The fresh and oligohaline tidal marshes constitute the primary vegetation between outer tidal salt and brackish marshes and inland non-tidally influenced vegetation (upland or wetland). Examples are found from Newfoundland to Texas along the Atlantic and Gulf coasts of the United States and adjacent Canada. They may grade into uplands or non-tidal freshwater marshes and swamps. Most examples of this vegetation are characterized by a mixture of annual and perennial grasses, forbs, sedges, rushes, other grass-like plants, floating or submerged aquatics, shrubs, and scattered tree saplings. Dominance patterns change seasonally, yearly, and geographically. In addition to regional variability, freshwater tidal marshes may also exhibit floristic zonation based on age, water depth, tidal regime, and other factors, into a low and high marsh. This vegetation supports broad-leaved emergent plants such as Nuphar advena, Nuphar orbiculata, Nuphar sagittifolia, Peltandra virginica, Pontederia cordata, and Sagittaria spp.; annual and perennial grasses such as Calamagrostis canadensis, Leersia oryzoides, Panicum hemitomon, Spartina cynosuroides, Spartina patens, Zizania aquatica, Zizaniopsis miliacea; sedges and rushes such as Carex spp., Cladium mariscus ssp. jamaicense, Eleocharis spp., Fuirena spp., Schoenoplectus pungens, Schoenoplectus tabernaemontani; other grass-like plants and annual and perennial forbs such as Acorus calamus, Amaranthus cannabinus, Ambrosia trifida, Bidens spp., Impatiens capensis, Hibiscus moscheutos, Kosteletzkya virginica, Polygonum spp., Sium suave, Typha spp.; and scattered shrubs such as Cephalanthus occidentalis and Morella cerifera.
Common taxa of the non-tidal marshes and wet prairies include species of Eleocharis, Fimbristylis, Panicum, Rhynchospora, Sarracenia, and Xyris. Also included are warm-temperate shrub swamps dominated by the shrubs Cephalanthus occidentalis, Vaccinium corymbosum, Vaccinium formosum, or Vaccinium fuscatum. These wetlands occur on the Atlantic and Gulf coastal plains in depressions and basins, seepage slopes, interdunal swales and poorly drained wet flats. The vegetation ranges from floating-leaved aquatics in deeper basins, to emergent marsh in semipermanent water, to drawdown zones with diverse small graminoid and forb vegetation, to shrub swamp and shrub edges. Wet prairie vegetation is also found on extensive wet flats and consists of primarily herbaceous wetland vegetation with relatively thick cover of graminoid species. Examples occupy low, flat plains on poorly drained soils, often saturated for 50-100 days per year. In addition to saturation or flooding, occasional to frequent fires, including during the early growing season, promote the maintenance of this vegetation.
Among the hundreds of species found in Atlantic and Gulf Coast freshwater tidal marshes (Tinder 2013), there appear to be few that are unique to this type, apart from Aeschynomene virginica, which is a federally threatened plant species (Odum et al. 1984). Tidal and non-tidal freshwater habitats share many species. Rather than the presence of specific freshwater species, this vegetation might better be differentiated by a mixture of wetland plants that are also tolerant of low salinity levels, including annual and perennial grasses, forbs, sedges, rushes, other grass-like plants, floating or submerged aquatics, shrubs, and scattered tree saplings. Dominance patterns change seasonally, yearly, and geographically. In addition to regional variability, freshwater tidal marshes may also exhibit floristic zonation based on age, water depth, tidal regime, and other factors, into a low and high marsh. Some species that occur in and may dominate or co-dominate various examples of this vegetation across its range include Bidens spp., Cladium mariscus ssp. jamaicense (= Cladium jamaicense), Eleocharis spp., Hydrocotyle spp., Panicum hemitomon, Peltandra virginica, Polygonum spp., Pontederia cordata, Sagittaria spp., Schoenoplectus pungens, Schoenoplectus tabernaemontani, and Spartina cynosuroides, Spartina patens, Zizania aquatica, and Zizaniopsis miliacea.
Atlantic and Gulf coastal non-tidal freshwater marshes are found in depressions, basins, and interdunal swales, and on seepage slopes, or on wet flats on the coastal plain of the southeastern United States, including some areas as far north as Cape Cod, Massachusetts. Common taxa include species of graminoids Aristida, Eleocharis, Panicum, Rhynchospora, Xyris, forbs Pontederia cordata and Sarracenia, and shrubs Hypericum, Vaccinium, Ilex, and Cephalanthus occidentalis.
