Piedmont Hardpan Forests
These deciduous and mixed forests occupy gentle to flat Piedmont uplands and ancient, never-flooded stream terraces with impermeable clay subsoils. On high bedrock terraces of the Potomac Gorge in northern Virginia and occasionally elsewhere, flat-lying bedrock underlying shallow soil acts as a surrogate "hardpan" and supports similar vegetation. Piedmont Hardpan Forests occur from Virginia south to Georgia. Sites are usually underlain either by mafic rocks such as diabase or by acidic slates. Surficial soils are silt or clay loams, with an abrupt transition to heavy, plastic clay hardpans at depths of 23 to 38 cm (9 to 15 in). These shrink-swell clay soils pond water for brief or, at a few sites, prolonged periods during rainy weather, but tend to be very hard and dry during significant portions of the growing season. Post oak (Quercus stellata) is the most typical overstory tree, growing in nearly pure stands or in variable mixtures with pignut hickory (Carya glabra), Carolina shagbark hickory (Carya carolinae-septentrionalis, only in Halifax County), white oak (Quercus alba), blackjack oak (Quercus marilandica var. marilandica), Virginia pine (Pinus virginiana), and white ash (Fraxinus americana). Virginia pine increases following cutting and may dominate on heavily disturbed, clear-cut sites. Winged elm (Ulmus alata), sweetgum (Liquidambar styraciflua), and eastern red cedar (Juniperus virginiana var. virginiana) are characteristic understory trees. Shrubs include eastern redbud (Cercis canadensis var. canadensis), black haw (Viburnum prunifolium), fringetree (Chionanthus virginicus), and blueberries (Vaccinium spp.). In closed stands, there is often little herbaceous growth, while more open stands support large patches of xerophytic graminoids such as poverty oatgrass (Danthonia spicata), eastern needlegrass (Piptochaetium avenaceum), and little-headed nutrush (Scleria oligantha).
Stands in which water ponds for longer periods contain peculiar mixtures of upland and wetland species, but their hydrological status is problematic and they are treated here as communities of the Terrestrial System. In these periodically wet variants, species such as willow oak (Quercus phellos), sweetgum (Liquidambar styraciflua), deciduous holly (Ilex decidua var. decidua), hairy highbush blueberry (Vaccinium fuscatum), St. Peter's-wort (Hypericum crux-andriae), and beaksedges (Rhynchospora spp.) are intermingled with the xerophytic species listed above.
Piedmont hardpan forests are scattered throughout the Piedmont in specialized soil environments and are considered uncommon to rare in Virginia.Reference: Fleming (2002a), Fleming (2007).
Click on the images below to open a larger image in a separate window.Mature Piedmont Hardpan Forest in Cumberland County, dominated by hickories (Carya spp.), post oak (Quercus stellata), and white oak (Quercus alba), with grassy herb layer domianted by eastern needlegrass (Piptochaetium avenaceum) and little-headed nutrush (Scleria oligantha). Some of the larger trees in the stand have been aged at > 240 years old. Turkey Ridge Natural Area, Cumberland State Forest. Gary P. Fleming / © DCR Natural Heritage.
Xeric hardpan forest on high bedrock terrace flanking the Potomac River downstream from Great Falls, Fairfax County (Great Falls Park). Pignut hickory (Carya glabra) and post oak (Quercus stellata) are the overstory dominants. Gary P. Fleming / © DCR Natural Heritage.
Piedmont Hardpan Forest on upland flats underlain by gabbro in Charlotte County (Hogan Creek Wildlife Management Area, John H. Kerr Reservoir). Gary P. Fleming / © DCR Natural Heritage.