Highelevation rock outcrop

Distinguishing Features

High-elevation rock outcrops differ from surrounding communities by having extensive areas of bare or lichen-covered rock scattered with herb-dominated mats on shallow soils. Trees are stunted and largely restricted to outcrop margins and crevices where deeper soils accumulate. Patches of shrubs are often present, but large areas dominated by shrubs are considered heath balds. High-elevation rock outcrops are differentiated from low-elevation rock outcrops not only by elevation (the boundary is about 3,000 ft.) but by vegetation as well. Annual plants and weedy species, for example, are common in the hot, dry, more frequently disturbed habitats associated with granite outcrops in the piedmont, but they are rare on high-elevation outcrops.

Introduction

This community occurs on rock outcrops on exposed ridges, peaks, and upper slopes at elevations above 3,000 ft. Outcrops consist of various types of rock, including granite, gneiss, schist, and amphibolite; soils vary from bare rock with no soil, to thin soils over rock, to deeper soils in crevices and forest borders. High rainfall, low temperatures, and frequent fog characterize the climate and keep the thin soils moist, reducing the incidence of drought stress. Species composition varies depending on soil depth and moisture, elevation, exposure, and geology. Two different types of high-elevation rock outcrop occur. Granitic domes consist of smooth, exfoliating granite or similar rock material that is largely free of crevices. Rocky summits, in contrast, have irregular, fractured rocks where cracks and crevices accumulate pockets of soil, providing additional habitat for plants to grow. The vegetation consists of lichens and mosses growing on bare rock, mat-forming species on thin soils, with shrubs and stunted trees on deeper soils. While many rock outcrop species also occur in surrounding forests and balds, a number of distinctive species occur, including rare endemics and northern disjuncts.

Vegetation

A patchwork of lichen and moss-covered rocks, low-growing, mat-forming herbaceous plants, scattered shrubs, and stunted trees characterize the vegetation. Lichens growing on bare rocks include brightly colored crustose lichens, whose circular patches appear to be "painted" on rocks, several species of Cladonia lichens (with grayish green upright branches), and the smooth rock tripe (Umbili-caria mammulata), whose large, leathery, leaf-like structure curls up and appears brittle when dry but which has a slimy, rubbery feel when wet. Two common mosses are dry rock moss (Grimmia laevigata) and the Appalachian haircap moss (Polytrichum appalachianum). The pincushion mounds of Grimmia are vivid green when actively growing, black when dormant. Twisted hair spikemoss (Selaginella tortipila) keeps a low profile by forming compact mounds of tightly packed, mosslike leaves. Cliff saxifrage (Micranthespetiolaris) commonly occurs on both moist and dry sites.

The most lush and species-rich sites are persistently wet due to seepage. Areas with deeper soil, such as older well-established mats, rock crevices in rocks, and outcrop margins support various shrubs and trees. Low-growing shrubs include Granite Dome St. John's wort (Hypericum buckleyi), mountain golden heather (Hud-sonia montana), and sand myrtle (Kalmia buxifolia). Curiously, sand myrtle, tur-keybeard (Xerophyllum asphodeloides), and pinebarrens death camas (Stenanthium leimanthoides) grow on high-elevation rock outcrops and near sea level in the New Jersey pine barrens. Scattered, mostly stunted trees include Fraser fir (Abiesfraseri), red spruce (Picea rubens), yellow birch (Betula alleghaniensis), table mountain pine (Pinus pungens), Carolina hemlock (Tsuga caroliniana), and smooth serviceberry (Amelanchier laevis). Tree flagging results from branches being buffeted by winds and ice, causing the buds to be killed, occurring to such an extent that there is little growth on the windward side of trees.

Seasonal Aspects

Common spring-flowering shrubs include Catawba rhododendron (Rhododendron catawbiense), mountain laurel (Kalmia latifolia), and sand myrtle, all of which are evergreen shrubs in the heath family (Ericaceae). Catawba rhododendron and mountain laurel produce spectacular floral displays at the tips of shoots with thick leathery leaves, whereas sand myrtle is a relatively short shrub with clusters of small, white flowers and narrow leaves. Deciduous heath shrubs include pinkshell azalea (Rhododendron vaseyi), a rare species whose showy pink flowers cover leafless stems in spring, and highbush blueberry (Vaccinium corymbosum), a medium to large multistemmed shrub whose small white to pinkish pendulous flowers develop into sweet, edible berries in summer.

