Southern Appalachian Valley Limestone Glade

This ecological system consists of open glades and surrounding woodlands on shallow, high pH soils of the Ridge and Valley region from southwestern Virginia southward. Examples of related calcareous vegetation from the Cumberland Plateau area of Alabama (231Cd of Ecomap 2007; 68e of EPA) are included here as well. These glades occur in broad valley bottoms, rolling basins, and adjacent slopes where soils are shallow over flat-lying limestone strata. The flat to rolling terrain and locally xeric soils may have been especially conducive to periodic fires that helped maintain the prairielike openings and savannalike woodlands. Today, much of the system is currently somewhat more closed and brushy, suggesting fire suppression. Quercus muehlenbergii and Quercus stellata are typical where the canopy is present. Dominant or abundant Juniperus virginiana var. virginiana is probably a result of the lack of fire.

The vegetation of typical examples could range from open woodlands of Quercus muehlenbergii and Juniperus virginiana, with interspersed grasslands dominated by perennial Schizachyrium scoparium, to patches dominated by annual grasses such as varieties of Sporobolus vaginiflorus (e.g., var. ozarkanus, var. vaginiflorus). Some other trees that may occur in stands include Quercus falcata, Quercus shumardii, Quercus stellata, as well as the understory woody plants Cercis canadensis, Salix humilis, and Viburnum rufidulum. Some characteristic herbs include Eryngium yuccifolium, Manfreda virginica, and Hypericum dolabriforme. Dominant or abundant Juniperus virginiana var. virginiana is probably a result of the lack of fire.

Examples occur on shallow, high pH soils, in broad valley bottoms, rolling basins, and adjacent slopes over limestone strata.

The flat to rolling terrain and locally xeric soils may have been especially conducive to periodic fires that helped maintain the grass-dominated openings and open woodlands. 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).

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).

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 from southwestern Virginia (roughly Roanoke) south through the southern Ridge and Valley into Georgia (as well as in the Cumberlands of Alabama).