Low-Elevation Outcrop Barrens
This group comprises very sparse woodlands, scrub, and herbaceous vegetation of exposed outcrops up to about 975 m (3200 ft) elevation in the Piedmont and mountain regions. The majority of documented occurrences are on mafic (e.g., diabase, amphibolite, gabbro) outcrops of the Piedmont and southern Blue Ridge, and metabasalt (greenstone) outcrops of the northern Piedmont and Blue Ridge. Fewer examples have been documented from igneous and felsic sedimentary rocks.
Low-elevation acidic barrens of exposed sandstone, quartzite, and granitic outcrops are much rarer in Virginia than mafic barrens. These communities are scattered over the western Piedmont, Blue Ridge, Cumberland Mountains, and Ridge and Valley strike ridges. Habitats are often steeply sloping pavements with warm aspects. They typically have very high cover of exposed bedrock, with vascular plants occupying crevices and locally developed organic mats. Soil environments are minimal and highly oligotrophic. Vegetation is usually a mosaic of shrub thickets, herbaceous patches, and lithophytic lichens. Composition varies to some extent with aspect and elevation. Woody scrub usually consists of scattered, highly stunted trees, bear oak (Quercus ilicifolia), and ericaceous shrub thickets. Herbaceous species, which are characteristically sparse or concentrated in discrete mats, include silverling (Paronychia argyrocoma), cliff saxifrage (Hydatica petiolaris), little bluestem (Schizachyrium scoparium var. scoparium), broomsedge (Andropogon virginicus var. virginicus), Pennsylvania sedge (Carex pensylvanica), poverty oatgrass (Danthonia spicata), and silky oatgrass (Danthonia sericea). A few sites support relatively dense cover of graminoids.
Habitats of the mafic barrens generally have high cover of exposed bedrock, but often have more extensive organic or soil mats, and thus more vascular plant cover, than do acidic outcrops. Soils usually consist of thin veneers and vary from moderately acidic to circumneutral, with moderately high base status. Vegetation is usually a patchwork of severely stunted trees, shrub thickets, herbaceous mats, and lithophytic lichens. Typical woody species include white ash (Fraxinus americana), eastern red cedar (Juniperus virginiana), fringetree (Chionanthus virginicus), common ninebark (Physocarpus opulifolius var. opulifolius), staghorn sumac (Rhus typhina), aromatic sumac (Rhus aromatica var. aromatica), and hoptree (Ptelea trifoliata var. trifoliata). Eastern prickly-pear (Opuntia humifusa) is present at most Piedmont sites. Typical herbs include nodding onion (Allium cernuum), roundleaf fameflower (Phemeranthus teretifolius), slender knotweed (Polygonum tenue), woodland sunflower (Helianthus divaricatus), hairy lip fern (Myriopteris lanosa), rusty woodsia (Woodsia ilvensis), little bluestem(Schizachyrium scoparium var. scoparium), Pennsylvania sedge (Carex pensylvanica), hair-awn muhly (Muhlenbergia capillaris), whorled milkweed (Asclepias verticillata), Appalachian phacelia (Phacelia dubia var. dubia), hoary mountain-mint (Pycnanthemum incanum var. incanum), dwarf skullcap (Scutellaria leonardii), blue waxweed (Cuphea viscosissima), upland flatsedge (Cyperus lupulinus ssp. lupulinus), lacegrass (Eragrostis capillaris), false pennyroyal (Trichostema brachiatum), Virginia dwarf-dandelion (Krigia virginica), pale corydalis (Capnoides sempervirens), mountains and foothills only), and American alumroot (Heuchera americana).
All of these small-patch communities are rare in Virginia and globally. Although many occurrences are in remote locations, degradation from trampling and invasive weeds is a serious threat to outcrops located near popular trails and overlooks. Perhaps because of their more fertile substrates, basic outcrop barrens are more prone to invasion by non-native weeds than are acidic barrens.
Reference: Copenheaver et al . (2004), Fleming (2002a), Fleming et al. (2007), Fleming and Coulling (2001), Fleming and Patterson (2004), Rawinski et. al. (1996), Rawinski and Wieboldt (1993).
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© DCR-DNH, Gary P. Fleming.Low-elevation outcrop barrens have been fairly intensively inventoried and 75 plots have been sampled (Fig. 1). Six community types have been classified, five of them supported by an average of 13 plots. The units segregate geographically and along elevation and substrate gradients. Only a single example of the Southern Blue Ridge Low-Elevation Granitic Barren is currently known. Few additional examples of vegetation in this group are likely to be found. Vegetation of low-elevation acidic barrens has been described in field notes from several sites in western Virginia, but only two plots have been sampled. At least some of our occurrences fit reasonably well into the broadly defined USNVC community type (CEGL004524) below. Click on any highlighted CEGL code below to view the global USNVC description provided by NatureServe Explorer.
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