Physiognomy and Structure: These are flooded riverine forests composed of tall trees, mostly broad-leaved deciduous trees and one needle-leaved deciduous tree (Taxodium).

Floristics: Stands of this group are primarily tall-statured forests dominated by Taxodium distichum (and occasionally Taxodium ascendens) and/or Nyssa aquatica (and occasionally Nyssa biflora). The ranges of Taxodium distichum and Nyssa aquatica are not entirely congruent, with Nyssa aquatica being more limited. Therefore, in some parts of the range of the group, one of these characteristic species may not be present. 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. Associations dominated by Quercus spp. are included in a different group. Also included here are some associations dominated by smaller trees, including Forestiera acuminata, Itea virginica, and Planera aquatica, as well as Cephalanthus occidentalis, which can range in lifeform from small tree to shrub. Cornus foemina and Decodon verticillatus may also be present. Terrestrial herbs 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, and Saururus cernuus. Aquatic and floating herbs include Ceratophyllum spp., Elodea spp., Lemna minor, Ludwigia peploides, Potamogeton spp., and Sagittaria lancifolia.

Dynamics:  Flooding is an important ecological factor in examples of this group, and the length of the flooding may be the most important factor separating this vegetation from that of other wetland groups. 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. 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, and most floodplain vegetation in this group 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. Stands of this group often have limited herbaceous layers, making fire less of an important factor, even during extended dry periods.

Some examples 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.

The associations in this group are generally of a longer hydroperiod than those in ~Oak - Sweetgum Floodplain Forest Group (G034)$$, but some in that group are of a long hydroperiod as well. Stands of this group, because of their longer hydroperiod, have been subject to more indirect disturbance effects than others, but they are still subject to a variety of indirect modern human influences. Many larger rivers have been dammed, and power generation and regulation of waterflow create unnatural flood regimes. Extensive erosion of uplands, caused by many years of poor agricultural practices, transported large amounts of sediment into floodplains. River bottoms were the focus of agriculture among Native Americans, so some members of this group have a long history of human clearing. A number of exotic plant species have invaded floodplains as well. Isolated wetlands such as depression ponds dominated by Taxodium are for the time being placed in nonriverine groups.