Mangroves are trees and shrubs that grow in saline coastal habitats in the tropics and subtropics – mainly between latitudes 25° N and 25° S. The saline conditions tolerated by various species range from brackish water, through pure seawater (30 to 40 ppt), to water of over twice the salinity of ocean seawater, where the salt has become concentrated by evaporation (up to 90 ppt).
There are many species of trees and shrubs adapted to saline conditions. Not all are closely related, and the term "mangrove" may be used for all of them, or more narrowly only for the mangrove family of plants, the Rhizophoraceae, or even more specifically just for mangrove trees of the genus Rhizophora.
Mangroves form a characteristic saline woodland or shrubland habitat, called mangrove swamp, mangrove forest, mangrove or mangal. Mangals are found in depositional coastal environments where fine sediments (often with high organic content) collect in areas protected from high energy wave action. They occur both in estuaries and along open coastlines. Mangroves dominate three quarters of tropical coastlines.
|Taiga, Boreal forests|
|Montane grasslands and shrublands|
|Temperate coniferous forests|
|Tropical and subtropical coniferous forests|
|Temperate broadleaf and mixed forests|
|Mediterranean forests, woodlands, and scrub|
|Tropical and subtropical moist broadleaf forests|
|Tropical and subtropical dry broadleaf forests|
|Temperate grasslands, savannas, and shrublands|
|Tropical and subtropical grasslands, savannas, and shrublands|
|Deserts and xeric shrublands|
|Flooded grasslands and savannas|
|Littoral, Intertidal zone|
Plants in mangals are diverse but all are able to exploit their habitat (the intertidal zone) by developing physiological adaptations to overcome the problems of anoxia, high salinity and frequent tidal inundation. About 110 species belong to the mangal. Each species has its own solutions to these problems; this may be the primary reason why, on some shorelines, mangrove tree species show distinct zonation. Small environmental variations within a mangal may lead to greatly differing methods for coping with the environment. Therefore, the mix of species is partly determined by the tolerances of individual species to physical conditions, like tidal inundation and salinity, but may also be influenced by other factors such as predation of plant seedlings by crabs.
Once established, mangrove roots provide an oyster habitat and slow water flow, thereby enhancing sediment deposition in areas where it is already occurring. The fine, anoxic sediments under mangroves act as sinks for a variety of heavy (trace) metals which colloidal particles in the sediments scavenged from the water. Mangrove removal disturbs these underlying sediments, often creating problems of trace metal contamination of seawater and biota.
Mangroves protect coastal areas from erosion, storm surge (especially during hurricanes), and tsunamis. The mangrove's massive root system is efficient at dissipating wave energy. Likewise, they slow down tidal water enough that its sediment is deposited as the tide comes in, leaving all except fine particles when the tide ebbs. In this way, mangroves build their own environment. Because of the uniqueness of mangrove ecosystems and the protection against erosion that they provide, they are often the object of conservation programs including national Biodiversity Action Plans.
However, mangroves' protective value is sometimes overstated. Wave energy is typically low in areas where mangroves grow, so their effect on erosion can only be measured over long periods. Their capacity to limit high-energy wave erosion is limited to events like storm surges and tsunamis. Erosion often occurs on the outer sides of bends in river channels that wind through mangroves, while new stands of mangroves are appearing on the inner sides where sediment is accreting.
The unique ecosystem found in the intricate mesh of mangrove roots offers a quiet marine region for young organisms. In areas where roots are permanently submerged, the organisms they host include algae, barnacles, oysters, sponges, and bryozoans, which all require a hard surface for anchoring while they filter feed. Shrimps and mud lobsters use the muddy bottom as their home. Mangrove crabs mulch the mangrove leaves, adding nutritients to the mangal muds for other bottom feeders. In at least some cases, export of carbon fixed in mangroves is important in coastal food webs.
Mangrove plantations in Vietnam, Thailand, the Philippines and India host several commercially important species of fish and crustaceans. Despite restoration efforts, developers and others have removed over half of the world's mangroves in recent times.
Of the recognized 110 mangrove species, only about 54 species in 20 genera from 16 families constitute the "true mangroves", species that occur almost exclusively in mangrove habitats. Demonstrating convergent evolution, many of these species found similar solutions to the tropical conditions of variable salinity, tidal range (inundation), anaerobic soils and intense sunlight. Plant biodiversity is generally low in a given mangal. This is especially true in higher latitudes and in the Americas. The greatest biodiversity occurs in the mangal of New Guinea, Indonesia and Malaysia.
