Present Plant Communities on Plummers Island
by Elizabeth F. Wells


            Plummers Island’s varied topography supports three main types of plant communities: riparian, terrace, and upland forest. The riparian communities have relatively low topographic relief, occur at the lower elevations along the river, and are flooded frequently, from several times a year to once every 2-3 years. They are dominated by a rich flora of species typically associated with floodplains, such as sycamore (Platanus occidentalis), silver maple (Acer saccharinum), red maple (A. rubrum), boxelder (A. negundo), green ash (Fraxinus pennsylvanica), slippery elm (Ulmus rubra), black walnut (Juglans nigra), spicebush (Lindera benzoin), and many other species tolerant of flooding. Sycamore, green ash, boxelder, and slippery elm form a closed canopy throughout the island’s floodplain, and pawpaw (Asimina triloba) is conspicuous in the riparian subcanopy over much of the island. Virginia-creeper (Parthenocissus quinquefolia), summer grape (Vitis aestivalis), winter grape (V. vulpina), and poison-ivy (Toxicodendron radicans) are common vines throughout the island. A few non-native invasive species such as garlic-mustard (Alliaria petiolata) and Japanese honeysuckle (Lonicera japonica) are common throughout most of the floodplain; poison-hemlock (Conium maculatum) and oriental bittersweet (Celastrus orbiculatus) are conspicuous in some areas.


            The herbaceous vegetation of the riparian communities varies with habitat, soil depth, and flooding frequency. The floodplains on the lower end and at the head of the island are relatively broad and gently rolling. The deep sandy soil is redeposited by rapidly flowing water during floods, leaving bare sand which is quickly revegetated by opportunistic herbaceous plants following floods. The lush herbaceous layer here is composed of native ramp (Allium tricoccum), wild blue phlox (Phlox divaricata), wingstem (Verbesina alternifolia), and southern flatseed-sunflower (V. occidentalis) and non-native annual wormwood (Artemisia annua), Asiatic water-pepper (Polygonum cespitosum), ground-ivy (Glechoma hederacea), and stinging nettle (Urtica dioica).


            A rocky channel in the floodplain, known as Upper Gap, on the northwest side of the smaller knoll has shallow silty soil overlying tilted plates of bedrock and supports a highly diverse flora. Trees include hackberry (Celtis occidentalis), slippery elm, black willow (Salix nigra), cottonwood (Populus deltoides), river birch (Betula nigra), boxelder, pawpaw, and other floodplain species. Herbs include many native species such as asters (Aster spp.) and goldenrods (Solidago spp.), Maryland figwort (Scrophularia marilandica), and American germander (Teucrium canadense), and non-native Maack’s honeysuckle (Lonicera maackii) and moneywort (Lysimachia nummularia), as well as many of the species common in the deep sandy floodplain on the island. Lizard’s-tail (Saururus cernuus) thrives in a shallow pool between rock outcrops. A small pond has developed nearby in a low area on the north side of the island.


            The river and lagoon banks of the island in most areas are steep and sandy. The bank on the south side receives full sun during the afternoon and bears an extremely dense assemblage of tall herbs such as tall coneflower (Rudbeckia laciniata), giant ragweed (Ambrosia trifida), wingstem, dame’s-rocket (Hesperis matronalis), and many others. The banks on the north side are deeply shaded and sparsely vegetated as compared to the south-facing bank, bearing species typical of the sandy floodplain. The species on the north bank include annual wormwood, Asiatic water-pepper, stinging nettle, wingstem, and southern flatseed-sunflower.


            Sandy and silty bars in a few locations around the shore of the island remain submerged until the river level falls in late summer and exposes them, at least during dry years. These bars support a rich but short-lived assemblage of herbaceous and woody species. The herbaceous species are extremely diverse and include tooth-cup (Ammannia coccinea), Koehne’s ammania (A. latifolia), spiny amaranth (Amaranthus spinosus), bur-cucumber (Sicyos angulatus), ditch stonecrop (Penthorum sedoides), carpetweed (Mollugo verticillata), false pimpernel (Lindernia dubia), winged monkey-flower (Mimulus alatus), water paspalum (Paspalum fluitans), and others, in addition to many found on the floodplain of the island.