The yellow trumpet-shaped flowers of southern bush honeysuckle (Diervilla sessilifolia) bloom at the tips of leafy stems in summer, whereas prostrate mats of Granite Dome St. John's wort produce bright yellow flowers and red cone-shaped fruits. Its flowers lack nectar but the numerous stamens attract pollen-collecting bees that function as pollinators. Another prostrate subshrub is the rare mountain golden heather—look for its yellow flowers among needle-like evergreen leaves from late spring through summer. Herbaceous plants that flower in summer include cliff saxifrage, whose open inflorescence of small but distinctive white flowers overtops a basal rosette of semisucculent green to red leaves with coarsely toothed margins. Rock alumroot (Heuchera villosa) has large maple-like basal leaves and upright stalks that produce numerous small whitish flowers from early summer to fall. Mountain dandelion (Krigia montana) is easily recognized by its basal leaves and dandelion-like yellow flower heads.

Three rare, summer-flowering herbs are spreading avens (Geum radiatum), Heller's blazing star (Liatris helleri), and mountain sandwort (Minuartiagroenlandica). Spreading avens has large, mostly basal leaves with conspicuous bowl-shaped yellow flowers. Heller's blazing star produces small upright stems culminating in showy clusters of lavender flower heads, and from late spring through early fall mountain sandwort forms prostrate "cushions" with numerous tiny white flowers.

Distribution

High-elevation rock outcrops occur mainly in the high peaks region of the southern Appalachians, primarily in North Carolina and Tennessee, less frequently in Virginia and Georgia.

Dynamics

Lichens and mosses are the first plants to colonize bare rock. These slow-growing plants collect small amounts of soil that allow hardy vascular plants to get established. The small mats of vegetation that gradually form go through a succession of species largely driven by the amount and rate of soil accumulation. As new species invade and previously established plants spread horizontally, the mat gets progressively larger. Once a mat has built up to a depth of several inches, seedlings of woody plants have a chance to colonize. Such seedlings have slow growth rates and high mortality. The mats themselves are often ephemeral as increasing weight causes them to slide downslope, unless their roots find anchorage in cracks or crevices. High winds can blow trees over, lifting up and destroying the mat in which they're rooted. Most outcrops remain relatively open habitats dominated by herbaceous plants, due to exfoliating rock, soil erosion, vegetation loss, and other factors.

Climate change, alpine relicts, and rare species. In today's climate, the southern Appalachians aren't high enough for an alpine zone, as the tallest peaks are covered with trees and grassy balds and outcrops aren't true alpine communities. Temperatures were apparently cold enough during the last ice age (in the Pleistocene epoch) as a treeline and a true alpine community occurred at elevations above 5,000 ft, as are indicated by pollen and macrofossil evidence. The alpine zone lasted until about 12,000 years ago, when temperatures began warming and trees were able to migrate up to the highest peaks. Remnants of an alpine flora still persist in certain small, isolated high-elevation outcrops where a combination of cooler temperatures, shallow soils, high light, and diminished competition closely resembles past alpine conditions. Among the ice age relicts are a small number of species whose distributions are centered in the alpine and arctic habitats of New England and Canada that have disjunct (isolated) populations at high elevations in the southern Appalachians, including mountain sandwort, three-tooth cinquefoil (Sibbaldiopsis tri-dentata), and deerhair bulrush (Trichophorum cespitosum). Conversely, a number of rare species most closely affiliated with the coastal plain, including Heller's blazing star and mountain golden heather, apparently colonized mountain outcrops during a previously hot, dry climate. Climatic change during the last ice age contributed to the large number of species endemic (restricted) to the southern Appalachians, as widespread population loss would have left some species with only a few surviving populations within a restricted geographic area. Currently, more than 40 species of rare plants occur on high-elevation rock outcrops.

