Sand / Gravel / Mud Bars and Shores
The intermittently exposed shores and depositional bars of rivers and large streams support specialized herbaceous vegetation. Communities in this group are small, localized in distribution, and distinctive both environmentally and floristically. They are widely scattered throughout Virginia, but are most frequent and best developed along large rivers. Habitats are typically submerged during the winter and spring, but draw down during the summer and may be extensively exposed during the late summers and falls of dry years. A small subset of these communities is associated with diurnally exposed, dense clay mud flats on tidal freshwater river shores. Substrates consist of fine-textured to moderately coarse alluvium. The vegetation is variable but generally dominated by herbaceous species that are specially adapted to cycles of flooding and exposure. Seed banks or perennial rootstocks of these species are capable of maintaining their viability during long periods of submersion, quickly germinating or sprouting when favorable draw-down conditions occur. Flood-battered shrubs and tree saplings occur sparsely in these habitats.
Species of relatively wide distribution on sandy or gravelly draw-down shores include creeping dayflower (Commelina diffusa), red-root flatsedge (Cyperus erythrorhizos), creeping lovegrass (Eragrostis hypnoides), small-flower halfchaff sedge (Lipocarpha micrantha), common water-willow (Justicia americana), low spikesedge (Kyllinga pumila), false pimpernels (Lindernia dubia var. dubia and var. anagallidea), seedboxes (Ludwigia spp.), spreading panic grass (Panicum dichotomiflorum var. dichotomiflorum), water paspalum (Paspalum fluitans), marsh yellow cress (Rorippa palustris ssp. palustris ), and toothcup (Rotala ramosior). Species characteristic of Piedmont and mountain draw-down shores include scarlet ammannia (Ammannia coccinea), nodding beggar-ticks (Bidens cernua), three-lobe beggar-ticks (Bidens comosa), spreading sandmat (Euphorbia humistrata), awned flatsedge (Cyperus squarrosus), dock-leaf smartweed (Persicaria lapathifolia), and Carolina leaf-flower (Phyllanthus caroliniensis ssp. caroliniensis). Species most typical of Coastal Plain draw-down shores include white-edge flatsedge (Cyperus flavicomus), coastal flatsedge (Cyperus polystachyos), shade mudflower (Micranthemum umbrosum), lax hornpod (Mitreola petiolata), American halfchaff sedge (Lipocarpha maculata), Bosc's bluets (Oldenlandia boscii), warty panic grass (Panicum verrucosum), and coastal rose-pink (Sabatia calycina).
References: DeBerry and Perry (2005), Fleming (2007), Lea (2000), Ludwig (1996).
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Exposed herbaceous sand bars along the Blackwater River in Isle of Wight County. Photo: © Irvine Wilson. Low herbaceous sand/gravel bar along the Potomac River in Fairfax County (Riverbend Park). Dominants here are creeping lovegrass (Eragrostis hypnoides), awned flatsedge (Cyperus squarrosus), and false pimpernel (Lindernia dubia var. dubia).Photo: Gary P. Fleming / © DCR Natural Heritage.
Sparse graminoid vegetation on gravel bars along Back Creek near Blowing Springs, Bath County (George Washington and Jefferson National Forests). Dominants include spreading panic grass (Panicum dichotomiflorum var. dichotomiflorum, in foreground) and redtop panic grass (Coleataenia stipitata, large clumps in background). Photo: © Gary P. Fleming.