Nashville Basin Limestone Glade

This system encompasses a range of plant communities associated with thin soils on flat areas of Ordovician limestone in the Nashville Basin of Tennessee (mostly inner basin, also outer basin), with a few disjunct occurrences in Kentucky. The vegetation of this system includes sparsely vegetated rock outcrops, annual Sporobolus spp.-dominated grasslands, Schizachyrium scoparium-dominated perennial grasslands, seasonally wet herbaceous washes and seeps, shrublands, as well as woodlands dominated by Juniperus virginiana and oaks. In addition, Echinacea tennesseensis and Astragalus bibullatus are completely endemic to this system. There are numerous other disjunct and near-endemic plants.

The vegetation of this system includes sparsely vegetated rock outcrops, annual Sporobolus spp.-dominated grasslands, Schizachyrium scoparium-dominated perennial grasslands, seasonally wet herbaceous washes and seeps, shrublands, as well as woodlands dominated by Juniperus virginiana and oaks. Other woody plants associated with this system include Quercus shumardii, Cercis canadensis, Ulmus alata, Fraxinus quadrangulata, and Acer saccharum. Characteristic shrubs include Forestiera ligustrina, Rhus aromatica, Hypericum frondosum, and Frangula caroliniana. Other herbaceous taxa include Andropogon gerardii, Bouteloua curtipendula, Silphium trifoliatum, Silphium terebinthinaceum, Helianthus mollis, Grindelia lanceolata, Liatris spp., Stenaria nigricans (= Hedyotis nigricans), Croton capitatus, Heliotropium tenellum, Trichostema brachiatum (= Isanthus brachiatus), Manfreda virginica, Ruellia humilis, Phemeranthus calcaricus (= Talinum calcaricum), Sedum pulchellum, and Panicum flexile. In addition, Echinacea tennesseensis and Astragalus bibullatus are completely endemic to this system. There are numerous other disjunct and near-endemic plants, including Astragalus tennesseensis, Dalea gattingeri, and Pediomelum subacaule (Somers et al. 1986). Small-scale seepage areas and washes may contain Dalea foliosa, Eleocharis bifida, Eleocharis compressa, Hypoxis hirsuta, Isoetes butleri, Mecardonia acuminata, Mitreola petiolata, Nothoscordum bivalve, and Schoenolirion croceum, as well as Juncus filipendulus, Carex crawei, Carex granularis, Leavenworthia torulosa, Leucospora multifida, Lobelia appendiculata var. gattingeri (= Lobelia gattingeri), Muhlenbergia schreberi, and Sisyrinchium albidum.

This system is associated with thin soils on flat areas of Ordovician limestone in the Inner Nashville Basin of Tennessee (Ecoregion 71i of Griffith et al. 1998 and EPA 2004; Subsection 222Ed of Keys et al. 1995), with a few disjunct occurrences in Kentucky.

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.

Periodic droughts, fire, historic grazing, and ice storms all play a role in the dynamics of the system by restricting woody growth and maintaining the more open glade structure. Historic grazing by wild and domestic ungulate species represented a significant disturbance regime in the past. Regionally significant drought cycles lead to death or decline of Juniperus virginiana, as well as affecting the severity of other disturbance regimes. Severe droughts kill tree saplings growing in cracks and potholes, helping to retain the open character of the glades (Quarterman et al. 1993). Fire carries best in zones or areas dominated by perennial grasses, which provide the most abundant and consistent fuel. This zone is also the most vulnerable to succession, with Juniperus virginiana and various native (and exotic) shrubs occupying these areas in periods without disturbance (Landfire 2007a).

The ecological processes that maintain these open grasslands and glades within a forested matrix are not completely understood. Clearly periodic drought cycles of varying lengths play a role, along with fire and free-ranging grazing livestock, at least until the 1940s, when open range laws were changed (DeSelm 1994). Livestock confinement, habitat fragmentation, and the ingrowth of exotic shrubs have caused many examples of these communities to become more densely covered by woody plants, including the native but weedy Juniperus virginiana var. virginiana.

Open range laws and the use of fire to clear native grass pastures worked to keep large parts of the rural Nashville Basin in an open, grass-dominated condition, either as open, prairie-like areas, or as oak woodlands with a native grass and forb understory. This combination of conditions persisted until about 1945 (DeSelm 1994). In a Missouri study of presettlement fire using composite fire scar chronologies, Guyette and McGinnes (1982 as cited in Frost 1998) reconstructed a presettlement fire frequency of 3.2 years in Missouri cedar glade vegetation.

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 Ailanthus altissima, Albizia julibrissin, and Lespedeza cuneata) 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 restricted to flat areas of Ordovician limestone in the Inner Nashville Basin of Tennessee (Ecoregion 71i of Griffith et al. (1998); Subsection 222Ed of Keys et al. (1995)), as well as limited and disjunct examples on flat Mississippian limestones in Kentucky.