There are no plant species unique to freshwater tidal habitats (Odum et al. 1984), and the list of possible components numbers in the hundreds (Tiner 2013). Most examples of this vegetation are characterized by a mixture of annual and perennial grasses, forbs, sedges, rushes, other grass-like plants, floating or submerged aquatics, shrubs, and scattered tree saplings (Tiner 2013). Dominance patterns change seasonally, yearly, and geographically. In addition to regional variability, freshwater tidal marshes may also exhibit floristic zonation based on age, water depth, tidal regime, and other factors, into a low and high marsh (Odum et al. 1984, Mitsch and Gosselink 1986c, Tiner 2013). Zonation in freshwater tidal marshes is not as pronounced as in salt marshes (Odum et al. 1984, Mitsch and Gosselink 1986c, Tiner 2013), and may often be more of a spatial mosaic driven by a diverse seed bank (Leck and Simpson 1987). Where present, low marsh is younger, more erodible, more deeply flooded for longer periods of time, has lower litter accumulation, lower organic matter in the soils, and lower primary productivity than high marsh (Odum et al. 1984, Mitsch and Gosselink 1986c, Tiner 2013).
The low tidal freshwater marsh is typically characterized by species such as Amaranthus cannabinus, Heteranthera reniformis, Nuphar advena, Nuphar orbiculata, Nuphar sagittifolia, Peltandra virginica, Pontederia cordata, Sagittaria spp., Schoenoplectus pungens, Schoenoplectus tabernaemontani, and Zizania aquatica. The high marsh is more diverse and often includes a mixture of annual and perennial grasses, forbs, sedges, rushes, other grass-like plants, and shrubs (Odum et al. 1984, Mitsch and Gosselink 1986c). In the mid-Atlantic a high marsh may be dominated by a mixed group of annuals in mid-season (e.g., Acorus calamus, Bidens laevis, Hibiscus moscheutos, Impatiens capensis, Iris versicolor, Polygonum spp. Pilea pumila, Amaranthus cannabinus, Symphyotrichum subulatum, Zizania aquatica and others); later in the season perennial grasses such as Spartina cynosuroides, Typha spp., and Zizaniopsis miliacea may become monodominant (Odum et al. 1984, Mitsch and Gosselink 1986c, Tiner 2013). Dominants in Louisiana freshwater tidal marshes include Bidens laevis, Eleocharis spp., Hydrocotyle spp., Panicum hemitomon, Sagittaria lancifolia, Schoenoplectus pungens, Spartina patens, and Zizaniopsis miliacea. Associates include Alternanthera philoxeroides (non-native), Bacopa monnieri, Cladium mariscus ssp. jamaicense (= Cladium jamaicense), Echinochloa walteri, Leptochloa fusca ssp. fascicularis (= Leptochloa fascicularis), Pluchea camphorata, Pontederia cordata, Sagittaria spp., Schoenoplectus californicus, and Vigna luteola (Gosselink et al. 1979, Gosselink 1984, Visser et al. 1998, 2000). Plant composition of freshwater tidal marshes in New Jersey generally occurs as a mosaic of patches dominated by a few or a single species and has dramatic seasonal variation. Tidal freshwater marshes (0-0.5 ppt) are characterized by Acorus calamus, Ambrosia trifida (levees), Bidens laevis, Eleocharis spp., Impatiens capensis, Peltandra virginica, Polygonum punctatum, Pontederia cordata, and Sagittaria spp. Freshwater tidal flats (0-0.5 ppt) are characterized by sparse, low-growing mats of Bidens eatonii, Crassula aquatica, Eriocaulon parkeri, Isoetes riparia, Lindernia dubia, Ludwigia palustris, Polygonum punctatum, and Sagittaria subulata. Freshwater tidal communities in this region often contain one or more regionally or globally rare plant species, such as Aeschynomene virginica, Eriocaulon parkeri, and Bidens bidentoides (Walz et al. 2007). In addition, some of these same dominants may comprise another type of freshwater tidal marsh, a floating marsh (flotant), where the marsh vegetation and the underlying mat of roots, organic matter, and sediments detach from and float above the mineral substrate (Sasser et al. 1995, 1996, 2009). Flotants are the most common freshwater marsh in the Louisiana's delta marshes, estimated to cover approximately 130,000 ha (Sasser et al. 2009).
