Mangrove swamp is an easily recognized habitat along tropical and subtropical coastlines and brackish estuaries and deltas, where evergreen trees and shrubs thrive in tideland mud or sand flats inundated daily with sea water. These flats are found mostly along bays and inlets protected from heavy waves. Some coral reefs on islands can support mangal in relatively high energy environments.
The plant community of a mangrove swamp is most commonly termed mangal, a forest with a dense canopy, also known as mangrove swamp forest or, simply, mangrove. Although mangal occurs along more than two-thirds of all saltwater tropical coastlines, parallel to the shoreline, this is a very narrow, fringing forest, and, hence, less than one-tenth of one percent of the earth's surface is inhabited by mangal.
The saturated mud flat of the typical mangrove swamp is a hostile environment for typical plants, because the soil has very low levels of oxygen for roots and toxic levels of sulfides. Ocean water generally has 33 to 38 parts of salt per thousand, and evaporation of water from the mangrove mud results in much higher salt concentrations experienced by the plants. All species that inhabit the outer (ocean-facing) portion of the mangrove swamp are halophytes, i.e., plants that are adapted to saline soils, and certain mangrove species can tolerate soils more than double the salinity of ocean water. Species that are less resistant to salt damage grow on the landward edge of mangal, where high tides reach only infrequently, or along river banks (estuarine mangrove), where freshwater mingles with sea water due to tidal influences.
Mangrove swamp is often adjacent to one of several other habitats, e.g., salt flats with its herbaceous to succulent forms of plants, sandy beach strand, freshwater swamp, or terrestrial forest or scrub. The transition can be very abrupt, but more than half of the species that can be present in mangal occur in mangal and also the adjacent plant community (mangal associates).
About 110 species are characteristic plants of mangrove vegetation, out of more than 250,000 species of vascular plants, indicating that this is a tough environment for plants. At a given mangal location, it would be highly unusual to collect three dozen species, and some sites, especially marginal subtropical locations and recently established plots, may have only one or two species. The mangrove species of the world are all perennials, and none can grow either where any freezing occurs or where the water temperature is seasonally cold. Hence, temperature factors limit the poleward extent of this type of forest. [Avicennia can tolerate up to twelve hours of freezing air temperatures.]
Mangrove species have evolved from nonmangrove plant lineages independently many different times, and therefore occur in more than 30 families of dicotyledons, as well as the monocotyledons Nypa (a palm, family Arecaceae), Crinum angustifolium (family Amaryllidaceae), and Pandanus (screwpines, family Pandanaceae) and ferns of the genus Acrostichum.
The mangrove family, Rhizophoraceae, is virtually always present at mangal sites. The genus Rhizophora is exceedingly common around the world, although none of the individual species is truly cosmopolitan. Red mangrove, R. mangle, is, of these, the most widespread species, occurring along the Pacific coast of the Americas from 28 degrees north latitude in Baja California and Sonora to northwestern South America and the Galapagos Archipelago, on the eastern side of the Americas from southernmost Florida to southern Brazil, and along tropical West Africa. In the Old World, Bruguiera gymnorrhiza is especially widespread, ranging from East Africa to eastern Australia, Samoa in the Pacific Basin, and the Ryukyu Islands in Asia. There are other exceedingly widespread species of this family in Old World mangal, including the two species of Ceriops and Kandelia candel.
Avicennia spp., black mangrove, is ubiquitous in mangal. In the Western Hemisphere, A. germinans is codominant with red mangrove, R. mangle, and a variant of this Avicennia occurs along the coastline of West Africa. Two other species in the genus may be found in the New World. Avicennia marina is the most widespread species of the Old World, extending from East Africa to Fiji in Polynesia and the North Island of New Zealand, and occurring at the coldest localities in New Zealand, subtropical China (26 degrees north latitude), and southeastern Australia (Victoria, at 38.45 degrees south latitude).
Sonneratia alba (family Sonneratiaceae) is characteristic of the tropical mangal of the Old World, generally appearing with Avicennia, the Rhizophoraceae, Excoecaria agallocha (family Euphorbiaceae), Xylocarpus granatum (family Meliaceae), Aegiceras corniculatum (family Myrsinaceae; a shrubby understory), Osbornia octodonta (family Myrtaceae), and Lumnitzera racemosa (family Combretaceae).
In mangal of the Americas, diversity of the woody species is much lower. In addition to Rhizophora mangle and Avicennia germinans, a visitor to a New World mangrove swamp is likely to find Laguncularia racemosa (family Combretaceae) and either Conocarpus erectus (family Combretaceae) or Pelliciera rhizophorae (family Pellicieraceae).
The nipa palm, Nypa fruticans, is a very aggressive colonizer of estuarine banks and lagoons of the Old World tropics. This plant crowds out all potential competitors by forming subterranean rhizomes in the mud, from which arise the pinnately compound leaves. The other monocotyledon of mangal, species of Pandanus, most commonly grows in coastal swamps than within the dense thicket of mangal. The only terrestrial ferns of mangal are species of Acrostichum, which tend to grow in less saline microhabitats and also can tolerate shade, but are still very tolerant of salinity.
Mangal along a tropical bay characteristically shows zonation. On the outfacing edge, fully exposed to high tides twice each day, is the seaward zone, which is inhabited by a small subset of tree species, Sonneratia alba, Avicennia spp., and Rhizophora spp. In the middle zone typically occur members of the Rhizophoraceae (mangrove family), the so-called Rhizophora zone or mesozone. The back, inland portion of mangal, also called the landward zone, which less frequently is covered by sea water and can receive freshwater from ground water or land runoff, is where the mangal associates can survive. There may be a gradual transition from mangal to terrestrial forest, but, in general, it does not appear that back mangal is merely a sere (stage) in succession from mangrove plants to a terrestrial forest.
Species that colonize the outer edge of mangal, such as Sonneratia alba and Avicennia alba, are able to remain firmly rooted as seedlings when they are regularly inundated by high tides. The species that first establish mangal most often possess a major system of aerial roots. Such roots not only help to anchor the plant and keep it raised out of water, but also create a tangle of crossed and looping roots. Within that tangle of roots collect sediments, including organic matter from decaying plant organs.
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