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Xeric Hardpan Forest

Sites

Upland flats and gentle slopes with an impermeable clay subsoil but which do not pond water for extended periods. Most commonly occurs on mafic rocks.

Soils

Various Piedmont soils with a clay hardpan or shallow rock. Series include Helena-Sedgefield, Iredell, and Orange.

Hydrology

Terrestrial. Impermeable subsoil makes soil very dry in summer and during drought periods, but wet in rainy periods.

Vegetation

Canopy

Somewhat stunted and open canopy characteristically dominated by Quercus stellata (Post Oak) and Q. marilandica (Blackjack Oak). A variety of other species may be present, including Fraxinus americana (White Ash), Quercus phellos (Willow Oak), Q. alba (White Oak), Carya carolinae-septentrionalis (Carolina Shagbark Hickory), C. glabra (Pignut Hickory), Pinus echinata (Shortleaf Pine), P. virginiana (Virginia Pine) and various other oaks.

Understory

Typical understory species include Ulmus alata (Winged Elm), Juniperus virginiana (Eastern Red Cedar), Diospyros virginiana (American Persimmon), Vaccinium arboreum (Farkleberry), Chionanthus virginicus (Fringe-tree), Cercis canadensis (Eastern Redbud) and sometimes Acer leucoderme (Chalk Maple). Shrubs may be sparse or dense, with Vaccinium stamineum (Common Deerberry), Viburnum prunifolium (Black Haw) and Rhus aromatica (Fragrant Sumac) typical.

Herb Layer

A common herb is Danthonia spicata (Poverty Oat-grass). Other herbs may include Piptochaetium avenaceum (Eastern Needlegrass), Hypericum hypericoides (St. Andrew’s Cross), Hieracium venosum (Veiny Hawkweed), Solidago spp. (Goldenrod), Lespedeza spp., Oenothera fruticosa (Southern Sundrops), Clematis ochroleuca (Curlyheads) and Sericocarpus linifolius (Narrow-leaf White-topped Aster). A few areas, all with the canopy removed or kept open by artificial disturbance, have a very diverse herbaceous flora, with a number of species of prairie affinities.

Dynamics

The natural structure and dynamics of these communities is uncertain. Most now have a nearly closed canopy. Reproduction occurs in canopy gaps. In disturbed sites pines may dominate, but these sites appear to succeed to post oak-blackjack oak dominance. Succession is slowed by the unfavorable site conditions. Although the natural fire frequency is not known, it was certainly greater than now. With normally dry conditions and a grassy herb layer, these sites would have been susceptible to fire almost any time there was ignition. Fire of even moderate frequency would likely have combined with the dry site conditions to reduce tree reproduction and increase grass dominance, producing a more open, or even prairie-like, vegetation structure than is now seen. Fire would have had greater effects on the vegetation structure on these unfavorable sites than in adjacent, more mesic hardwood forests which would have burned with the same frequency. The occurrence of a number of species associated with Xeric Hardpan Forests but occurring largely in pastures, roadsides, and other chronically disturbed areas supports the idea of a naturally more open vegetation structure.

Associations

Usually associated with Upland Depression Swamp Forest and Basic Oak-Hickory Forest or Dry Oak-Hickory Forest.

Comments

This community type is relatively rare. The conditions necessary for the formation of the impermeable subsoil seem to be fairly specialized and many areas of apparently suitable substrate, and even of the appropriate soil series, do not support Xeric Hardpan Forests.