Riverside Outcrop Barrens
Exposed, xeric outcrops within the flood zone of major Piedmont and mountain-region rivers provide the habitats for communities in this group. The very few documented examples of this group in Virginia are located along the Potomac, Shenandoah, and James Rivers, especially in gorges. Occurrences are known from several bedrock types, including dolomite, calcareous shale, charnockite, and acidic schists and metagrawacke. Habitats are subject to occasional flood-scouring, as well as edaphic stresses, with flood return intervals ranging from about one to more than ten years.
Lichens and mosses are the dominant life-forms of these outcrops, with vascular plants limited to crevices, gravel-filled depressions, and humus-covered shelves. Shrubs and herbs that occur in riverside barrens of various bedrock types include eastern red cedar (Juniperus virginiana var. virginiana), common ninebark (Physocarpus opulifolius var. opulifolius), shrubby St. John's-wort (Hypericum prolificum), big bluestem (Andropogon gerardii), little bluestem (Schizachyrium scoparium var. scoparium), field chickweed (Cerastium velutinum var. velutinum), and riverbank goldenrod (Solidago rupestris). Species confined to basic or calcareous outcrops include hoptree (Ptelea trifoliata var. trifoliata), nodding onion (Allium cernuum), and plains muhly (Muhlenbergia cuspidata). Species confined to acidic schists and metagrawacke of the Potomac Gorge, west of Washington, D.C., include Nantucket serviceberry (Amelanchier nantucketensis), running serviceberry (Amelanchier spicata), fetterbush (Eubotrys racemosa), moss phlox (Phlox subulata), stiff-leaved aster (Ionactis linariifolia), balsam ragwort (Packera paupercula var. paupercula , = Senecio pauperculus), and sticky goldenrod (Solidago racemosa).
Riverside Outcrop Barrens often occur in patch-mosaics with (or as small inclusions within) Riverside Prairies, but are readily distinguished by their sparse vegetation (vs. dense tall-grass dominance in the prairies). Communities in this group are very rare and localized in Virginia and range-wide.Reference: Fleming (2007), Lea (2000).
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Sparsely vegetated metagrawacke and schist on bedrock terrace along the Potomac River below Great Falls. Sticky goldenrod (Solidago racemosa) is blooming in the foreground crevices. Gary P. Fleming / © DCR Natural Heritage.