Piedmont / Mountain Small-Stream Alluvial Forests
Forests in this group occupy temporarily flooded habitats along smaller-order streams, where distinct alluvial landforms (e.g., levees, terraces, backswamps) occur at very small scales. These communities are found along many small rivers and streams throughout the Piedmont and mountain-region valleys; forests occurring along relatively high-gradient small rivers and large streams in the mountains were formerly recognized as a distinct group ("montane alluvial forests") but have now been merged into a broader treatment. Soils, flooding regimes, and floristic composition are diverse. Habitats generally consist of narrow floodplains with fine to coarse alluvial soils; bouldery or cobbly alluvium and rocky streambeds are typical in and at the foot of the mountains.
Characteristic trees in the Piedmont and larger mountain valleys include sycamore (Platanus occidentalis), eastern boxelder (Acer negundo var. negundo), American elm (Ulmus americana), green ash (Fraxinus pennsylvanica), river birch (Betula nigra), red maple (Acer rubrum), yellow buckeye (Aesculus flava; southwestern Virginia only), black walnut (Juglans nigra), tulip-tree (Liriodendron tulipifera), and black willow (Salix nigra). Shrubs include spicebush (Lindera benzoin var. benzoin), black haw (Viburnum prunifolium), American hornbeam (Carpinus caroliniana ssp. virginiana), and American hazelnut (Corylus americana). Herbaceous composition varies greatly with site conditions and geography, but generally includes such characteristic species as mistflower (Conoclinium coelestinum), hollow joe-pye weed (Eutrochium fistulosum), thin-leaved sunflower (Helianthus decapetalus), cardinal flower (Lobelia cardinalis), great blue lobelia (Lobelia siphilitica var. siphilitica), American bugleweed (Lycopus americanus), Virginia bugleweed (Lycopus virginicus), fringed loosestrife (Lysimachia ciliata), sensitive fern (Onoclea sensibilis var. sensibilis), fall phlox (Phlox paniculata), hispid hedge-nettle (Stachys hispida), tall meadow-rue (Thalictrum pubescens), and wingstem (Verbesina alternifolia) .
Along montane floodplains, most of the lower, streamside terraces are probably flooded briefly at least annually. More powerful, catastrophic floods occur at irregular intervals and may be very destructive to stream channels and vegetation. The forests of these habitats are characterized by a mixture of bottomland and mesophytic species. Among the most common trees are sycamore (Platanus occidentalis), eastern hemlock (Tsuga canadensis), eastern white pine (Pinus strobus), white oak (Quercus alba), birches (Betula alleghaniensis and Betula lenta var. lenta), and tulip-tree (Liriodendron tulipifera). Other trees that may be locally important are American beech (Fagus grandifolia), yellow buckeye (Aesculus flava), red maple (Acer rubrum), American elm (Ulmus americana) and, in southwestern Virginia only, sweetgum (Liquidambar styraciflua), (Carpinus caroliniana ssp. virginiana), great rhododendron (Rhododendron maximum), smooth alder (Alnus serrulata), willows (Salix sericea and Salix nigra), and witch hazel (Hamamelis virginiana var. virginiana). Montane floodplain habitats are relatively well drained and support a great variety of mostly mesophytic herbaceous species such as hog-peanut (Amphicarpaea bracteata), common jack-in-the-pulpit (Arisaema triphyllum ssp. triphyllum), green-and-gold (Chrysogonum virginianum var. virginianum), and golden ragwort (Packera aurea = Senecio aureus). More frequently flooded and hydric microhabitats along the stream channels, however, usually support some wetland species, e.g., twisted sedge (Carex torta), nodding sedge (Carex gynandra), fowl mannagrass (Glyceria striata var. striata), hooked buttercup (Ranunculus recurvatus var. recurvatus), and marsh blue violet (Viola cucullata).Compared to Piedmont / Mountain Floodplain Forests, communities of this group occupy smaller, more topographically heterogeneous floodplains and contain a lower diversity of vegetation. They are poorly documented in Virginia and are mostly in degraded condition because of extensive past clearing, grazing, catastrophic flooding, and invasive introduced weeds.
References: Coulling (1999), Fleming (2002a), Fleming (2007), Fleming and Coulling (2001), Fleming and Moorhead (2000), Gemborys (1974), Hupp (1982), Hupp (1986), Olson and Hupp (1986), Rawinski et al . (1994), Rawinski et al . (1996).
Click on the images below to open a larger image in a separate window.
Lush graminoid vegetation in a floodplain forest along Long Branch, in the western Piedmont of Patrick County. Photo: Irvine Wilson / © DCR Natural Heritage.
Sycamores (Platanus occidentalis) characterize a narrow floodplain forest along the Cowpasture River south of Williamsville in Bath County (George Washington and Jefferson National Forests). Photo © Gary P. Fleming.
A montane alluvial forest along the headwaters of the Jackson River in Hidden Valley, Bath County (George Washington and Jefferson National Forests). The forest contains sycamore (Platanus occidentalis), birches (Betula spp.), and eastern hemlock (Tsuga canadensis), with an understory of American hornbeam (Carpinus caroliniana) and smooth alder (Alnus serrulata). Photo: Gary P. Fleming / © DCR Natural Heritage.