Wednesday, June 10, 2009

Granitic Flatrocks

Exposed, gently sloping, granitic outcrops of the Piedmont physiographic region support distinctive communities characterized by lichens and sparse vascular plants. This community group ranges from eastern Alabama to Virginia and is probably best represented in Georgia, with another noteworthy cluster in north-central North Carolina. Granitic Flatrocks in Virginia range in elevation from about 70 to 100 m (230 to 320 ft) and occur on true granites and a range of related rocks such as granitic gneisses and granodiorites. Most examples are located on gentle slopes along streams, where the erosive power of water over time has worn rock surfaces smooth and created small, gravel-filled depressions.
Lichens (e.g. ., Xanthoparmelia conspersa, Cladonia caroliniana, and other Cladonia spp.) and the bryophyte (Grimmia laevigata) are dominant biota on granitic flatrocks, covering much of the exposed bedrock. However, vascular plants dominate locally in crevices, flats, and depressions where moisture and thin layers of detritus accumulate. Among the vascular plants, Small's stonecrop (Diamorpha smallii), Small's purslane (Portulaca smallii), and granite loving flat sedge (Cyperus granitophilus) are globally rare and endemic to these habitats. Other typical vascular plants include roundleaf fameflower (Talinum teretifolium), Appalachian sandwort (Minuartia glabra), rock spike-moss (Selaginella rupestris), buttonweed (Diodia teres), elliptical rushfoil (Croton willdenowii), broomsedge (Andropogon virginicus var. virginicus), little bluestem (Schizachyrium scoparium), fork-tip three-awn grass (Aristida dichotoma), purple three-awn grass (A. purpurascens), common hair sedge (Bulbostylis capillaris), open-flower panic grass (Dichanthelium laxiflorum), and orange-grass (Hypericum gentianoides). These small-patch communities often cover less than 0.4 ha (1 ac) and are considered globally rare. Major threats include invasive non-native weeds and quarrying.

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