Southern Appalachian Dark Rock Glade and Barrens

This Southern and Central Appalachian system consists of vegetation associated with shallow soils over predominantly mafic bedrock, usually with significant areas of rock outcrop. Bedrock includes a variety of igneous and metamorphic rock types such as greenstone and amphibolite. These areas support a patchy mosaic of open woodland and grassy herbaceous vegetation sometimes with a predominant woody short-shrub community present.

Vegetation is a fine mosaic of different physiognomies, with open woodland and grassy herbaceous vegetation or short shrubs predominating. Some instances may have closed canopies of small trees or large shrubs, but no examples have large canopy trees with a closed canopy. Bare rock outcrops are usually present in a minority of the area. The canopy species are species tolerant of dry, shallow soils, most commonly Quercus montana (= Quercus prinus), Pinus spp., and Juniperus virginiana var. virginiana. Basic examples may also have Carya glabra, Fraxinus americana, and other species abundant. Shrubs may be dense, with species determined by soil chemistry. The herb layer is usually fairly dense and dominated by grasses, both in treeless areas and beneath open canopy. An abundant forb component is also usually present, especially in the more basic examples. The forbs include species characteristic of other rock outcrops and grassland species, with a smaller number of forest species present. Other species that may be present include the woody plants Carya tomentosa (= Carya alba), Diervilla lonicera, Gaylussacia baccata, Kalmia latifolia, Aronia melanocarpa (= Photinia melanocarpa), Physocarpus opulifolius, and Quercus stellata; and herbaceous plants Aletris farinosa, Allium cernuum, Allium cuthbertii, Carex pensylvanica, Croton michauxii var. ellipticus (= Croton willdenowii), Cheilanthes tomentosa, Deschampsia flexuosa, Helianthus divaricatus, Hylotelephium telephioides, Muhlenbergia sobolifera, Packera paupercula, Phacelia dubia, Pycnanthemum tenuifolium, Saxifraga michauxii, Schizachyrium scoparium, Selaginella rupestris, Sibbaldiopsis tridentata, Solidago simplex var. randii, Solidago ulmifolia, Sorghastrum nutans, and Thalictrum revolutum.

This system occurs on upper to mid slopes, usually on gentle to moderate slopes but occasionally steeper. The ground is mostly shallow soil over bedrock, usually with significant areas of rock outcrop. The rock usually has few fractures but may have a pitted or irregular surface. This rock structure supports more extensive and deeper soil development than in Southern Appalachian Granitic Dome (CES202.297), but has few of the crevices and deeper rooting sites available in Southern Appalachian Rocky Summit (CES202.327). Micro-scale soil depth and presence of seepage are important factors in determining the vegetation patterns. Shallow soil, unable to support a closed tree canopy, separates this system from forest systems. Bedrock includes a variety of igneous and metamorphic rock types. Some examples are on mafic substrates such as amphibolite, some are on felsic rock such as granitic gneiss but have flora that suggests a basic influence, and a few occur on felsic rocks and are clearly acidic. Rock or soil chemistry appears to be the most important factor affecting different associations on sites that have the physical structure to belong to this system. Elevation may also be an important factor causing variation.

The dynamics of this system are complex and poorly understood. These sites, with their shallow soil, would likely be affected by fire, drought, and windstorms. Severe droughts kill tree saplings growing in cracks and potholes, helping to retain the open character of the glades (Quarterman et al. 1993). Fire may be an important influence on the vegetation structure, and may function to keep the vegetation more open in the long run. The patchy distribution of vegetation in examples of this system may limit fire intensity. These glades do not appear to be undergoing the kind of cyclic succession that has been described for granitic domes, but some balance of soil accumulation and destruction may be occurring on a longer term or coarser scale. There may be a zonation or patchiness to glade/barren vegetation, with different zones that may be identified by their characteristic plant species (Quarterman et al. 1993). These zones are apparently relatively stable, with woody plant encroachment evident only in relation to the invasion of shrubs and trees into potholes or crevices where soil accumulates more rapidly. It is possible that the slightly irregular curved surface of some examples represents a late stage in the weathering of old exfoliation surfaces that once supported granitic domes, but most known examples are not spatially associated with existing granitic domes.

