Blue Ridge Low-Elevation Graywacke Outcrops
Southern Appalachian Rocky Summit (NatureServe 2015)
Graywacke outcrops are found in Southeastern Tennessee in Blount, Monroe, and Polk Counties. They occur within the Blue Ridge Mountain ecoregion, on generally north-facing side slopes and summits of ridge lines, where rivers cut through the landscape.
These outcrops are small patch communities that can be distinguished from surrounding communities by the characteristic bare or lichen-encrusted rocks. The vegetation is limited to low-growing grasses and herbs, no more than 1 meter (less than 3 feet) in height, which grow in shallow soil pockets and crevices along the rocks. The surface of the bedrock support only non-vascular plants such as mosses and lichens.
They are found within the Level IV Southern Metasedimentary Mountains Ecoregion. They are typically found on north-facing side slopes of low-elevation ridge lines, where rivers cut through the Blue Ridge Mountains. They are steeply sloping, ranging from 35-90% slopes. The elevation ranges from 300-500 meters (1000-1700 feet), although most occur around 400 meters (1300 ft.) above sea level. These outcrops vary greatly in shape and size, with some being as small as 10 square meters and others as large as 1400 square meters. The bedrock is Cambrian aged Walden Creek Group, including formations of Sandsuck, Wilhite, Shields, and Licklog. The soil type is Cataska-rock outcrop complex, a channery residuum weathered from metamorphic and sedimentary rock. Due to the steep slopes, graywacke outcrops are excessively drained.
Outcrops are formed by erosion of the soil, which exposes the bedrock and creates areas where vascular plant growth is limited. Wind and rain weather the soil and rock, keeping it exposed to high amounts of solar insolation, while the steep slopes keep the area well-drained. These are all factors that limit the ability of vascular plant growth and result in the open nature of the outcrops.
Due to a lack of research for graywacke outcrops, almost no floristic information exists on these communities.
Agalinis plukenetii (Chattahoochee false foxglove), Agalinis setacea (threadleaf false foxglove), Amelanchier sanguinea (roundleaf serviceberry), Gelsemium sempervirens (yellow jassamine), Monotropsis odorata (sweet pinesap), Polygonum cilinode (fringed black bindweed), Pseudognaphalium helleri (Heller’s cudweed)
A floristic survey is necessary to document the invasive species that may be present on graywacke outcrops.
Community Variation and Subtypes
This community has a main subtype consisting of the characteristic bare or lichen-encrusted rock that may also include other non-vascular plants such as liverworts and mosses. The second subtype is comprised of small forbs and herbaceous plants that take root in the shallow soil pockets or crevices that occur along the rock surfaces. This community also supports a third subtype that includes the surrounding edges of the rocks which contain deeper soils and may support small trees and shrubs.
Associated Natural Communities
Presettlement Distribution and Size
The distribution of graywacke outcrops is likely the same as pre-settlement, although the size of the outcrops were probably larger due to the presence of wildfire that once kept the edges more open.
This is a rare community type that is small in size and limited to the sides of mostly north-facing ridges along the Blue Ridge Mountain ecoregion.
Blount Co., Northern bank of Calderwood Lake on the Tennessee River: (35.501802ᵒ, -83.971023ᵒ)
Monroe Co., Northern bank of the Tellico River on Turkey Creek Mountain: (35.333058ᵒ, -84.179255ᵒ)
Polk Co., Northern bank of the Ocoee River on Brock Mountain: (35.076711ᵒ, -84.502422ᵒ)
Residential and/or commercial development of the ridges pose the biggest threat to the preservation of graywacke outcrops.
These outcrops will be maintained by natural processes if left undisturbed, although the surrounding forest may be fire dependent.
Future Research Needs
Graywacke outcrops are in need of botanical research to document the species that grow there. Further research would also be helpful for addressing any conservation issues that may arise in the future.
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.
Checklist of Plant Species known from this community