Acidic riverbank outcrop (S3)
Acidic riverbank outcrops are open, flood-scoured bedrock exposures along medium-sized and large-sized rivers, typically along river narrows (where ice and water scour are intense enough to expose considerable amounts of bedrock). Vegetation is usually extremely sparse, but may include a variety of forbs and grasses such as pointed auricle path rush, Allen's poverty oatgrass, hawkweeds, and goldenrods, as well as several ferns such as northern lady fern. Lichens and woody plants are notably absent. Height above the river channel appears to influence species composition and plant size. Flood-tolerant forbs and grasses are the dominant life forms. 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.
Emergent seepage is absent, as are the species characteristic of seeps, although the community may occur in conjunction with seep communities. 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.
Characteristic vegetation: Vegetation is usually extremely sparse, but characterized by a diverse array of forbs and grasses. Mosses may be abundant and help stabilize the soil. Characteristic vascular species include path rush (Juncus tenuis), hawkweeds (Hieracium spp.), northern lady fern (Athyrium filix-femina), little bluestem (Schizachyrium scoparium), big bluestem (Andropogon gerardii), upland bent-grass (Agrostis perennans), blue-green hairgrass (Deschampsia caespitosa), New York aster (Aster novi-belgii), grass-leaved goldenrod (Euthamia graminifolia), and a variety of other composites and graminoids. Twisted sedge (Carex torta) and seedling-sized willows (Salix spp.), mountain ashes (Sorbus spp.), and speckled alder (Alnus incana) may survive here, but remain scattered.
Northern New Hampshire examples are likely to have tufted oat-grass (Danthonia compressa), dwarf bilberry (Vaccinium cespitosum), three-toothed cinquefoil (Sibbaldiopsis tridentata), spiked false oats (Trisetum spicatum), and less frequently, seedling-sized balsam poplar (Populus balsamifera) and northern white cedar (Thuja occidentalis). Dwarf bilberry and three-toothed cinquefoil are otherwise restricted to alpine areas and high-elevation outcrops. The rare Robinson’s hawkweed (Hieracium robinsonii) may occur in this community (known only from one site in New Hampshire).
Good examples of this community occur along the
Acidic riverbank outcrop at Garvins Falls (photo by Ben Kimball)
Acidic riverbank outcrop along the Merrimack River (photo by Ben Kimball)
Acidic riverbank outcrop at Livermore Falls along the Pemigewasset River (photo by Dan Sperduto)
Acidic riverbank outcrop at Hellgate along the Dead Diamond River (photo by Dan Sperduto)