Piedmont Hardpan Woodland

This system of the southern Piedmont occurs in places where a particularly dense clay hardpan has developed over a range of typically mafic rocks, sometimes with more limited areas of shallow glade-like vegetation. In the deeper soil portions of this system, the density of the clay, in combination with its shrink-swell properties, limits water and root penetration into the soil and creates xeric conditions for plants despite the presence of deep soil. Possibly the most typical expression of this system in North and South Carolina is an open forest or woodland of Quercus stellata, with Quercus marilandica as a characteristic associate. The open canopy leads to a better developed herb layer than in most Piedmont forests, one that is usually grassy. In Virginia, typical canopy trees include Quercus alba, Carya glabra, and Fraxinus americana. Some of these sites may have once supported open prairies or prairie savannas when they burned more frequently. Fire was probably once the most important natural dynamic process, but the universal elimination of fire in the Piedmont makes this difficult to observe on most of the modern landscape.

Vegetation consists of xerophytic species, most typically consisting of open forests or woodlands of Quercus stellata, with Quercus marilandica as a characteristic associate in North and South Carolina. In Virginia and adjacent Maryland, Quercus alba, Fraxinus americana, and Carya glabra are common canopy components. The open canopy leads to a better developed herb layer than in most Piedmont forests, one that is usually grassy. Some of these sites may have once supported open prairies or prairie savannas when they burned more frequently. A significant flora of shade-intolerant herbs with prairie affinities is present in open areas on these soils. Typical understory species include Juniperus virginiana, Cercis canadensis, Diospyros virginiana, Vaccinium arboreum, Ulmus alata, and Chionanthus virginicus. Shrubs may be sparse or dense, with Viburnum rafinesqueanum, Viburnum prunifolium, Vaccinium stamineum, and Vaccinium pallidum being typical. The most common herbs are Danthonia spicata and Schizachyrium scoparium. Other herbs include Clematis ochroleuca, Sericocarpus linifolius (= Aster solidagineus), Hieracium venosum, Hieracium gronovii, Hypericum hypericoides, Symphyotrichum dumosum (= Aster dumosus), Lespedeza spp., Oenothera fruticosa, Liatris pilosa (= Liatris graminifolia), and Solidago spp. (Schafale and Weakley 1990). In contrast to upland forests of adjacent portions of the Virginia Piedmont, there seems to be a greater abundance of hickory in examples of this ecological system (Farrell and Ware 1991, Ware 1992).

This system occurs in places in the southern Piedmont where a particularly dense clay hardpan, typically composed of Montmorillonite, has developed. The substrate is typically mafic igneous or metamorphic rock (gabbro, basalt, diabase, or amphibolite) but occasionally is slate. The density of the clay, or its shrink-swell properties, limits penetration of water into the soil and limits penetration of roots, creating xeric conditions for plants despite the presence of deep soil. These areas generally occur on unusually flat uplands but may occur on tops of narrower ridges. Only a minority of these substrates form the distinctive soil conditions of this system. Local topography that promotes runoff is important to forming this system. Areas with these soil conditions but with concave topography perch water and support Piedmont depressional wetlands. Soils in most examples are basic or circumneutral, but those formed from slate are somewhat acidic. In Virginia and adjacent Maryland, this system occupies one of the largest Triassic basins in eastern North America. It includes a mix of sedimentary rocks, especially siltstone, mixed with igneous intrusions. The igneous rocks weather to form more mafic soils, while the sedimentary rocks are more acidic. The local landscape may best be thought of as a lowland, in comparison with the surrounding and prevailing topography.

Fire was probably once the most important natural dynamic process, but the universal elimination of fire in the Piedmont makes this difficult to determine. Both the drier character of the sites and the distinctive soil conditions interact with one another to retard woody succession. These factors would presumably have interacted with the fire regime to promote more open vegetation on these sites. This would presumably lead to a greater probability that these open woodland conditions would prevail for a longer period than they would on more typical soils. Fire would have kept canopies open and would have promoted a more diverse, grass-dominated herb layer. Bison may have once been a significant grazing influence on this system. These sites are now of limited extent and it is harder to determine how these past disturbance factors operated in the larger landscape.

Conversion of this type has primarily resulted from removal of the characteristic dominant hardwoods (primarily Quercus species and Carya species) through logging, and the subsequent development of the sites to human uses. This is also the most critical anthropogenic threat. This may result in a stand dominated by wind-blown or bird-dispersed tree species, including Acer rubrum, Celtis spp., Fraxinus americana, Juglans nigra, Juniperus virginiana, Liquidambar styraciflua, Liriodendron tulipifera, Prunus serotina, Robinia pseudoacacia, Sassafras albidum, Ulmus americana, and the exotic Ailanthus altissima. These and other fire-intolerant species persist and increase in the absence of fire (Edwards et al. 2013). Logging which is not carefully done can lead to soil erosion, and then conversion to Pinus taeda plantations or succession to Acer rubrum, Liquidambar styraciflua, Pinus taeda ruderal forest (Nordman 2013). Patches dominated by Pinus taeda are artifacts of past disturbance and succession in the absence of fire. These are likely to eventually succumb to drought, fire or insect damage. Another major threat is conversion to human-created land uses, including residential development, industrial development, and infrastructure development. Feral hog (Sus scrofa) activity can eradicate the native ground and shrub flora (Engeman et al. 2007). In addition, invasive exotic species, including Ligustrum sinense, Lonicera japonica, Microstegium vimineum, Pueraria montana var. lobata, Rosa multiflora, and others can become dominant in the ground and shrub layers following canopy disturbance and are threats to the natural species diversity of these habitats (Edwards et al. 2013).

As currently known, this system is found in the Piedmont of Maryland, Virginia, North Carolina, South Carolina and Georgia. Its status in Alabama is not known. Its occurrence may be more frequent in the Triassic basins, but it is not restricted to them.