding areas. One is Amphicarpus pusillus, a dwarf member of the snapdragon family whose pinkish white flowers appear in mid-March. The other is a quillwort, or Isoetes, which reproduces through male and female spores and thus is considered more closely related to ferns than to flowering plants. These spores are formed by the thousands in the swollen pouch at the base of each of the leaves, which resemble blades of grass.

In depressions that do not sustain water year-round, the depth of the soil determines which flowering plants appear. If it is less than four inches, usually the only one is Diamorpha smallii, a tiny member of the same family as sedum (and named for the botanist John Kunkel Small, not for its size). Its fleshy leaves enable it to survive and conditions. Where the soil is four to six inches deep, providing more nutrients and more room for roots, a few additional species appear: a delicate bent grass and a delicate sedge, both with threadlike leaves; sandwort, a tiny plant related to chickweed; rushfoil, whose leaves are covered with silvery scales; and a type of Saint John's-wort called orange-grass, whose minute leaves give off an orange scent when crushed.