Montane Depression Wetlands
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). Physiognomy varies from a patchwork of buttonbush thickets and herbaceous openings to wholly herbaceous. Other characteristic species, varying from site to site, include broadleaf arrowhead (Sagittaria latifolia var. latifolia), northeastern bulrush (Scirpus ancistrochaetus), inflated sedge (Carex vesicaria), bladderworts (Utricularia spp.), common mermaid-weed (Proserpinaca palustris), mild water pepper (Polygonum hydropiperoides var. hydropiperoides), pale mannagrass (Torreyochloa 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), swamp beggar-ticks (Bidens discoidea), silvery sedge (Carex canescens ssp. disjuncta), hop sedge (Carex lupulina), tussock sedge (Carex stricta), sharp-scaled mannagrass (Glyceria acutiflora), eastern mannagrass (Glyceria septentrionalis), slender 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 (Woodwardia virginica) and cinnamon fern (Osmunda cinnamomea var. cinnamomea) 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 their requisite habitats 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 (Panicum verrucosum), tall flat panic grass (Panicum rigidulum var. rigidulum), and least 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, = Scirpus 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 state 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, = Scirpus torreyi). In addition to Barratt's sedge, notable Coastal Plain disjuncts include dwarf burhead (Echinodorus tenellus), 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 on the images below to open a larger image in a separate window.
|Three-way sedge (Dulichium arundinaceum) and buttonbush (Cephalanthus occidentalis) in Browns Pond, on Tower Hill Mountain, Bath County (George Washington and Jefferson National Forests). Photo: Gary P. Fleming / © DCR Natural Heritage.|
|Green Pond, a 0.4 ha (one acre) ridge-top pond on the northern Blue Ridge at Big Levels, Augusta County (George Washington and Jefferson National Forests). The dominant graminoid is water sedge (Carex aquatilis), a northern species known in Virginia only from this site.Photo: Tom Rawinski / © DCR Natural Heritage.|
|Horseshoe Swamp, an unusual Shenandoah Valley Sinkhole Pond containing an herbaceous community dominated by the Coastal Plain disjunct Barratt's sedge (Carex barrattii). Near Maple Flats, Augusta County (George Washington and Jefferson National Forests).Photo: Gary P. Fleming © DCR Natural Heritage.|
|Autumnal view of Mount Joy Pond, a Shenandoah Valley Sinkhole Pond near Steele's Tavern, Augusta County (Mount Joy Pond Natural Area Preserve). Photo: Irvine Wilson / © DCR Natural Heritage.|
|Exsiccated bed of Oak Pond at Maple Flats, Augusta County (George Washington and Jefferson National Forests). Low herbaceous species covering the pond bed include warty panic grass (Panicum verrucosum), northern St. John's-wort (Hypericum boreale), and least spikerush (Eleocharis acicularis). Photo: Gary P. Fleming / © DCR Natural Heritage.|
|Spring Pond, an unusual Shenandoah Valley sinkhole pond that is permanently flooded with cold seepage inputs. Golden-club (Orontium aquaticum) dominates much of the pond, while Virginia chain fern (Woodwardia virginica) colonizes the edge.Photo: Tom Rawinski / © DCR Natural Heritage.|
|Spatterdock (Nuphar advena) is the only vascular plant in the deeply flooded central zone of a Shenandoah Valley sinkhole pond near Harriston, Augusta County. Photo: Gary P. Fleming / © DCR Natural Heritage.|
|Forested outer zone of a Shenandoah Valley sinkhole pond near Harriston, Augusta County. Barratt's sedge (Carex barratii ) forms large swards under an overstory of red maple (Acer rubrum), black gum (Nyssa sylvatica), and pin oak (Quercus palustris). Photo: Irvine Wilson / © DCR Natural Heritage.|
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