            The woody species on these bars are all first-year seedlings of trees such as river birch, southern catalpa (Catalpa bignonioides), cottonwood, green ash, empress-tree (Paulownia tomentosa), sycamore, silver maple, and boxelder that die when the bar is flooded during the following winter. A small marsh persists near the south shore of the island in one location, where swamp milkweed (Asclepias incarnata), cardinal flower (Lobelia cardinalis), golden-club (Orontium aquaticum), and common arrowhead (Sagittaria latifolia) are found in some years. Rock outcrops that jut into the river at the west end provide habitat for rose-mallow (Hibiscus moscheutos) and buttonbush (Cephalanthus occidentalis) as well as sycamore and willow.


            Terrace communities occur at the higher elevations along the river and are flooded less frequently, having a flood return interval greater than every 3 years, in most cases as great as 25-30 years. Their soils are deep, sandy, mature with a developed soil profile, and rarely redeposited by flooding, in contrast with immature riparian soils which typically have little time for soil profile development between fast-moving floods. Their topographic relief varies from almost level to gently rolling. They are dominated by species typically associated with moist upland sites, such as sugar maple (Acer saccharum) in the canopy, bladdernut (Staphylea trifolia) in the tall shrub layer, and wild ginger (Asarum canadense), ostrich fern (Matteuccia struthiopteris), and Christmas fern (Polystichum acrostichoides) in the herb layer.


            The vegetation of terraces also contains many species common to floodplains, such as red maple, tulip-tree (Liriodendron tulipifera), hackberry, pawpaw, and a few upland species, such as fringe-tree (Chionanthus virginicus) and northern red oak (Quercus rubra). One sloping terrace supports leatherwood (Dirca palustris), basswood (Tilia americana), lowland brittle fern (Cystopteris protrusa), blue cohosh (Caulophyllum thalictroides), black cohosh (Cimicifuga racemosa), and Virginia waterleaf (Hydrophyllum virginianum). On the island, terraces offer the best display of showy spring ephemerals, such as Virginia bluebells (Mertensia virginica), Dutchman’s-breeches (Dicentra cucullaria), squirrel-corn (D. canadensis), sessile trillium (Trillium sessile), and creamy violet (Viola striata).


            Upland forest communities occur on the two high knolls and among the large boulders and rock outcrops of the island well above the river, and they never flood. Their soils are relatively thin, clay-rich, with a developed soil profile, and are derived from the weathered granitic gneiss bedrock. Their topography ranges from rolling vegetated slopes to very steep, barren, craggy cliffs and rock outcrops. The cliffs on the north side of the higher knoll, known as Cabin Hill, are almost vertical, and their crevices and north-facing ledges provide habitat for basswood, sugar maple, marginal shield fern (Dryopteris marginalis), Christmas fern, and rock polypody (Polypodium virginianum).

  

            Trees such as chestnut oak (Quercus montana), northern red oak, white oak (Q. alba), pignut hickory (Carya glabra), eastern red cedar (Juniperus virginiana), hop hornbeam (Ostrya virginiana), slippery elm, American elm (U. americana), and red maple form a somewhat open canopy over both knolls. Shrubs include deerberry (Vaccinium stamineum), lowbush blueberry (V. pallidum), and others. The herb layer provides dense cover under the open canopy, with species such as white wood aster (Aster divaricatus), rattlesnake-weed (Hieracium venosum), blue-stem goldenrod (Solidago caesia), black-edged sedge (Carex nigromarginata), variable panic grass (Dichanthelium commutatum), and others, including many grasses and sedges. Vines such as Virginia-creeper, summer grape, winter grape, and poison ivy completely cover some of the cliffs and large boulders, especially where they receive full afternoon sun on the south side and on the high, massive rock (“Rock of Gibraltar”) at the upper end of the island. Tree-of-heaven (Ailanthus altissima) is established on the smaller knoll, and several trees have reached the canopy.




23 February 2004