Physiognomy and Structure: These are most typically closed-canopy flooded, riverine, and bottomland forests (and more rarely woodlands) dominated by a combination of tall broad-leaved deciduous trees and a needle-leaved deciduous tree (Taxodium). The understory is also quite variable. Shrubs and herbaceous plants can be virtually absent to dense. Vines are common.

Floristics: The dominant species in stands of floodplain forest primarily vary with hydroperiod, with species sorting on a rough gradient from wet to mesic. Bald-cypress - tupelo forests (G033) are primarily dominated by Taxodium distichum (occasionally Taxodium ascendens) with Nyssa aquatica (occasionally Nyssa biflora). Oak - sweetgum forests (G034) are dominated by Liquidambar styraciflua, Quercus spp., and Ulmus spp. There is also some floristic variation based on hydrology; shorter- hydroperiod examples will contain Quercus michauxii, Quercus pagoda, Quercus shumardii, and Quercus texana and longer-hydroperiod ones will have Nyssa biflora, Quercus laurifolia, Quercus lyrata, and Quercus phellos. Other dominant species include Acer negundo, Celtis laevigata, Fraxinus pennsylvanica, and Platanus occidentalis. Ash - elm - willow forests (G759) are typically dominated by Fraxinus pennsylvanica, Populus deltoides, Salix caroliniana, Salix nigra, and Ulmus americana.

Some other trees which may be associated with stands of this group include Acer rubrum var. drummondii, Carya aquatica, Fraxinus pennsylvanica, Gleditsia aquatica, Salix nigra, and other trees tolerant of flooding. Some inclusions are dominated by small trees or large shrubs, including Carpinus caroliniana, Cephalanthus occidentalis, Cornus foemina, Decodon verticillatus, Forestiera acuminata, Ilex decidua, Ilex opaca var. opaca, Itea virginica, Planera aquatica, and Viburnum dentatum. Except in the very wet examples, understory, shrub and herb layers are generally well-developed and woody vines are also prominent. Vines such as Nekemias arborea (= Ampelopsis arborea), Berchemia scandens, Campsis radicans, Parthenocissus quinquefolia, Smilax bona-nox, Toxicodendron radicans, and Vitis spp., may be conspicuous. The perennial graminoid bamboo Arundinaria gigantea (= ssp. gigantea) is a common understory component in these forests on natural levees and higher point bars, and may become dominant after thinning or removal of the overstory. Some other examples have very diverse herbaceous layers, including Bidens aristosa, Carex cherokeensis, Carex debilis, Carex digitalis, Carex joorii, Chasmanthium latifolium, Geum canadense, Glyceria striata, Leersia virginica, and Polygonum hydropiperoides. Terrestrial herbs in wetter examples are usually limited due to long hydroperiods, and are often only found on elevated logs and stumps, but can include Boehmeria cylindrica, Carex decomposita, Carex grayi, Carex intumescens, Carex joorii, Carex lupulina, Commelina virginica, Leersia lenticularis, Onoclea sensibilis, and Saururus cernuus. Aquatic and floating herbs include Ceratophyllum spp., Elodea spp., Lemna minor, Ludwigia peploides, Potamogeton spp., and Sagittaria lancifolia.

Floodplain forests, at least in the Southern Appalachian region, are much more prone to invasion from exotics than are uplands, with average cover of 22% (Brown and Peet 2003). Matthews et al. (2011) note the following exotics are common in Piedmont floodplain forests: shrubs Ligustrum sinense, Lonicera japonica; and herbs Glechoma hederacea, Microstegium vimineum, Murdannia keisak. Additional information is needed on the typical exotics found in this macrogroup. See ~Southeastern North American Ruderal Flooded & Swamp Forest Macrogroup (M310)$$ for a summary of exotic species that may also occur in this type.

Dynamics:  Flooding is an important ecological factor in examples of this macrogroup, and the length of the flooding may be the most important factor distinguishing this vegetation. In addition to disturbance, floods bring nutrient input, deposit sediment, exclude non-flood-tolerant species and disperse plant seeds. Flooding is most common in the winter but may occur in other seasons. The small flows, low gradient, and binding of sediment by vegetation limit channel shifts and sediment movement, but floods may cause local disturbance by scouring. Flood waters may have significant energy in higher-gradient systems, but scouring and reworking of sediment rarely affect more than small patches. Most of these forests would exist naturally as multi-aged old-growth forests driven by gap-phase regeneration.

Most of these forests exist naturally as multi-aged old-growth forests driven by gap-phase regeneration. Windthrow is probably the most important cause of gaps, as wind disturbance is perhaps more important than in uplands because of frequently wet soils, less dense soils, and more shallow-rooted trees.

Fire does not appear to be a dominant factor in this floodplain vegetation, and it is not very flammable. Fire is probably more important in small stream examples than in larger river ones, because distances to uplands are short and stream channels and sloughs are smaller and less effective as firebreaks. However, most of the vegetation is not very flammable and usually will not carry fire. However, historical references to canebrakes dominated by Arundinaria gigantea suggest that at least in some portions of stands, fire may have once been more of a factor. Examples often have limited herbaceous layers, making fire less important, even during extended dry periods.

Some tidal examples are included in ~Bald-cypress - Tupelo Floodplain Forest Group (G033)$$; these are affected by diurnal tidal flooding or by storm tides. In these cases, infrequent intrusion of saltier water, which is stressful or fatal to many of the plant species, is an important periodic disturbance created by storms. Natural fire is not frequent in these tidal examples, but may sometimes be important in determining the boundary between tidal swamps and tidal marshes. The tidal forest examples generally appear to be in a shifting relationship with adjacent tidal freshwater marshes. Many of these tidal marshes have standing dead trees in them, suggesting they recently were swamps. But some marshes are being invaded with trees and may be turning into swamps. Rising sea level is causing more inland swamps to develop tidal characteristics and causing some stands to turn into marshes. In areas not too strongly affected by salt intrusion, drowning by rising sea level, or fire, the communities can be expected to exist as old-growth, multi-aged forests.

Southern ash - elm - willow floodplain forests (G759) are subject to frequent disturbances, including flooding, erosion, and deposition of sediment. This leads to a physiognomically diverse group. Stands that have recently been subjected to severe flooding may be relatively open. Most stands have moderate to closed tree canopies, though.

Matthews et al. (2011), in their summary of historic changes to Piedmont floodplain forests, capture much of what has occurred across the range of this type; namely, that these forests occur in a highly fragmented landscape, where the natural hydrologic regime has been altered by anthropogenic activities for hundreds of years, where a long history of selective tree harvesting has reduced the abundance of certain species; and where non-native invasive species are increasingly common. Finally, they note that, in the Piedmont, extensive sediment deposition occurred following European agriculture on the uplands during the period 1700-1940.