Many of the dominant and characteristic species of tidal freshwater wetlands (e.g., Asclepias incarnata, Cephalanthus occidentalis, Echinochloa walteri, Hibiscus moscheutos, Kosteletzkya virginica, Eleocharis palustris, Eleocharis quadrangulata, Leersia oryzoides, Mikania scandens, Panicum virgatum, Peltandra virginica, Polygonum arifolium, Polygonum sagittatum, Polygonum punctatum, Polygonum densiflorum, Polygonum hydropiperoides, Pontederia cordata, Rosa palustris, Rumex verticillatus, Sagittaria latifolia, Sagittaria lancifolia ssp. media (= Sagittaria falcata), Saururus cernuus, Schoenoplectus tabernaemontani, Schoenoplectus americanus, Scirpus cyperinus, Spartina cynosuroides, Typha latifolia, Typha domingensis, Typha angustifolia, Typha x glauca, and Zizania aquatica) occur across a range of salinities that span fresh (<0.5 ppt) to oligohaline (0.5-5.0 ppt) (Odum et al. 1984). While some species that characterize this vegetation (e.g., Spartina patens) are tolerant of mesohaline water (5.0-18.0 ppt), they occur in freshwater tidal habitats in a mixture with species most common in oligohaline or freshwater habitats. Once salinity becomes greater than 5.0 ppt, freshwater marshes tend to turn over to saltwater marshes (Mitsch and Gosselink 1986c).
Non-tidal wetlands of the Atlantic and Gulf coasts commonly include species of Eleocharis, Fimbristylis, Panicum, Rhynchospora, Sarracenia, and Xyris. Graminoids include Andropogon glomeratus, Aristida beyrichiana, Aristida palustris, Aristida stricta, Calamovilfa spp., Carex striata, Ctenium aromaticum, Cyperus haspan, Cyperus virens, Dichanthelium erectifolium, Dichanthelium wrightianum, Eleocharis elongata, Eleocharis equisetoides, Eleocharis microcarpa, Eleocharis quadrangulata, Fuirena scirpoidea, Fuirena squarrosa, Juncus abortivus, Juncus effusus, Juncus repens, Leersia hexandra, Panicum hemitomon, Panicum rigidulum, Panicum verrucosum, Panicum virgatum, Rhynchospora chapmanii, Rhynchospora corniculata, Rhynchospora filifolia, Rhynchospora harperi, Rhynchospora inundata, Rhynchospora tracyi, Saccharum spp., and Steinchisma hians. Forbs include Bartonia verna, Centella erecta, Lachnanthes caroliana, Lachnocaulon minus, Ludwigia glandulosa, Ludwigia linearis, Ludwigia spp., Proserpinaca spp., Rhexia alifanus, Rhexia cubensis, Rhexia spp., Sabatia angularis, Sagittaria longiloba, Sagittaria papillosa, Symphyotrichum subulatum, and Xyris jupicai. Large wetland ferns such as Osmunda cinnamomea and Osmunda regalis also often dominate. Some examples have a very sparse tree component of Magnolia virginiana, Pinus elliottii var. elliottii, Pinus palustris, Pinus serotina, or Taxodium spp. and scattered shrubs, such as Morella cerifera, Morella pensylvanica, Vaccinium corymbosum, or Clethra alnifolia. Other woody plants may include Acer rubrum, Cephalanthus occidentalis, Cyrilla racemiflora, Hypericum chapmanii, Hypericum fasciculatum, Hypericum reductum, Ilex coriacea, Ilex glabra, Ilex myrtifolia, Lyonia lucida, Nyssa biflora, and Vaccinium spp. Other characteristic and often dominant species for the northern part of the range (Massachusetts to New York or New Jersey) include Cyperus dentatus, Dichanthelium meridionale, Eleocharis acicularis, Eleocharis robbinsii, Eriocaulon aquaticum (= Eriocaulon septangulare), Euthamia caroliniana (= Euthamia tenuifolia), Gratiola aurea, Juncus militaris, Juncus pelocarpus, Lobelia dortmanna, and Xyris difformis. Along rivers in northeastern, central and southern Florida, Cladium mariscus ssp. jamaicense or Panicum hemitomon and Polygonum punctatum were apparently the historical dominant plant species.
Climate: South of Virginia, the climate is humid, warm-temperate. From eastern Virginia to Cape Cod, Massachusetts, the climate is humid, cool-temperate.