Red mangroves, which can survive in the most inundated areas, prop themselves above the water level with stilt roots and can then absorb air through pores in their bark (lenticels). Black mangroves live on higher ground and make many pneumatophores (specialised root-like structures which stick up out of the soil like straws for breathing) which are also covered in lenticels. These "breathing tubes" typically reach heights of up to thirty centimeters, and in some species, over three meters. There are four types of pneumatophore—stilt or prop type, snorkel or peg type, knee type, and ribbon or plank type. Knee and ribbon types may be combined with buttress roots at the base of the tree. The roots also contain wide aerenchyma to facilitate oxygen transport within the plant.
Red mangroves exclude salt by having significantly impermeable roots which are highly suberised, acting as an ultra-filtration mechanism to exclude sodium salts from the rest of the plant. Analysis of water inside mangroves has shown that 90% to 97% of salt has been excluded at the roots. Salt which does accumulate in the shoot concentrates in old leaves which the plant then sheds. Red mangroves can also store salt in cell vacuoles. White (or grey) mangroves can secrete salts directly; they have two salt glands at each leaf base (hence their name—they are covered in white salt crystals).
Because of the limited freshwater availability in salty intertidal soils, mangroves limit the amount of water that they lose through their leaves. They can restrict the opening of their stomata (pores on the leaf surfaces, which exchange carbon dioxide gas and water vapour during photosynthesis). They also vary the orientation of their leaves to avoid the harsh midday sun and so reduce evaporation from the leaves. Anthony Calfo, a noted aquarium author, observed anecdotally that a red mangrove in captivity only grows if its leaves are misted with fresh water several times a week, simulating the frequent tropical rainstorms.
The biggest problem that mangroves face is nutrient uptake. Because the soil is perpetually waterlogged, there is little free oxygen. Anaerobic bacteria liberate nitrogen gas, soluble iron, inorganic phosphates, sulfides, and methane, which makes the soil much less nutritious and contributes to mangroves' pungent odor. Prop root systems allow mangroves to absorb gases directly from the atmosphere, and other nutrients such as iron, from the inhospitable soil. Mangroves store gases directly inside the roots, processing them even when the roots are submerged during high tide.
In this harsh environment, mangroves have evolved a special mechanism to help their offspring survive. Mangrove seeds are buoyant and therefore suited to water dispersal. Unlike most plants, whose seeds germinate in soil, many mangroves (e.g. Red Mangrove) are viviparous, whose seeds germinate while still attached to the parent tree. Once germinated, the seedling grows either within the fruit (e.g. Aegialitis, Avicennia and Aegiceras), or out through the fruit (e.g. Rhizophora, Ceriops, Bruguiera and Nypa) to form a propagule (a ready-to-go seedling) which can produce its own food via photosynthesis. The mature propagule then drops into the water which can transport it great distances. Propagules can survive desiccation and remain dormant for over a year before arriving in a suitable environment. Once a propagule is ready to root, its density changes so that the elongated shape now floats vertically rather than horizontally. In this position, it is more likely to lodge in the mud and root. If it does not root, it can alter its density and drift again in search of more favorable conditions.
The following listing (modified from Tomlinson, 1986) gives the number of species of mangroves in each listed plant genus and family.
|Family||Genus, number of species||Common name|
|Acanthaceae, Avicenniaceae or Verbenaceae
(family allocation disputed)
|Avicennia, 9||Black mangrove|
|Combretaceae||Conocarpus, 1; Laguncularia, 11; Lumnitzera, 2||Buttonwood, White mangrove|
|Arecaceae||Nypa, 1||Mangrove palm|
|Rhizophoraceae||Bruguiera, 6; Ceriops, 2; Kandelia, 1; Rhizophora, 8||Red mangrove|
|Lythraceae||Sonneratia, 5||Mangrove apple|
|Family||Genus, number of species|
|Acanthaceae||Acanthus, 1; Bravaisia, 2|
Mangroves occur in numerous areas worldwide. See List of mangrove ecoregions.
Nigeria has Africa's largest mangrove concentration, spanning 36,000 km2. Oil spills and leaks have destroyed many in the last fifty years, damaging the local fishing economy and water quality.
Along the coast of the Red Sea both on the Egyptian side and in the Gulf of Aqaba, mangroves composed primarily of Avicennia marina and Rhyzophora mucronata in about 28 stands cover about 525 hectares. Almost all Egyption mangrove stands are now protected.
Mangroves live in many parts of the tropical and subtropical coastal zones of North, South and Central America.
