Physiognomy and Structure: No Data Available

Floristics: The vegetation is dominated by dense, and occasionally pure, stands of Nyssa aquatica but often in association with Taxodium distichum (never very abundant in this type), Liquidambar styraciflua, Planera aquatica, Nyssa biflora, Gleditsia aquatica, Fraxinus profunda, and Cephalanthus occidentalis. The herbaceous layer is conspicuously sparse, and density is wholly dependent upon the extent and duration of flooding. Where water is permanent, herbaceous plants rely on substrates found on rotting logs, stumps, terraces, and buttresses of trees. Some herbs which may be present at low densities on these elevated places include Phanopyrum gymnocarpon, Pluchea camphorata, Boehmeria cylindrica, Rudbeckia laciniata, Sagittaria latifolia, Onoclea sensibilis, Triadenum walteri, Carex joorii, Carex glaucescens, Asclepias perennis, Saururus cernuus, Justicia ovata, Leersia lenticularis, and others. The exotic plant species Eichhornia crassipes may a problem. Subcanopy density and forest tree recruitment are poor due to fluctuating water levels (TNC 1995a).

At Shiloh National Military Park (western Tennessee) this occurs with vegetation open-canopied, due to beaver activity (both impounding and chewing of bark which has killed trees). The dominant canopy tree is Nyssa aquatica. Salix nigra is also present in the canopy. Acer rubrum is the only subcanopy tree. Shrubs are fairly dense; Acer rubrum, Alnus serrulata, Cornus foemina, and Itea virginica are the most important, with Nyssa aquatica, Cephalanthus occidentalis, and Salix nigra. Polygonum sagittatum, Leersia oryzoides, Microstegium vimineum, Impatiens capensis, and Polygonum punctatum are the dominant herbaceous plants. Other herbs are Scirpus cyperinus, Mikania scandens, Boehmeria cylindrica, Cicuta maculata, Apios americana, Triadenum walteri, Cinna arundinacea, Sagittaria latifolia (= Sagittaria longirostra), Erechtites hieraciifolius, Ludwigia decurrens, Peltandra virginica, Carex crinita, Commelina virginica, Bidens aristosa (= Bidens polylepis), Carex lupulina, Panicum dichotomiflorum, Bidens frondosa, Ceratophyllum demersum, Pilea pumila, Heteranthera reniformis, Saururus cernuus, Cuscuta gronovii, Gratiola virginiana, Ludwigia peploides ssp. glabrescens, Eclipta prostrata (= Eclipta alba), Galium tinctorium, Lobelia cardinalis, and the exotic Arthraxon hispidus.

This community most commonly occurs in seasonally flooded riparian wetlands or lakes, where standing water is absent during parts of the year. Timing, duration, and depth of flooding are the major environmental factors governing vegetative composition within and among occurrences of forested swamps. Water tupelo is more commonly associated with riparian floodplain wetlands or margins of lakes or ponds where standing water occurs for only a few months of each year (usually winter and spring). Bald-cypress is more commonly associated with permanent deep-water forested wetlands.

Dynamics:  This is a climax wetland community. Flooding frequency is approximately 100% of years, and flooding duration is approximately 100% of the growing season. Seasonal (spring and fall) flood pulses contribute to channel scouring and deposition of organic matter (where slackwater and pooling of floodwaters occur). Flooding also provides water which persists during dry periods, although dewatering during summer months is not uncommon. Seasonal flooding and permanent water require special adaptations by vegetation to exist in this extremely dynamic ecosystem. Many plants have highly specialized methods to facilitate acquisition and transport of oxygen during periods of prolonged inundation. Neither bald-cypress nor water tupelo are able to germinate in water and require dry periods to produce seedlings. Seedlings are unable to survive complete submergence, and prolonged flooding with excessive water levels can kill even mature trees. Subcanopy and herbaceous components of this community are very dynamic, and densities fluctuate with availability of substrate.

Nyssa aquatica swamps occur most commonly in riparian zones subject to seasonal hydrologic pulses. Inundation is usually more than 6 months of the year and is greatest during spring and fall. Timing, duration, depth, and extent of flooding greatly affect community composition. Deeper, more permanent flooding conditions seem to select for bald-cypress. Prolonged drought can create conditions favorable for tupelo recruitment. In addition, Nyssa aquatica may replace Taxodium distichum on some sites because bald-cypress has minimal sprouting ability, erratic reproduction, and slower growth (Wharton et al. 1982). Prolonged flooding can increase senescence of seedlings and saplings, removing all but older mature timber. These open areas, while quickly colonized during subsequent dry years, are very dynamic until the hydrology stabilizes sufficiently long to select for a specific species.