High-Energy Tidal River Shores
This ecological group encompasses small-patch herbaceous vegetation of narrow tidal flats, shores, bars, and channel edges in the low intertidal zone along major tidal rivers and their larger tributaries. Tidal fluctuations, as well as turbulence, flushing, and substrate scouring from waves, wind, and ice, are more pronounced in these habitats than in the adjacent marshes and appear to be stronger correlates of vegetation composition than does prevailing salinity regime. Substrates occupied by vegetation of this group are relatively coarse for the tidal class, ranging from mixtures of sand, silt, clay, and pebbles to coarse gravel and cobbles with low organic matter content. However, the soils of somewhat lower-energy channel edges may consist of mucky clay and very fine silt with higher organic matter. The habitats are generally exposed at low tide but remain inundated during spring tides.
Vegetation of high-energy tidal shores varies from sparse to dense, but tends to be low-statured (< 1 m tall, often much less). Unlike most marshes, these communities occur in linear patches, chiefly < 10 m wide, and most occur under both freshwater and oligohaline regimes. Composition appears to vary with substrate and other environmental dynamics. Species of bulrush, particularly common threesquare (Schoenoplectus pungens var. pungens) and soft-stem bulrush (Schoenoplectus tabernaemontani), are characteristic of most sites. Common threesquare dominates gravelly shores of the tidal Potomac River in northern Virginia, where ice scouring is most frequent. Vegetation of sandy habitats subject to greater periodic turbulence tends to be sparse and contain low annuals and perennials such as river-bank quillwort (Isoetes riparia), Parker's pipewort (Eriocaulon parkeri), dotted smartweed (Polygonum punctatum), strap-leaf arrowhead (Sagittaria subulata), grassleaf arrowhead (Sagittaria graminea var. graminea), longstem waterwort (Elatine americana), flatsedges (Cyperus spp.), marsh seedbox (Ludwigia palustris), and golden club (Orontium aquaticum). Common water-willow (Justicia americana) often forms dominance patches on clayey mudflats and bars. Vegetation of mucky channel edges is usually taller and contains Common threesquare, soft-stem bulrush, and squarestem spikerush (Eleocharis quadrangulata), along with pickerelweed (Pontederia cordata), arrow-arum (Peltandra virginica), wild rice (Zizania aquatica), waterhemp pigweed (Amaranthus cannabinus), Walter's barnyard grass (Echinochloa walteri), and other more typical marsh species.Reference: Coulling 2002, Walton et al. 2001.
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|Narrow tidal shore community dominated common threesquare (Schoenoplectus pungens var. pungens) and dotted smartweed (Polygonum punctatum), along the Potomac River at Dyke Marsh, Fairfax County (George Washington Memorial Parkway). Ice-scouring is an important environmental factor along the upper tidal Potomac , resulting in locally coarse, gravelly-cobbly shores. Photo: Kristin Taverna.|
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