Conservation Aspects

Most high-elevation rock outcrops in the southern Appalachians have some level of protection as they occur in National Parks, National Forests, state and local parks, and private reserves. Nonetheless, protected areas with public access often have problems with trampling by hikers attracted to rock outcrops by the good views the outcrops provide. Low growth forms and shallow soils make rock outcrop plants particularly vulnerable to trampling damage by unwitting nature enthusiasts and rock climbers. Other threats include habitat destruction by development activities such as ski slopes, resorts, private homes, and rock quarries. And plants restricted to cool microclimates on mountain summits are particularly vulnerable to global climate change.

SUGGESTED READING

Cogbill, C. V., P. S. White, and S. K. Wiser. 1997. Predicting Treeline Elevation in the Southern Appalachians. Castanea 62:137-46.

Wiser, S. K., and P. S. White. 1999. High-Elevation Outcrops and Barrens of the Southern Appalachian Mountains. In Anderson, R. C., J. S. Fralish, and J. M. Baskin, eds., Savannas, Barrens and Rock Outcrop Communities of North America, 119-32, New York: Cambridge University Press.

Wiser, S. K., R. K. Peet, and P. S. White. 1996. High Elevation Rock Outcrop Vegetation of the Southern Appalachian Mountains. Journal of Vegetation Science 7:703-22.

HIGH-ELEVATION ROCK OUTCROP: CHARACTERISTIC

PLANTS

Trees

OCCASIONAL TO LOCALLY ABUNDANT SPECIES

Abies fraseri

Fraser fir

p. 219

Amelanchier laevis

Smooth serviceberry

pp. 225-26

Betula alleghaniensis

Yellow birch

pp. 226-27

Picea rubens

Red spruce

p. 248

Pinus pungens

Table mountain pine

pp. 249-50

Quercus rubra

Northern red oak

pp. 259-60

Sorbus americana

Mountain ash

pp. 262-63

Tsuga caroliniana

Carolina hemlock

pp. 265-66

Shrubs

OCCASIONAL TO LOCALLY ABUNDANT SPECIES

Aronia melanocarpa

Black chokeberry

p. 272

Diervilla sessilifolia

Southern bush honeysuckle

p. 282

Kalmia buxifolia

Sand myrtle

pp. 290-91

Kalmia latifolia

Mountain laurel

pp. 291-92

Menziesia pilosa

Minniebush

pp. 295-96

^Pieris floribunda

Mountain fetterbush

Rhododendron catawbiense

Catawba rhododendron

pp. 301-2

Vaccinium corymbosum

Highbush blueberry

p. 317

Herbs

OCCASIONAL TO LOCALLY ABUNDANT SPECIES

Asplenium montanum

Mountain spleenwort

p. 339

Avenella flexuosa

Wavy hairgrass

pp. 341-42

Danthonia compressa

Mountain oat grass

pp. 359-60

Gaultheria procumbens

Wintergreen

pp. 376-77

Grimmia laevigata

Dry rock moss

p. 381

Heuchera villosa

Rock alumroot, Crag jangle

pp. 385-86

'Krigia montana

Mountain dandelion

Liatris helleri

Heller's blazing star

p. 396

Micranthes petiolaris

Cliff saxifrage

p. 403

Minuartia groenlandica

Mountain sandwort

p. 405

Oclemena acuminata

Whorled aster

p. 409

Schizachyrium scoparium

Little bluestem

pp. 436-37

Selaginella tortipila

Twisted hair spikemoss

p. 437

Solidago glomerata

Skunk goldenrod

pp. 443-44

Xerophyllum asphodeloides

Turkey beard, Beargrass

Lichens

Umbilicaria mammulata Rare Plants

Smooth rock tripe pp. 461-62

Geum radiatum Hudsonia montana Hypericum buckleyi Packera millefolium Rhododendron vaseyi Sibbaldiopsis tridentata

Spreading avens p. 379

Mountain golden heather pp. 286-87

Granite dome St. John's wort pp. 287-88

Blue Ridge ragwort pp. 415-16

Pinkshell azalea pp. 304-5

Three-tooth cinquefoil pp. 438-39

t = plant not included in the species profiles (Part IV)

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