Soil/substrate/hydrology: Tidal fresh marshes exist along low-relief coastlines and upper reaches of tidal rivers and creeks where there is sufficient freshwater input from rain and rivers, and enough tidal amplitude to reach upstream into marshes along bays and rivers (e.g., Hudson River, Delaware River, Cape Fear River). Both lunar and wind tides are important, though wind tides may be more important in areas where tidal amplitude is low (e.g., the Gulf coast and the embayed region of North Carolina and Virginia). Along the Atlantic coast this vegetation is often found on rivers where there is a geomorphological constriction that increases tidal amplitude (Odum et al. 1984). Within any specific region tidal marshes occur along elevational gradients that result in varying water depths. Tidal freshwater marshes tend to be common along the coastal edge of river systems with large watersheds and no dams (Tiner 2013). Most of the Atlantic coast freshwater tidal marshes are riverine (Odum et al. 1984). They formed as sea level rose after the last glaciation. Sediment carried by streams and rivers filled drowned river valleys that were downcut during the Pleistocene glaciations. Marshes built up and expanded as streams and rivers deposited their sediment load and the tides helped to extend the area of available habitat. Similar marshes are present along the Gulf coast, but in Louisiana where the majority of freshwater tidal marshes in the U.S. are found, freshwater marshes were formed through the deltaic processes of the Mississippi River (Gosselink 1984, Mitsch and Gosselink 1986c, Visser et al. 1998, 2000). Hydrology and salinity are the most important driving processes determining the range of potential vegetation in this division. Variations in flooding, sedimentation rates, erosion, scouring, wrack deposition and salinity are regular disturbances (Tiner 2013).
Tidal freshwater marshes occur in a variety of settings, including "mature marshes," marshes that may be more than 500 years old with a well-developed peat substrate (found most commonly on the Atlantic coast), "floating marshes," marshes that have broken loose from the mineral substrate and float on the water surface (found on the northern Gulf coast), and "new marshes" in areas where sedimentation by rivers is exceeding erosion and subsidence such as prograding deltas (found throughout the range) (Odum et al. 1984, Mitsch and Gosselink 1986c, 2000, Mitsch et al. 2009, Tiner 2013). In some areas, marshes have expanded in the recent past as a result of streams and rivers carrying and depositing higher sediment loads because of inland erosion (Odum et al. 1984). Soils in older marshes tend to be high in organic matter and those in younger marshes high in clays and silts (Odum et al. 1984).
Common characteristics of tidal freshwater marshes include variable rates of sedimentation and vertical accretion of sediments and organic matter (Perry et al. 2009). In many places, this is often offset by subsidence, reduced sedimentation due to water diversion, and a rising sea level. The substrate of tidal wetlands varies from primarily black, fibrous organic muck over sandy or silt clay loam occasionally mixed with woody peat, underlain by deep coastal plain quartzite sand deposits in New Jersey (Tedrow 1986), to thin or thick mats of floating roots and peat over muck and clay in Louisiana (Sasser et al. 2009). The New Jersey Geological Survey publication by Waksman et al. (1943) entitled "The Peats of New Jersey and Their Utilization" distinguishes marine salt marsh peats found along the coast and in bays that have fine mud rich in organic matter derived of decomposed grasses from the freshwater tidal marshes of drowned estuarine streams with freshwater alluvial peat and sedge-and-reed-peat characterized by coarse fibrous organic material often with wood particles and large amounts of mineral sediment (Walz et al. 2007). Odum et al. (1984) described the substrate underlying most tidal freshwater marshes in the eastern U.S. as "a dark, mucky soil" with high levels of silts and clays in the low marsh and higher levels of organic matter in the high marsh.
Tidal freshwater and oligohaline marshes are complex entities that are affected by movement of the salt line resulting from seasonal fluctuations in precipitation as well as changes in the periodicity and amplitude of tidal inundation (lunar and wind). Sea level rise due to global warming, and the resulting change in salt intrusion, vegetation composition, marsh acreage, and function, are serious concerns for the future of these critically important estuarine wetlands (Walz et al. 2007).
Non-tidal, freshwater marshes, wet prairie, and shrub swamp occur along rivers and in different types of depressions such as former lake basins, shallow peat-filled valleys, and zones around existing natural lakes (Kushlan 1990). Sites also includes oligotrophic wetlands maintained by seepage at the zone between an overlaying, permeable sandy layer and a lower layer of relatively impermeable material such as sandstone or clay. This vegetation also includes wetlands of low, flat plains on poorly drained soils, such as Ultisols, Spodosols, Inceptisols, and Entisols (Collins et al. 2001); some of these soils have an argillic horizon which impedes drainage and contributes to high water tables. The low areas where this vegetation occurs on barrier islands and similar immediate coastal areas are dune swales or other basins. These ponds have standing water well into the growing season, and many are permanently saturated. The vegetation also occurs in small basins and depressions, primarily in sandy terrain of the Atlantic Coastal Plain, from southeastern Virginia to Florida, including the Lake Wales Ridge area of central Florida. Most southeastern basins are formed by subsidence of surface sediments caused by solution in underlying limestone, but origins may be different from Delaware northward.