Blue Ridge High-Elevation Gneiss Outcrops
Southern Appalachian Rocky Summit (NatureServe 2015))
This type of outcrop can be found in Eastern Tennessee, in Carter County near the border of North Carolina, where it occurs infrequently. It is restricted to the summit of Roan and Yellow Mountains. The few occurrences of gneiss outcrops are all located within Roan Mountain State Park of the Cherokee National Forest.
Blue Ridge high-elevation gneiss outcrops are small patch communities dominated by grasses and small herbs. They are typically surrounded by a Picea-Abies forest matrix, however the dominant vegetation for the outcrops is approximately 0.1-1 m (3 ft or less) in height. The grasses and small herbs that occur on the outcrops are restricted to shallow pockets of soil and cracks along the bedrock where soil has accumulated. The surfaces of the rock outcrops lack vascular plants, but are covered with lichens and mosses.
This community type lies within the Level IV Ecoregion known as the South Igneous Ridges and Mountains. The outcrops occur on the summits of high-elevation ridges along Roan and Yellow Mountains. These summits where they occur are flat to gently sloping with a range of 8-15% slopes. All gneiss outcrops are found at high-elevations of approximately 1800 meters (5900 feet) above sea level. There are approximately only six gneiss outcrops in Tennessee. Most of them are small in size, averaging 10 square meters (30 square feet). However, there are two outcrops that are notably larger, one is about 45 square meters (150 square feet) atop Yellow Mountain, and the largest is approximately 190 square meters (620 square feet) at the summit of Roan Mountain. The bedrock is comprised of migmatitic biotite-hornblende gneisses that is Middle Proterozoic aged. The soil, limited to shallow pockets and crevices, is a Burton-Craggey complex (gravelly loam) with residuum weathered from granite and gneiss. It is windswept and well-drained with no possibilities of ponding or flooding.
Gneiss outcrops, like others, are developed from erosional processes that expose the bedrock and limit the amount of soil present. The runoff that occurs with rain, and the windswept nature of these outcrops, keep the bedrock exposed and allow for increased solar insolation. All of these factors limit the ability for vascular plant growth and maintain the open nature of the outcrops.
Abies fraseri (Fraser fir), Agrostis perennans (upland bent grass), Carex misera (wretched sedge), Heuchera villosa (hairy alumroot), Polypodium appalachianum (Appalachian polypody), Rhododendron catawbiense (purple laurel), Saxifraga michauxii (Michaux’s saxifrage), Solidago glomerata (skunk goldenrod), Sorbus americana (American mountain ash), Trichophorum caespitosum (tufted bulrush), Vaccinium corymbosum (highbush blueberry)
Abies fraseri (Fraser fir), Alnus viridis ssp. crispa (mountain alder), Anastrophyllum saxicola (liverwort), Bazzania nudicaulis (liverwort), Carex misera (wretched sedge), Corydalis sempervirens (rock harlequin), Helianthus glaucophyllus (whiteleaf sunflower), Hypericcum mitchellianum (Blue Ridge St. John’s wort), Lilum grayi (Gray’s lily), Menziesia pilosa (minnie bush), Minuartia groenlandica (Greenland sandwort), Packera schweinitziana (Schweinitz’s ragwort), Paronychia argyrocoma (silverling), Plagiochila exigua (petty featherwort), Radula tenax (liverwort), Silenecaroliniana ssp. pensylvanica (Pennsylvania catchfly), Solidago spithamaea (Blue Ridge goldenrod), Sphenolobopsis pearsoni (liverwort), Thuja occidentalis (white cedar)
This information requires more extensive research of these rare gneiss outcrop communities.
Community Variation and Subtypes
This community’s characteristic subtype is the bare or lichen-encrusted rock that may also include other non-vascular plants such as liverworts and mosses. The second subtype is comprised of grasses and small herbaceous plants that grow in the shallow soil pockets and cracks that occur in the surface of the rock. A third subtype includes the edges surrounding the bare rock, which contains deeper soils and may support small trees and shrubs.
Associated Natural Communities
Presettlement Distribution and Size
It is thought to be relatively the same, although presettlement the edges of these outcrops may have been more open due to the occurrence of wildfires.
This community type is rare, with approximately six gneiss outcrops occurring in Tennessee. They are typically small in size and are restricted to the summit of Roan and Yellow Mountain in Carter County.
Carter Co., Yellow Mountain: (36.104983ᵒ, -82.080607ᵒ)
Carter Co., Roan Mountain: (36.106093ᵒ, -82.094329ᵒ)
Residential and/or commercial development of mountain summits may pose a threat to the existence of gneiss outcrops. Fire suppression has likely altered, and may continue to alter, the vegetative composition along the edges surrounding the outcrops.
These outcrops are self-maintaining when left undisturbed, although the surrounding forest may be fire dependent.
Future Research Needs
Additional research would be beneficial to compile more detailed information on the plants occurring on Blue Ridge high-elevation gneiss outcrops, and document any invasive species that may be a threat to the native vegetation.
These high-elevation gneiss outcrops are referenced in a study by Susan Wiser, Robert Peet, and Peter White, titled High-Elevation Rock Outcrop Vegetation of the Southern Appalachian Mountains.
Hardeman, W.D., and others, 1966, Geologic map of Tennessee: Division of Geology, Tennessee Department of Environment and Conservation, scale 1:250,000
NatureServe. 2014. NatureServe Explorer: An online encyclopedia of life [web application]. Version 7.1 NatureServe, Arlington, Virginia. Available http://explorer.natureserve.org
Soil Survey Staff, Natural Resources Conservation Service, United States Department of Agriculture. Web Soil Survey. Available online at http://websoilsurvey.nrcs.usda.gov/. Accessed February/2/2015.
Susan K. Wiser, R. K. Peet, and P. S. White, High-Elevation Rock Outcrop Vegetation of the Southern Appalachian Mountains, Journal of Vegetation Science, Vol 7, No. 5, Oct., 1996, pp. 703-722, Blackwell Publishing.
Checklist of Plant Species known from this community