Salt-tolerant plants (halophytes) include herbaceous forbs, graminoids, and dwarf or subshrubs. Many of the forbs are succulent (e.g., Sarcocornia and Salicornia spp.). Graminoids often dominate Arctic salt marshes, while subshrubs dominate salt marshes in Mediterranean and
Table 1 Representative species of global salt marshes based on a summary by Paul Adam Arctic Puccinellia phryganodes dominates the lower elevations
Boreal Triglochin maritima and Salicornia europea are widespread. Brackish conditions have extensive cover of Carex spp. Temperate
Europe: Puccinellia maritima dominated lower elevations historically (but Spartina anglica often replaces it). Juncus maritimus dominates the upper marsh; Atriplexportulacoides is widespread USA:
Atlantic Coast: Spartina alterniflora is extensive across seaward marsh plain; S. patens occurs more inland Gulf of Mexico: Spartina alterniflora and Juncus roemerianus dominate large areas Pacific Northwest: Distichlis spicata in more saline areas, Carex lyngbei in less saline areas California: Spartina foliosa along bays, Sarcocornia pacifica inland Japan: Zoysia sinica dominates the mid-marsh
Australasia: Sarcocornia quinqueflora dominates the lower marsh, Juncus kraussii the upper marsh
South Africa: Sarcocornia spp. are abundant in the lower marsh, Juncus kraussii in the upper marsh. Spartina maritima is sometimes present
Dry coasts vegetation tends toward subshrubs, such as Sarcocornia, Suaeda, Limoniastrum, and Frankenia species Tropical Sporobolus virginicus and Paspalum vaginatum form extensive grasslands. Batis maritima, Sesuvium portulacastrum, and Cressa cretica are also found subtropical climates. Many salt marshes support mono-typic stands of cordgrass (Spartina spp.) (Table 1).
Floristic diversity of salt marshes is low because few species are adapted to saline soil. Members of the family Chenopodiaceae comprise a large proportion of the flora (e.g., species of Arthrocnemum, Atriplex, Chenopodium, Salicornia, Sarcocornia, and Suaeda). In contrast to the flowering plants, salt marsh algae are diverse in both species and functional groups (green macroalgae, cyanobacteria, diatoms, and flagellates).
NaCl is a dual stressor, as it challenges osmotic regulation and sodium is toxic to enzyme systems. Salt marsh halophytes cope with salt by excluding entry into roots, sequestering salts intracellularly (leading to succulence), and excreting salt via glands, usually on leaf surfaces. One succulent, Batis maritima, continually drops its older salt-laden leaves, which are then washed away by the tide. I. Mendelssohn has attributed moisture uptake from seawater to the ability of some species to synthesize prolines.
Prolonged inundation reduces the supply of oxygen to soils, causing anoxia and stressing vascular plants. In addition, abundant sulfate in seawater is reduced to sulfide in salt marsh soil, with high sulfide concentrations, which are toxic to roots.
Salt marsh vascular plants withstand brief inundation but do not tolerate prolonged submergence, as occurs when a lagoon mouth closes to tidal flushing and water levels rise after rainfall. Salt marshes in lagoons thus experience irregular episodes of dieback and regeneration in relation to ocean inlet condition.
Regular inundation benefits halophytes by importing nutrients and washing away salts. Salts that accumulate on the soil surface during daytime low tides and salts excreted by halophytes are removed by tidal efflux.
Thus, soil salinities are relatively stable where tidal inundation and drainage occur frequently. Inland salt marshes, however, experience infrequent reductions in salinity during rainfall, and soils can become extremely hypersaline (e.g., >10% salt). In between irregular inundation events, halophytes and resident animals endure hypersaline drought.
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