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Table of ContentsCoastal Plain / Piedmont Seepage Swamps
This group contains forested vegetation of braided headwaters stream bottoms and seeping toe-slopes saturated by abundant groundwater discharge. Classified units in the group separate along a gradient of soil fertility and groundwater chemistry, which range from extremely acidic and nutrient-poor to highly calcareous. Despite the wide variation in substrate status, the communities in the group share hydrologically and topographically similar habitats, as well as many wetland species that tolerate a wide range of soil conditions.
The more acidic and nutrient-poor seepage swamps, occurring primarily in the Coastal Plain, are characterized by Sphagnum-covered hummocks in a sandy or peaty substrate. The habitats are generally wet and somewhat protected from fire, although adjacent upland forests are generally pyrophytic; there is little doubt that prior to widespread exclusion of wildfires, fire occasionally burned into these habitats during dry periods. Dominant overstory species are red maple (Acer rubrum) and blackgum (Nyssa sylvatica), with tulip-tree (Liriodendron tulipifera) and loblolly pine (Pinus taeda) also locally important. Common small trees and shrubs are sweetbay magnolia (Magnolia virginiana var. virginiana), sweet pepperbush (Clethra alnifolia), highbush blueberries (Vaccinium spp.), swamp azalea (Rhododendron viscosum var. viscosum), smooth winterberry (Ilex laevigata), and southern wild raisin (Viburnum nudum). Compact dodder (Cuscuta compacta) is often abundantly attached to the stems of shrubs in these swamps. Common herbaceous species include cinnamon fern (Osmundastrum cinnamomeum), netted chain fern (Lorinseria areolata), and the sedges Carex lonchocarpa and Carex seorsa. Less widespread herbaceous species that could be considered more or less diagnostic (especially on the Coastal Plain) include skunk cabbage (Symplocarpus foetidus), kidney-leaved grass-of-parnassus (Parnassia asarifolia), Collins's sedge (Carex collinsii), twining bartonia (Bartonia paniculata ssp. paniculata), and the federally listed swamp-pink (Helonias bullata).
Seepage swamps occupying the bottoms of Coastal Plain ravines that have downcut into Tertiary shell deposits or limesands have strongly calcareous soils and groundwater. These very rare, small-patch communities are known from the dissected inner Coastal Plain of Surry, Isle of Wight, York, James City, Gloucester, and Lancaster Counties. Hummock-and-hollow microtopography is prevalent, and exposed shells are common in springs and rills. Green ash (Fraxinus pennsylvanica), red maple (Acer rubrum), and tulip-tree (Liriodendron tulipifera) are common overstory trees in most occurrences, but a subset of ravines on the south side of the James River features the unusual co-dominance of bald cypress (Taxodium distichum) or swamp tupelo (Nyssa biflora). Small trees and shrubs include stiff dogwood (Cornus foemina ), spicebush (Lindera benzoin var. benzoin and var. pubescens), and wax myrtle (Morella cerifera). A number of noteworthy mountain disjuncts have been documented in the herbaceous flora of these communities, including marsh marigold (Caltha palustris var. palustris), rigid sedge (Carex tetanica), bog twayblade (Liparis loeselii; state-rare), swamp lousewort (Pedicularis lanceolata), and American false-hellebore (Veratrum viride). Reaching their northern limits are the southern species Florida adder's-mouth (Malaxis spicata), shadow witch orchid (Ponthieva racemosa), and drooping bulrush (Scirpus lineatus). Other characteristic herbs include lizard's-tail (Saururus cernuus), golden ragwort (Packera aurea), blackfruit clearweed (Pilea fontana), smooth bur-marigold (Bidens laevis), Carolina buttercup (Ranunculus carolinianus), common brome sedge (Carex bromoides spp. bromoides), and common wood reedgrass (Cinna arundinacea). The damp, fertile habitats are particularly susceptible to invasion by the introduced Japanese stiltgrass (Microstegium vimineum). The globally rare Tidewater interstitial amphipod (Stygobromus araeus) appears to be closely associated with groundwater in shell marl deposits.
Related seepage swamps have been documented in the Piedmont and inner Northern Coastal Plain of both Maryland and Virginia, and appear to represent a type that occurs north to southern New England. This type often occurs in edge-zones or abandoned oxbows of floodplains, where groundwater is discharged from the base of an adjoining slope. Lab analysis of soil samples have confirmed that these Piedmont/Northern Coastal Plain habitats are intermediate in soil chemistry between the extremely acidic and highly calcareous Coastal Plain swamps - i.e., strongly acidic but with moderately high calcium, magnesium, and total base saturation levels. In Virginia, the most characteristic species of this type appear to be red maple, green ash, white ash (Fraxinus americana), tulip-tree, spicebush, skunk cabbage (Symplocarpus foetidus), cinnamon fern (Osmundastrum cinnamomeum), orange jewelweed (Impatiens capensis), and clearweed (Pilea pumila).
Several uncommon odonates (dragonflies and damselflies) depend on forested seeps for breeding habitat. Coastal Plain / Piedmont seepage swamps are relatively small in size and threatened by beaver activities, agricultural pollutants, hydrologic disturbances, and logging. The ash component of these swamps is currently under major stress from outbreaks of the insect pathogen Emerald Ash Borer. The extensively tree mortality and gap disturbances associated with this pest will almost certainly lead to long-term compositional changes and increased susceptibility to invasive weeds.References: Fleming (2002a), Fleming (2007), McCoy and Fleming (2000), Rawinski (1995).
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© DCR-DNH, Gary P. Fleming.
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