Southern Appalachian Mountain Bald

This ecological system consists of dense herbaceous and shrubland communities in the highest elevational zone of the Southern Appalachians, generally above 1524 m (5000 feet) but occasionally to 1220 m (4000 feet), and at slightly lower elevations at its northern limit in Virginia and West Virginia, and in the Cumberland Mountains along the Virginia-Kentucky border. Vegetation consists either of dense shrub-dominated areas (heath balds) or dense herbaceous cover dominated by grasses or sedges (grassy balds). Heath balds are most often dominated by Rhododendron catawbiense, but substantial examples are also dominated by Rhododendron carolinianum, Kalmia latifolia, or a mixture of shrubs. One large example, dominated by Alnus viridis ssp. crispa, has been regarded as related to the heath balds, but is better treated separately due to much greater herbaceous diversity and coverage which is clearly different from typical heath balds. Grassy balds are characteristically dominated by Danthonia compressa, Deschampsia flexuosa, or Carex spp. Large areas have also become dominated by Rubus allegheniensis and/or Rubus canadensis, and by mixtures of native grasses with exotic pasture grasses. Most examples of grassy balds have some invading shrubs and trees, often dense enough to threaten the herbaceous vegetation. Heath balds may contain sparse stunted trees barely larger than the shrub canopy. The combination of high-elevation, non-wetland sites and dense herbaceous or shrub vegetation without appreciable rock outcrop conceptually distinguishes this system from all others in the Southern Appalachians. However, the widespread areas of degraded spruce-fir with grass and shrub cover and the invasion of grassy balds by trees blur the distinction somewhat.

Vegetation consists either of dense shrubs (heath balds or blackberry) or dense herbaceous cover dominated by grasses or sedges (grassy balds). Heath balds are most often dominated by Rhododendron catawbiense, but substantial examples are also dominated by Rhododendron carolinianum, Kalmia latifolia, or a mixture of other shrubs, including Prunus pensylvanica, Sorbus americana, Corylus cornuta, Gaylussacia baccata, Pieris floribunda, Vaccinium corymbosum, and Leiophyllum buxifolium. One large example, dominated by Alnus viridis ssp. crispa, is generally also regarded as being related to the heath balds. Grassy balds are characteristically dominated by Danthonia compressa, Carex pensylvanica, or other Carex spp. with forbs including Minuartia groenlandica, Paronychia argyrocoma, Saxifraga michauxii, Solidago glomerata, Solidago rugosa ssp. aspera, Sibbaldiopsis tridentata, and others. Large areas have also become dominated by Rubus allegheniensis, possibly with other brambles (Rubus canadensis, Rubus idaeus ssp. strigosus) and by mixtures of native grasses with exotic pasture grasses (e.g., Phleum pratense). Most examples of grassy balds have some invading shrubs and trees, often dense enough to threaten the herbaceous vegetation. Heath balds may contain sparse stunted trees barely larger than the shrub canopy.

This system generally occurs at elevations above 1524 m (5000 feet) but may range as low as 1220 m (4000 feet) in the Southern Blue Ridge, with most examples from 1600-1780 m (5200-5800 feet) elevation (Mark 1958). It is also of limited extent above 1035 m (3400 feet) in the Cumberland Mountains along the Virginia-Kentucky border. It occurs on broad ridgetops and narrow spur ridges. Elevation and orographic effects (winds cooling as they rise to create increased condensation) make the climate cool and wet, with heavy moisture input from fog and cloud interception as well as high rainfall and snowfall. Convex slopes and exposure to wind offset the moisture input to some extent. The high peaks of the Southern Appalachians are not above the treeline; balds occur well below the elevation which would be a treeline today. Concentration of air pollutants has been implicated as an important anthropogenic stress in this elevational range in recent years. Soils range from shallow and rocky to fairly deep residual soils. Any kind of bedrock may be present, but most sites have erosion-resistant felsic igneous or metamorphic rocks, with slate and quartzite particularly frequent. Alder bald tends to occur on areas with thinner and rockier soils than nearby grassy bald (Brown 1941, J. Donaldson pers. comm. 2013), and is distinct from heath bald (Harshberger 1903b, Schafale 2012). The sites that support balds are not obviously different from similar sites that support spruce-fir forests, so the origin of the balds continues to be fodder for debate. Grazing and/or exposure to the elements may help maintain balds. Grass balds occur on less than one percent of the sites suitable for them (White and Sutter 1999b), and heath balds occur on 4-9% of the sites suitable for them (White et al. 2001). Forests occur on most of these sites, such as northern hardwood, high-elevation oak, or spruce-fir forests.

The dynamics that maintain and that created the communities in this system have been a major topic of debate, so far without resolution. Most grassy bald occurrences show a strong tendency to succeed to shrub or forest vegetation under present conditions, suggesting that some important maintenance process has been lost. Northern hardwood, high-elevation oak, or spruce-fir forests may occur adjacent to balds. Grazing by native herbivores (elk and bison) and periodic fire have both been suggested as natural mechanisms to keep out woody vegetation. Others have suggested that all grassy balds are of anthropogenic origin and were never ecologically stable. The most definitive grassy balds have been documented as present at the time of the first European settlement, making documentation of their origin impossible. The presence of shade-intolerant endemic or disjunct herbaceous plant species in some suggests even greater age. These include Lilium grayi, Geum radiatum, Packera schweinitziana, and Houstonia purpurea var. montana. Some areas of the spruce-fir system degraded by a combination of logging, slash fires, and grazing resemble grassy balds, but most do not. The common practice of cattle grazing in grassy balds by early settlers has further obscured their presettlement character and evidence of presettlement disturbance processes.

Heath balds (not including alder balds) are more prone to disturbance by fire (Conkle 2004). However, heavy organic accumulations in the soil suggest great age for some. Most heath balds show limited tendency to succeed to forest, suggesting that the dense heath shrub layer is very competitive with tree seedlings. Spruce-fir forest stands which burned in historical times have not usually developed vegetation identical to heath balds.

The lack of grazing, in combination with acid deposition, very high levels of nitrogen deposition, warmer winter temperatures, and higher levels of atmospheric carbon dioxide are factors which influence the loss of native graminoid species diversity, increases in weedy species and the transition of many grassy balds to tree- and shrub-dominated vegetation (Nodvin et al. 1995, Weiss 1999, Stevens et al. 2004, Boggs et al. 2005, Sturm et al. 2005, Post 2013). Over long periods of time in the past, native large grazing mammals (such as deer, elk, bison, and perhaps other now extinct large mammals) and domesticated livestock (cattle, sheep, and goats) influenced the vegetation by maintaining lower levels of vegetation biomass and ecosystem nutrient levels. Nutrient cycling in balds is faster when grazing animals are present.

Mechanical management (such as mowing or bush-hogging) presents different selection factors for plants compared to animal grazing or fire. There are differences regarding biomass accumulation (thatch buildup), sunlight, nutrient cycling, and plant selectivity. Mechanical management may, over time, create a markedly different community composition compared to historic species compositions.

This system ranges from the Balsam Mountains and Great Smoky Mountains of North Carolina and Tennessee northward to Virginia and West Virginia. The system is also of limited extent in the Cumberland Mountains along the Virginia-Kentucky border. The current status in Georgia is open to question and the ecological system was apparently never extensive in any case. The distribution and classification of grassy balds and high-elevation pastures has been documented (Gersmehl 1970). Heath balds could be mapped separately from grassy balds as has been done for the Great Smoky Mountains (White et al. 2001). Alder bald can also be mapped separately, but it requires more field verification to map correctly.