Plant Life on Granite Outcrops
Granite outcrops stand in stark contrast to the surrounding matrix of old-field areas and oak-hickory-pine forest. They are populated by a unique, highly specialized group of plant species that differ strikingly in aspect from adjacent areas that support zonal vegetation. Perhaps the most obvious differences are seen in the terrestrial plants that occur in areas associated with the bare granite.
A few shrubs and woody vines, though by no means limited to granite outcrops, may grow especially well in the thin woods
It is the herbaceous plants, however, that give the granite outcrops their distinctive character. These plants are often classified into four categories, based on the maximum depth of soil over rock. The "diamorpha community," existing in soil depths of one to three inches, consists of a single vascular plant, Diamorpha smallii. This diminutive winter annual is able to invade and dominate because of its ability to tolerate low moisture levels in the soil and because it is free from competition in this stressful microenvironment. Occasionally a few cryptogams (plants that reproduce through
Lichen–annual herb communities occupy depressions over granite where soil depths reach four to six inches. In the spring these areas are dominated by such annual plants as sandwort (Arenaria uniflora), toadflax (Nuttallanthus canadensis), and bentgrass (Agrostis elliottiana). In the fall these communities are dominated by the Confederate daisy (Viguiera porteri). More constant, but rarely dominating these areas, are lichens.
Annual-perennial herb communities occur on soils seven to sixteen inches deep. An important soil builder in these areas is the haircap moss (Polytrichum commune). On thinner soil the Confederate daisy and the succulent perennial rock pinks (Talinum mengesii or T. teretifolium) may dominate.
The extremely hot, dry conditions in certain microenvironments on granite has led to a number of adaptations by the characteristic plants. Some perennial herbs—for example, prickly pear cactus (Opuntia humifusa) and rock pinks—tolerate drought by storing water in succulent stems or leaves. A number of annuals, including diamorpha, also have succulent tissues. Many plants on outcrops are significantly more hairy than their counterparts growing elsewhere. It seems likely that a dense covering of white hairs acts to increase the reflectivity of the plant's leaves and thereby decrease the heat load. The hairs may also help to reduce evapotranspiration. The conspicuous livery-white hairpoints of the dominant moss on bare granite surfaces, Grimmia laevigata, may similarly work to increase albedo (the ability to reflect light) and reduce water loss.
Most of the endemic flowering plants on the outcrops have evolved to survive the extremes of summer and winter. Seeds germinate after late September or early October rains to produce rosettes of frost-resistant leaves. When temperatures begin to rise in late March or early April, the plants bolt and flower profusely. By early May, when the shallow soil depressions have become bone-dry, the winter annuals have already matured their crop of seeds, which typically require an after-ripening period of four to five months. Thus they are able to survive the extremely hot, dry summer months as dormant populations of drought-resistant seeds.
Rogers McVaugh, "The Vegetation of the Granite Flatrocks of the Southeastern United States," Ecological Monographs 13 (1943): 119-66.
William H. Murdy and M. Eloise Brown Carter, Guide to the Plants of Granite Outcrops (Athens: University of Georgia Press, 2000).
Robert Wyatt and James R. Allison, "Flora and Vegetation of Granite Outcrops in the Southeastern United States," in Inselbergs: Biotic Diversity of Isolated Rock Outcrops in Tropical and Temperate Regions, ed. Stefan Porembski and Wilhelm Barthlott (New York: Springer, 2000).
Robert Edward Wyatt, Highlands Biological Station
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