Interdune Swales and Ponds
Swales, depressions, and very low flats in maritime dunes support a variety of saturated to seasonally flooded herbaceous and shrub-dominated wetlands . Communities in this group are highly variable in composition and difficult to classify because of seasonal change in the flora, succession of stands over time, temporal hydrological variation, and rapid geomorphic changes in dynamic dune systems. They are known to range from New Jersey to North Carolina, and some may extend further south. In Virginia, these wetlands are confined to zones behind barrier beaches from the Eastern Shore (Accomack and Northampton Counties) to Cape Henry and False Cape (City of Virginia Beach). The vegetation of interdune swales and ponds tends to sort out along geographic, physiographic, and hydrological gradients.Swales and low dune hollows that have perched water tables and shallow, seasonal or temporary flooding support maritime wet grasslands or shrublands. The swales are predominantly influenced by fresh water from rainstorms, but some may be periodically flooded by salt water from ocean storm surges. A thin, organically enriched, surficial soil layer often contributes to moisture retention. Hydrologic regime and distance from salt spray appear to exert considerable influence on floristic composition. Typically, occurrences are densely vegetated by one or more species of grasses (e.g., saltmeadow cordgrass [Spartina patens]); rushes (e.g., Juncus scirpoides, Juncus dichotomus, Juncus acuminatus, Juncus megacephalus, or Juncus canadensis); or sedges (e.g., Cyperus odoratus var. odoratus, Fimbristylis caroliniana, or Schoenoplectus pungens var. pungens). In smaller, temporarily flooded swales where rushes are dominant, slender flat-top goldenrod (Euthamia caroliniana), long-leaved aster (Symphyotrichum novi-belgii, = Aster novi-belgii), Richard's yellow-eyed grass (Xyris jupicai) and slender bladderwort (Utricularia subulata) may also be characteristic. Swales further inland contain additional assemblages of species, including dwarf umbrella-sedge (Fuirena pumila), ladies'-tresses (Spiranthes spp.), water sundew (Drosera intermedia), southern bog clubmoss (Lycopodiella appressa), white-top fleabane (Erigeron vernus), whorled nutrush (Scleria verticillata), and narrow-leaf whitetop sedge (Rhynchospora colorata) may also be characteristic. Successional invasion of these herbaceous assemblages by shrubs increases with distance from salt spray and stability of the habitat. Wax myrtle (Morella cerifera) is the most characteristic interdune shrubland species, with high-tide bush (Baccharis halimifolia) frequently associated and northern bayberry (Morella pensylvanica), red bay (Persea palustris), and yaupon (Ilex vomitoria) present at some sites.
Interdune ponds are the most permanently and deeply flooded interdune wetlands, encompassing both freshwater ponds, in which rainwater and groundwater quickly dilutes infrequent salt-water inputs, and slightly brackish ponds subject to more frequent salt water inputs. The latter, which appear to have salinity regimes that vary over time from entirely fresh to slightly mesohaline, are probably best characterized as oligohaline ponds. Community composition varies with geography, topographic position, exposure to storm surges and salt spray, hydroperiod, and soil properties. Seasonally flooded, freshwater ponds usually contain large cover of bulrushes (e.g., Scirpus cyperinus, Schoenoplectus pungens var. pungens, Schoenoplectus tabernaemontani), grasses (e.g., Panicum virgatum var. virgatum, Coleataenia rigidula ssp. condensa, Spartina patens), or square-stem spikerush (Eleocharis quadrangulata). The marginal zones of some freshwater ponds maybe dominated by nearly pure stands of twig-rush (Cladium mariscoides). Seasonally flooded oligohaline ponds may be dominated by narrow-leaf cattail (Typha angustifolia), swamp rose-mallow (Hibiscus moscheutos), or saltmarsh bulrush (Bolboschoenus robustus, = Scirpus robustus), compositionally resembling Tidal Oligohaline Marshes. Semipermanently flooded oligohaline ponds are dominated by coastal water-hyssop (Bacopa monnieri), white spikerush (Eleocharis albidum), and sago pondweed (Stuckenia pectinata, = Potamogeton pectinatus).
All types within the group are uncommon to rare, small-patch communities existing in fragile settings. They also support several state rare insects including a tiger beetle (Cicindella trifasciata ascendens) and a dragonfly (Anax longipes). In common with most maritime communities, threats to this group include development and sea-level rise.Reference: The Nature Conservancy (1997).
Click on the images below to open a larger image in a separate window.A maritime grassland occupying a large back-dune swale near the southern end of Assateague Island, Accomack County (Chincoteague National Wildlife Refuge). Characteristic plants here are saltmeadow cordgrass (Spartina patens , dominant), common threesquare (Schoenoplectus pungens var. pungens), and Carolina fimbry (Fimbristylis caroliniana). Photo: Gary P. Fleming / © DCR Natural Heritage.
Dense saltmeadow cordgrass (Spartina patens) in a saturated swale behind frontal dunes at the southern tip of the Eastern Shore in Northampton County (Fisherman's Island National Wildlife Refuge).Photo: Irvine Wilson / © DCR Natural Heritage.
Switchgrass (Panicum virgatum var. virgatum) and giant plumegrass (Saccharum giganteum) in a freshwater pond among eroded back dunes near Ragged Point on Assateague Island, Accomack County (Chincoteague National Wildlife Refuge). Photo: Gary P. Fleming / © DCR Natural Heritage. Common spatterdock (Nuphar advena) and swamp loosestrife (Decodon verticillatus) in an interdune pond First Landing / Seashore State Park, City of Virginia Beach. In some years, the exposed, peaty shores of this pond are carpeted with bog-loving species such as water sundews (Drosera intermedia). Photo: Gary P. Fleming / © DCR Natural Heritage. A large, oligohaline dune pond behind the frontal dunes near Swan Cove on Assateague Island, Accomack County (Chincoteague National Wildlife Refuge). The vegetation here is depauperate, with huge mats of coastal water-hyssop (Bacopa monnieri) dominating and only three additional species occurring in small numbers. Photo: Gary P. Fleming / © DCR Natural Heritage.