Montane Depression Swamps and Ponds
This group includes saturated, seasonally flooded, and semipermanently flooded vegetation of basin wetlands situated on broad ridge crests, landslide benches and mountain-foot alluvial fans of the Ridge and Valley and Blue Ridge provinces. Similar communities are known from the Ridge and Valley portions of West Virginia and Pennsylvania. These rare natural wetlands range in patch-size up to about 1.5 ha (3.7 ac) and are thought to have formed from the sagging or solution of underlying bedrock strata or landslide masses. Hydrologic regime is variable from one depression to another, and many sites exhibit pronounced seasonal water-level fluctuations, as well as strong zonation in flooding depth from center to edge.
The most widely distributed community type over the Virginia range of these wetlands is a seasonally or semipermanently flooded pond variably dominated by buttonbush (Cephalanthus occidentalis) and three-way sedge (Dulichium arundinaceum var. arundinaceum). Physiognomy varies from a patchwork of buttonbush thickets and herbaceous openings to almost entirely herbaceous. Other characteristic species, varying from site to site, include broad-leaved arrowhead (Sagittaria latifolia), northeastern bulrush (Scirpus ancistrochaetus), inflated sedge (Carex vesicaria), bladderworts (Utricularia spp.), common mermaid-weed (Proserpinaca palustris), mild water pepper (Persicaria hydropiperoides), pale mannagrass(Torreyochloa pallida var. pallida), creeping spikerush (Eleocharis palustris, = Eleocharis smallii), and Canada mannagrass (Glyceria canadensis). Additional species more typical of ponds or zones with shallower flooding and/or shorter hydroperiods include winterberry (Ilex verticillata), highbush blueberry (Vaccinium corymbosum), common greenbrier (Smilax rotundifolia), small beggar-ticks (Bidens discoidea), silvery sedge (Carex canescens var. disjuncta), hop sedge (Carex lupulina), tussock sedge (Carex stricta), sharp-scaled mannagrass (Glyceria acutiflora), eastern mannagrass (Glyceria septentrionalis), dwarf St. John's-wort (Hypericum mutilum), and rushes (Juncus spp.).
Forested depression wetlands frequently occupy the outermost zones of basins supporting more open ponds, and also occur in isolated patches at both low and high elevations. Dominant trees are typically red maple (Acer rubrum) and blackgum (Nyssa sylvatica), while ericads (especially highbush blueberries [Vaccinium corymbosum and Vaccinium fuscatum]) tend to be prevalent in the understory. The herbaceous flora of these forests is typically depauperate, although Virginia chain fern (Anchistea virginica) and cinnamon fern (Osmundastrum cinnamomeum) often form substantial dominance patches.
Intermittently to permanently flooded basin wetlands that occur on broad, acidic alluvial fan deposits along the western foot of the Blue Ridge are colloquially known as Shenandoah Valley Sinkhole Ponds. These wetlands and the geomorphic conditions that have produced them are strictly endemic to a narrow zone that stretches through eastern Augusta, Rockingham, and Page Counties in the central Shenandoah Valley. Here, local solution of deep underlying carbonate rocks and reworking of surficial material by streams have resulted in the development of numerous natural ponds varying in size from less than 0.04 ha (0.1 ac) to over 1.5 ha (3.7 ac). The extraordinary combination of solution features overlain by acidic colluvium and alluvium from metasedimentary rocks of the Blue Ridge has created wetlands with edaphic conditions similar to habitats in the Coastal Plain. Pollen profiles from bottom sediments from two Augusta County ponds demonstrate the continuous existence of wetlands over the past 15,000 years (Craig 1969). Most ponds in the Shenandoah Valley complex experience seasonally fluctuating water levels. Clay weathered from metasiltstone and phyllite alluvium impedes drainage, but most ponds draw down as the growing season progresses and evapotranspiration increases. The hydroperiods of many ponds are irregular and unpredictable, varying with size and depth of basin, degree of shading, and local groundwater conditions. Soils vary from organic to clay-rich. Low pH, calcium, and magnesium levels combined with high aluminum levels may impair the assimilation of macronutrients by plants.
Three rare, and apparently endemic community types occur in the Shenandoah Valley ponds. By far the most prevalent is a seasonally flooded vegetation type characterized by scattered pin oak (Quercus palustris) and herbaceous species such as warty panic grass (Kellochloa verrucosa), tall flat panic grass (Coleataenia rigidula ssp. rigidula), and needle spikerush (Eleocharis acicularis), which are well adapted to a regime of seasonal flooding and draw-down on mineral soils. More unusual is Spring Pond, a cold, permanently flooded pond with water levels constantly replenished by groundwater inputs. Dominants here are golden-club (Orontium aquaticum), seven-angled pipewort (Eriocaulon aquaticum), and water bulrush (Schoenoplectus subterminalis). At Horseshoe Swamp, a one ha (2.5 ac) seasonally flooded pond is overwhelmingly dominated by the Coastal Plain disjunct Barratt's sedge (Carex barrattii).
The flora of Shenandoah Valley sinkhole ponds is noteworthy for its high percentage of rarities and disjuncts with various biogeographic affinities. Virginia sneezeweed (Helenium virginicum) is endemic to these habitats and similar ponds in Missouri, while Virginia quillwort (Isoetes virginica) is a two-state (Virginia-North Carolina) endemic also found in the Piedmont. Northern plants isolated here include toothed flatsedge (Cyperus dentatus), slender sedge (Carex lasiocarpa var. americana), northern St. John's-wort (Hypericum boreale), and Torrey's bulrush (Schoenoplectus torreyi). In addition to Barratt's sedge, notable Coastal Plain disjuncts include dwarf burhead (Helanthium tenellum), black-fruited spikerush (Eleocharis melanocarpa), Robbins' spikerush (Eleocharis robbinsii), and maidencane (Panicum hemitomon). The fauna of these wetlands is also remarkable, with disjunct populations of the state-listed tiger salamander (Ambystoma tigrinum) and a very diverse assemblage of odonates (dragonflies and damselflies) containing a number of state-rare and/or disjunct species.
Montane Depression Wetlands are important breeding habitats for amphibians and odonates (dragonflies and damselflies). Although some ponds are located on U.S. Forest Service land, many remain unprotected and threatened by development, hydrologic alterations, off-road vehicles, and trash dumping. Beavers, which have severely damaged the natural communities at Spring Pond in recent years, pose an additional threat to these wetlands.
References: Craig (1969), Fleming and Coulling (2001), Fleming and Moorhead (2000), Fleming and Van Alstine (1999), Rawlinson and Carr (1937).Click here for more photos of this ecological community group.
© DCR-DNH, Gary P. Fleming.