Cumberland Plateau Sandstone Glade

This system encompasses a complex of sparsely vegetated rock outcrops, perennial grasslands, and woodlands on shallow soils on the Cumberland Plateau of Kentucky, Tennessee, Alabama, and Georgia. Herbaceous plants, including Diamorpha smallii and Minuartia glabra, are typical of the outcrops in Tennessee. In Alabama, Bigelowia nuttallii and Schizachyrium scoparium are important. Pinus virginiana and Acer rubrum are typical of the current condition of many of the woodlands surrounding these outcrops on the Cumberland Plateau. This dominance pattern may be due to lack of disturbance. Pinus rigida, Pinus echinata, and/or Quercus montana may also occur. Scattered shrubs, such as Gaylussacia spp., Vaccinium arboreum, and Chionanthus virginicus, occur on the margins in patches of deeper soil. Various mosses and fruticose lichens such as Cladonia spp. may be prominent in some examples. To the west, in the Interior Highlands (Ozark, Ouachita, and Interior Low Plateau regions), this system is replaced by Central Interior Highlands Dry Acidic Glade and Barrens (CES202.692) (both are found in Kentucky, with the latter in the Shawnee Hills of the Interior Low Plateau).

These sandstone glades occur in a matrix of pine-oak forests. Pinus virginiana and Acer rubrum are typical of the woodlands surrounding these outcrops on the Cumberland Plateau (Perkins 1981). This dominance pattern may be due to lack of disturbance. Other trees may include Pinus rigida, Pinus echinata, and/or Quercus montana (= Quercus prinus). A perennial grass zone is typically present, which contains Schizachyrium scoparium, Andropogon virginicus, Danthonia sericea, and Dichanthelium dichotomum, in varying proportions. Herbaceous plants which are typical of the outcrops in Tennessee include Diamorpha smallii and Minuartia glabra. In Alabama, Bigelowia nuttallii is important, forming stands with other forbs (Perkins 1981, A. Schotz pers. comm.). Other herbaceous plants which may be found include Liatris microcephala, Coreopsis pulchra, Eurybia surculosa (= Aster surculosus), Hypericum gentianoides, Phemeranthus mengesii (= Talinum mengesii), Nuttallanthus canadensis (= Linaria canadensis), Opuntia humifusa (= var. humifusa), Sporobolus vaginiflorus, and Erigeron strigosus. Nonvascular plants include Aulacomnium palustre, Campylopus pilifer, Grimmia spp., Polytrichum commune, Polytrichum juniperinum, and fruticose lichens such as Cladonia spp. (= Cladina spp.). Scattered shrubs, such as Gaylussacia spp., Vaccinium arboreum, and Chionanthus virginicus, along with oak and hickory regeneration, occur on the margins of more open areas, in patches of deeper soil.

This suite of glade, barren, and rock outcrop communities are found on flat to gently sloping expanses of sandstone and conglomerate (Edwards et al. 2013) on the surface of the Cumberland Plateau and related formations from Virginia south and west to Alabama. As the cement that holds the sand and conglomerate particles together dissolves and is transported away, sandy particles may collect in crevices and depressions to form sandy soil (Quarterman et al. 1993, Edwards et al. 2013). The sites of this system may be saturated for short times after rainfall, but also experience high temperatures in the summer, creating harsh conditions. Some examples of this system may occur adjacent to sandstone cliff faces.

Severe droughts kill tree saplings growing in cracks and potholes, helping to retain the open character of the glades (Quarterman et al. 1993). There is an apparent 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.

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. Trampling from human disturbance and over-use is a threat to the vegetation (Perkins 1981).

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 is found in the Cumberland Plateau of Kentucky, Tennessee, Virginia, Alabama, and Georgia.