Circumneutral riverbank outcrop (S1)
Circumneutral riverbank outcrops occur in open, flood-scoured bedrock exposures along medium-sized and large rivers, typically at river narrows. These settings are similar to those where acidic riverbank outcrops occur, but this community is less common and has substrates with a more neutral pH, presumably from higher calcium levels in certain bedrock types. In New Hampshire, this community is restricted to the Connecticut River valley, and it is rare elsewhere in New England.
Height above the river channel appears to influence species composition and plant size. Wetland species are often restricted to the lower 2–3 m above the channel. Emergent seepage is absent, although the community may occur in conjunction with seep communities. In contrast with seeps, sedges are often absent. This community differs from rocky ridge communities by the paucity of lichens and woody plants intolerant of flooding, and by the presence of flood-tolerant species. Exotics may be common. Plants may be stressed or killed during periods of drought. Sand, silt, and turf can accumulate in rock crevices and pockets.
Characteristic Vegetation: Plants present that are probably indicative of circumneutral or more enriched conditions may include harebell (Campanula rotundifolia), dwarf ragwort (Senecio pauperculus), Siberian chives (Allium schoenoprasum), rusty woodsia (Woodsia ilvensis), Christmas fern (Polystichum acrostichoides), and the globally rare Jesup’s milk vetch (Astragalus robbinsii var. jesupii). Species found on acidic riverbank outcrops may also be found in this community, including Aster spp. (asters), Solidago spp. (goldenrods), Viola spp. (violets), Schizachyrium scoparium (little bluestem), Andropogon gerardii (big bluestem), Onoclea sensibilis (sensitive fern), and scattered woody shrubs and tree seedlings. Non-native species may be common. In contrast with seeps, sedges are often absent.
Good examples of this community occur scattered along the middle-third of the Connecticut River (
Circumneutral riverbank outcrops can occur as part of moderate-gradient sandy-cobbly riverbank systems or high-gradient rocky riverbank systems.
Circumneutral riverbank outcrop at Sumners Falls along the CT River (photo by Dan Sperduto)
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