The most critical anthropogenic threat to native glade and rock outcrop vegetation is their conversion to human-created land uses, including residential development, quarries, industrial development, infrastructure development, and others (TNC 1996c). Rocky glades and outcrops may be the last areas to be converted to development and housing due to the unsuitability of the soil to septic tanks. Other common threats and stressors include both the removal of disturbance and the effects of inappropriate or too intensive or constant disturbance. These areas often attract off-road-vehicle use.

Fire plays a critical role in the maintenance of most native grasslands, which may surround or interfinger with rocky glades. In the absence of fire and appropriate disturbance in the landscape matrix, the areas with the most shallow soils (e.g., the glades) may be the only open areas persisting in a series of woody shrub thickets. Without fire or other disturbance, Juniperus species, Quercus species and other hardwoods quickly regenerate, shading out the herbaceous plants, and leading to a shift in species diversity from the ground layer to the upper woody strata, resulting in a net loss of species diversity (Taft et al. 1995). At sites with intermediate levels of woody encroachment, a signal of restoration potential is an inverse relationship between woody stem density and ground layer species richness (Taft 2009). More information is needed about the particular appropriate ranges of fire-return times and intensities in the various systems. The actual rocky or gravelly glades may not support sufficient fuel to consistently carry fire, but in the adjacent or interpenetrating perennial grasslands, occasional surface fire will retard woody plant encroachment and help maintain herbaceous diversity, as will, to an extent, grazing or mowing (Duffey et al. 1974). In addition to occasional fire, periodic drought may also be important in regulating woody plant encroachment into native grasslands. It is believed that these native glade-grassland systems have evolved under a combined system of grazing, drought, and periodic fire (Duffey et al. 1974, Estes et al. 1979, Noss 2013).

Fragmentation of glades and their accompanying native grasslands, barrens, and savannas occurs with the development of housing and industrial sites, as well as the construction of roads, which not only function as firebreaks, limiting the areas that can be burned with one ignition event, but which make it more difficult to mitigate the effects of smoke on human populations and their activities. In many cases, these glade-grassland systems were once extensive on the landscape, but have now been reduced to scattered and isolated remnant patches, presenting conservation and management challenges. These disturbances have had damaging effects on fragile soil profiles and plant and animal species. These combined impacts also foster a trend toward biotic homogenization, which results in the gradual replacement of ecologically distinct natural communities by those dominated by weedy generalists (McKinney and Lockwood 1999). In other cases, the grassland and/or glade system naturally occurs in small isolated patches occurring within an otherwise forested matrix.

Many glade sites, have been used as pastures, or as dumping grounds for trash (Quarterman et al. 1993). The spread of invasive exotic plants (particularly Ligustrum species and Lonicera species shrubs, as well as Lespedeza cuneata, Ailanthus altissima, and Albizia julibrissin) will fundamentally alter the character of glades and their accompanying native grasslands. Some of these exotics are allelopathic, thereby presenting a greater threat to native species (N. Murdock pers. comm.). Opportunistic native increaser plant species (e.g., Juniperus virginiana) can also shade out light-requiring herbaceous plants (TNC 1996c).

The most significant potential climate change effects over the next 50 years include shifts to dramatically drier or moister climate regimes. A cooler and wetter regime would most likely accelerate the trend toward woody plant encroachment, removing drought as a factor in its inhibition. A moderately drier regime during the growing season could favor the characteristic native grasses and forbs, which are adapted to these conditions better than the generalists. An extremely drier regime for an extended period of time could ultimately have negative effects.

This system occurs in scattered clusters in the Southern Blue Ridge and adjacent portions of the upper Piedmont and Central Appalachians.