Mountain / Piedmont Calcareous Cliffs
Cliff communities consist of woodland, scrub, and herbaceous vegetation of very steep to precipitous bedrock faces. Calcareous cliffs develop on outcrops of limestone, dolomite, shale, mudstone, and metabasalt in the mountains (most frequently in the Ridge and Valley province), and occasionally on Triassic-age siltstone in Mesozoic basins of the Piedmont. Habitats are usually formed by incision of high-order streams and feature rugged and complex microtopography, with very high cover of exposed bedrock.
On south and west-facing cliffs of carbonate formations, the surficial bedrock, hot microclimates, and associated edaphic stresses limit overall vegetation cover, woody growth, and species richness. Scattered scrub growth of eastern red cedar (Juniperus virginiana var. virginiana), chinkapin oak (Quercus muhlenbergii), hairy mock-orange (Philadelphus hirsutus), poison ivy (Toxicodendron radicans ssp. radicans), and other shrubs is typical. Prevalent among herbaceous species are lithophytic calciphiles such as black-stemmed spleenwort (Asplenium resiliens), wall-rue (Asplenium ruta-muraria var. cryptolepis), ebony sedge (Carex eburnea), rocktwist (Draba ramosissima), three-flowered melic (Melica nitens), rock sandwort (Minuartia michauxii var. michauxii), plains muhly (Muhlenbergia cuspidata), cliff-brakes (Pellaea atropurpurea and Pellaea glabella ssp. glabella), moss phlox (Phlox subulata), and aromatic aster (Symphyotrichum oblongifolium, = Aster oblongifolius).
On north-facing limestone or dolomite cliffs, habitats are open but sheltered, with limited direct solar exposure. Local zones of ephemeral seepage are frequent. Woody vegetation of these more mesic calcareous cliffs is comparatively diverse and often achieves larger size than on similar xeric sites. Typical woody plants include northern white-cedar (Thuja occidentalis), basswoods (Tilia americana var. americana and var. heterophylla), slippery elm (Ulmus rubra), and wild hydrangea (Hydrangea arborescens). Characteristic herbs include ebony sedge (Carex eburnea), bulblet fern (Cystopteris bulbifera), cliff stonecrop (Sedum glaucophyllum), northern bedstraw (Galium boreale), white-flowered leafcup (Polymnia canadensis), walking fern (Asplenium rhizophyllum), smooth rock-cress (Arabis laevigata var. laevigata), lyre-leaf rock-cress (Arabis lyrata), fernleaf phacelia (Phacelia bipinnatifida), and Carolina saxifrage (Saxifraga caroliniana). This is the principal habitat in Virginia for the globally rare, evergreen sub-shrub Canby's mountain-lover (Paxistima canbyi).
Cliffs on metamorphic and non-carbonate sedimentary rocks are poorly documented and known from relatively few siltstone, metasiltstone, and metabasaltic (greenstone) cliffs in the western Piedmont and Blue Ridge. Ordovician red mudstone/shale cliffs of the Ridge and Valley Province also support communities of this group. All require additional investigation. Habitats probably vary with aspect and other microhabitat conditions. Vegetation is generally dominated by umbilicate, foliose, and/or crustose lichens, with relatively sparse representation of vascular plants. Stunted trees and shrubs, e.g ., white ash (Fraxinus americana), eastern red cedar (Juniperus virginiana var. virginiana), and chestnut oak (Quercus montana, = Quercus prinus), occur in crevices or on cliff shelves. Herbaceous lithophytes such as wild columbine (Aquilegia canadensis), rock-cresses (Arabis spp.), Allegheny stonecrop (Sedum telephioides), field chickweed (Cerastium arvense var. velutinum), maidenhair spleenwort (Asplenium trichomanes ssp. trichomanes), and blunt-lobed woodsia (Woodsia obtusa ssp. obtusa) also find scattered footholds.
These small-patch communities are generally considered state-rare, but their conservation status needs further investigation. Because of inaccessible locations, stands seem immune from many types of anthropogenic disturbance. Scattered individuals of non-native weeds sometimes find footholds on xeric cliffs but are largely excluded by the hot, rocky substrates. On more mesic calcareous cliffs, Japanese honeysuckle (Lonicera japonica), Japanese stiltgrass (Microstegium vimineum), and other non-native plants find more favorable growing conditions and pose a serious threat to the sparse indigenous flora of these habitats.Reference: Fleming (1999), Fleming and Coulling (2001).
Click on the images below to open a larger image in a separate window.
|Sheer limestone cliff undercut by Buffalo Creek in Rockbridge County Photo: Tom Rawinski.|
|Ordovician mudstone cliffs below the summit of Redrock Mountain in Smyth County (Redrock Mountain Natural Area Preserve). Gary P. Fleming / © DCR Natural Heritage.|
|Wild hydrangea (Hydrangea arborescens), marginal woodfern (Dryopteris marginalis), and wild stonecrop (Sedum ternatum ) cover a north-facing metasiltstone cliff along Bull Run in Prince William County (Manassas National Battlefield Park). Gary P. Fleming / © DCR Natural Heritage.|
|Xeric woodland on massive dolomite cliff along the Roanoke River in Roanoke County. Photo: Tom Rawinski.|
Calcareous cliff vegetation is poorly inventoried and represented by only five plots in Virginia (map). The three classified types are conceptually broad and subject to modification if more data become available. However, because of dangerous, mostly inaccessible habitats, it may not be possible to acquire many more quantitative samples of this group. Click on any highlighted CEGL code below to view the global USNVC description provided by NatureServe Explorer.
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