Because of their sensitivity to sub-freezing temperatures, mangroves in the continental United States are limited to the Florida peninsula (see Florida mangroves) and isolated growths of Black Mangrove (Avicennia germinans) along the coast of southern Louisiana and south Texas
Mangroves occur on the west coast of Costa Rica, on the Pacific and Caribbean coasts of Nicaragua, Belize, Guatemala, Honduras, and Panama and on many Caribbean Islands, such as Curacao, Bonaire, Antigua, the Bahamas, Saint Kitts and Nevis and St. Lucia. Significant mangals include the Marismas Nacionales-San Blas mangroves in Mexico. Mangroves can also be found in Puerto Rico, Cuba, the Dominican Republic, Haiti, Jamaica, Trinidad, Barbados, and the Pacific coast of El Salvador.
Brazil contains approximately 26,000 km2 of mangals, 15% of the world's total of 172,000 km2.
Colombia possesses large mangrove forests on both its Caribbean and Pacific coasts.
Mangroves occur on Asia's south coast, throughout the Indian subcontinent, in all southeast Asian countries, and on islands in the Indian Ocean, Arabian Sea, Bay of Bengal, South China Sea and the Pacific.
The Pichavaram Mangrove Forest near Chidambaram, South India, by the Bay of Bengal is the world's second largest mangrove forest. Notably, it has actually increased by 90% in size between 1986 and 2002.
In Vietnam, mangrove forests grow along the southern coast, including two forests: the Can Gio Mangrove Forest biosphere reserve and the U Minh mangrove forest in the Sea and Coastal Region of Kien Giang, Ca Mau and Bac Lieu province.
The mangrove forests of Kompong Sammaki in Cambodia are of major ecological and cultural importance, as the human population relies heavily on the crabs and fish that live in the roots.
The three most important mangrove forests of Taiwan are: Tamsui River in Taipei, Jhonggang River in Miaoli and the Sihcao Wetlands in Tainan. According to research, there are four existing types of mangrove in Taiwan. Some places have been developed as scenic areas, such as the log raft routes in Sihcao.
In Pakistan, the mangrove forest are located on the coasts of Sindh and Balochistan provinces. Pakistan's mangrove ecosystem is one of the largest found in an arid climate. Without realising their global significance, the local communities continue to use mangroves as fuelwood and fodder. In urban areas, mangroves are being cut away for developmental activities on the coast. Pakistan project include rehabilitation of mangrove-degraded areas at Sonmiani and Jiwani in Balochistan, and Sandspit in Karachi, Sindh. In Karachi, land reclamation projects are cutting down mangrove forests and filling then earth and selling them for commercial and urban development.
Oman, near Muscat, supports large areas of mangroves, in particular at Shinas, Qurm Park and Mahout Island. In Arabic, mangrove trees are known as qurm, thus the mangrove area in Oman is known as Qurm Park. Mangroves are also present extensively in neighboring Yemen.
Iranian mangrove forests occur between 25°11′N to 27°52′N. These forests exist in the north part of the Persian Gulf and Oman Sea, along three Maritime Provinces in the south of Iran. These provinces respectively from southwest to southeast of Iran, include Bushehr, Hormozgan and Sistan & Balouchestan.
Australia has approximately 11,500 km2 of mangroves primarily on the northern and eastern coasts of the continent, with occurrences as far south as Millers Landing in Wilsons Promontory, Victoria (38°54′S) and Barker Inlet in Adelaide, South Australia.
New Zealand also has mangrove forests extending to around 38°S (similar to Australia's southernmost mangrove incidence): the furthest geographical extent on the west coast is Raglan Harbour (37°48′S); on the east coast, Ohiwa Harbour (near Opotiki) is the furthest south that mangroves are found (38°00′S).
Mangroves are not native to Hawaii, but the Red mangrove, Rhizophora mangle, and Oriental mangrove, Bruguiera sexangula, have been introduced and are now naturalized. Both species are classified as pests by the University of Hawaii Botany Department.
Red mangroves are the most common choice, used particularly in marine aquariums in a sump to reduce proteins and other minerals in the water. Mangroves also appear in home aquariums, and as ornamental plants, such as in Japan.
Grassroots efforts to save mangroves from development are becoming more popular as the benefits of mangroves become more widely known. In the Bahamas, for example, active efforts to save mangroves are occurring on the islands of Bimini and Great Guana Cay. In Trinidad and Tobago as well, efforts are underway to protect a mangrove threatened by the construction of a steelmill and a port. In Thailand, community management has been effective in restoring damaged mangroves.
Approximately 35% of mangrove area was lost during the last several decades of the twentieth century (in countries for which sufficient data exist, which encompass about half of the area of mangroves).
It has been cited that Mangroves can help buffer against Tsunami, cyclones, and other storms. One village in Tamil Nadu was protected from Tsunami destruction - the villagers in Naluvedapathy planted 80,244 saplings in order to get into the Guinness Book of World Records. This created a kilometre wide belt of trees of various varieties. When the Tsunami struck, much of the land around the village was flooded, but the village itself suffered minimal damage.