Coastal salt marsh, also termed tidal salt marsh, is the dominant, mostly herbaceous vegetation of seacoast mudflats, especially outside the tropics and subtropics, including around estuaries of all sizes, coastal plain, and deltas, and characterizing vegetation along huge bays, inlets, and fjords in subpolar and polar zones. For just the United States, salt marsh extends, interrupted only by beaches, from New Jersey to northern Florida along the Atlantic Ocean, rimming the Gulf of Mexico, and at scattered, nonrocky stretches from Washington to San Diego. These are incredibly productive biological systems, and are major feeding and nesting grounds of birds. In tropical and subtropical latitudes, coastal salt marsh is instead mangal, i.e., mangrove vegetation.
Like mangal, salt marsh is populated by halophytes, plants that can live under saline conditions. In the lowest portion of a typical marsh (lower marsh), especially one that is open to the sea, plants generally are flooded twice each day at high tide. These areas require plants to be able to withstand regular inundation by saltwater. Higher there may be an upper marsh zone, where inundation may not be as often, for example, less than one time per day. If inundation is uncommon and evaporation is intense, of if a marsh is closed from direct contact with seawater, a different type of halophytic vegetation may occur, such as salt flat succulent. High evaporation in dry coastal locations may produce muddy salt flats so toxic that no plants will grow there, at soil salt concentrations exceeding one hundred parts per thousand, i.e., three times that of ocean water. At the other end of the scale, river discharge of large volumes of freshwater into the upper salt marsh may have make some zones weakly brackish, supporting yet a different set of wetland plants.
Salt marshes of eastern and southern United States coastlines are dominated by cordgrasses, either Spartina alterniflora or S. patens or both. Spartina alterniflora is a tall species, and thrives on the outer edge of the marsh that regularly receives inundation twice every day. Higher in the marsh
In the warmer waters of the Gulf of Mexico, S. alterniflora typically forms the outer edge, but a rush, Juncus roemerianus, commonly fills in much of the salt marsh. The wetter the soils, the taller the stands of Juncus, so that there can be considerable different in vegetation height from lower to upper marsh, due entirely to water availability rather than to species composition.
Giant reed, Phragmites australis, can form monospecific stands on the landward edge of eastern salt marshes, where freshwater to brackish water dominates, as along major stretches of the eastern coastline of the United States.
In arctic and subarctic latitudes, coastal salt marsh is dominated by species of alkali grass, Puccinellia, as around Hudson Bay, Alaska, Iceland, the northern British Isles, and Scandivania.
In warm temperate regions like Southern California or the hot, arid zones of Arabia and northeastern Africa, where high evaporation have a heavy influence of salinity and soil water quality, grasses are much less abundant, being replaced by dicotyledonous halophytes. Widespread species with fleshy leaves or stems include:
Some common plants that are salt excluders are species of Atriplex, Frankenia, and Limonium (dicotyledons), and Distichlis spicata and Monanthochloe littoralis (monocotyledons). In Southern California salt marshes, C4 species are especially common, including Salicornia, Suaeda, Atriplex, Distichlis, and Monanthochloe. The C4 species of high light environments can yield much higher photosynthetic rates at higher temperatures and lower water loss than their related C3 species, which are more typical of temperate and colder latitudes.
It must be pointed out that variations in plant composition and zonation of salt marshes varies greatly around the world, even regionally, and there are no simple explanations to account for all the variations in the general theme.
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