|
|
|
|
Mangrove
|
| |
|
| |
Mangroves (generally) are trees and shrubs that grow in saline (brackish) coastal habitats in the tropics and subtropics. The word is used in at least three senses: (1) most broadly to refer to the habitat and entire plant assemblage or mangal, for which the terms mangrove swamp and mangrove forest are also used, (2) to refer to all trees and large shrubs in the mangal, and (3) narrowly to refer to the mangrove family of plants, the Rhizophoraceae, or even more specifically just to mangrove trees of the genus Rhizophora.
Discussion
Ask a question about 'Mangrove' Start a new discussion about 'Mangrove' Answer questions from other users |
Encyclopedia
Mangroves (generally) are trees and shrubs that grow in saline (brackish) coastal habitats in the tropics and subtropics. The word is used in at least three senses: (1) most broadly to refer to the habitat and entire plant assemblage or mangal, for which the terms mangrove swamp and mangrove forest are also used, (2) to refer to all trees and large shrubs in the mangal, and (3) narrowly to refer to the mangrove family of plants, the Rhizophoraceae, or even more specifically just to mangrove trees of the genus Rhizophora. Mangals are found in depositional coastal environments where fine sediments, often with high organic content, collect in areas protected from high energy wave action.
Ecology
A mangrove is a plant and mangal is a plant community and habitat where mangroves thrive. They are found in tropical and sub-tropical tidal areas, and as such have a high degree of salinity. Areas where mangals occur include estuaries and marine shorelines.
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 have been identified as belonging to the mangal. Each species has its own capabilities and 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 of coping with the environment. Therefore, the mix of species at any location within the intertidal zone 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, roots of mangrove plants provide a habitat for oysters and help to impede water flow, thereby enhancing the deposition of sediment in areas where it is already occurring. Usually, the fine, anoxic sediments under mangroves act as sinks for a variety of heavy (trace) metals which are scavenged from the overlying seawater by colloidal particles in the sediments. In areas of the world where mangroves have been removed for development purposes, the disturbance of these underlying sediments often creates problems of trace metal contamination of seawater and biota.
Mangroves protect the coast from erosion, surge storms (especially during hurricanes), and tsunamis. Their 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 and is not re-suspended when the tide leaves, except for fine particles. As a result, mangroves build their own environment. Because of the uniqueness of the mangrove ecosystems and their protection against erosion, they are often the object of conservation programs including national Biodiversity Action Plans.
Despite their benefits, the protective value of mangroves is sometimes overstated. Wave energy is typically low in areas where mangroves grow, so their effect on erosion can only be measured in the long-term. Their capacity to limit high-energy wave erosion is limited to events like storm surges and tsunamis. Erosion often still occurs on the outer sides of bends in river channels that wind through mangroves, just as new stands of mangroves are appearing on the inner sides where sediment is accreting.
Mangroves support unique ecosystems, especially on their intricate root systems. The mesh of mangrove roots produces a quiet marine region for many young organisms. In areas where roots are permanently submerged, they may host a wide variety of organisms, including algae, barnacles, oysters, sponges, and bryozoans, which all require a hard substratum for anchoring while they filter feed. Shrimps and mud lobsters use the muddy bottom as their home. Mangrove crabs improve the nutritional quality of the mangal muds for other bottom feeders by mulching the mangrove leaves. In at least some cases, export of carbon fixed in mangroves is important in coastal food webs.
The habitats also host several commercially important species of fish and crustaceans. In Vietnam, Thailand, the Philippines, and India, mangrove plantations are grown in coastal regions for the benefits they provide to coastal fisheries and other uses. Despite replanting programs, over half of the world's mangroves have been lost in recent times. Mangrove trees can be found in the Florida Keys.
Biology
A wide variety of plant species can be found in mangrove habitat, but of the recognized 110 species, only about 54 species in 20 genera from 16 families constitute the "true mangroves", species that occur almost exclusively in mangrove habitats and rarely elsewhere. Convergent evolution has resulted in many species of these plants finding similar solutions to the problems of variable salinity, tidal ranges (inundation), anaerobic soils and intense sunlight that come from living in the tropics. Plant biodiversity is generally low in a given mangal—more than twenty species are uncommon. This is especially true in higher latitudes and in the Americas. The greatest biodiversity occurs in the mangal of New Guinea, Indonesia and Malaysia.
Adaptations to low oxygen
Red mangroves, which can live in the most inundated areas, prop themselves up above the water level with stilt roots and can then take in 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 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.
Limiting salt intake
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 mangrove plants has shown that anywhere from 90% to 97% of salt has been excluded at the roots. Any salt which does accumulate in the shoot is concentrated in old leaves which are then shed, as well as stored away safely 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).
Limiting water loss
Because of the limited availability of freshwater in the salty soils of the intertidal zone, mangrove plants have developed ways of limiting 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, has 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 rainstorms in the tropics.
Nutrient uptake
The biggest problem that mangroves face is nutrient uptake. Because the soil is perpetually waterlogged, there is little free oxygen. Thus anaerobic bacteria liberate nitrogen gas, soluble iron, inorganic phosphates, sulfides, and methane, which makes the soil much less nutritious and contributes to a mangrove's pungent odor. Prop root systems allow mangroves to take up gases directly from the atmosphere, and various other nutrients, like iron, from the inhospitable soil. Gases are quite often stored directly inside the roots and processed even when the roots are submerged during high tide.
Increasing survival of offspring
In this harsh environment, mangroves have evolved a special mechanism to help their offspring survive. All mangroves have buoyant seeds suited to dispersal in water. Unlike most plants, whose seeds germinate in soil, many mangrove plants (e.g. Red Mangrove) are viviparous, i.e., their seeds germinate while still attached to the parent tree. Once germinated, the seedling grows either within the fruit (e.g. Aegialitis, Acanthus, Avicennia and Aegiceras), or out through the fruit (e.g. Rhizophora, Ceriops, Bruguiera and Nypa) to form a propagule (a seedling ready to go), which can produce its own food via photosynthesis. When the propagule is mature it drops into the water where it can then be transported great distances. Propagules can survive desiccation and remain dormant for weeks, months, or even over a year until they arrive in a suitable environment. Once a propagule is ready to root, it will change its density so that the elongated shape now floats vertically rather than horizontally. In this position, it is more likely to become lodged in the mud and root. If it does not root, it can alter its density so that it floats off again in search of more favorable conditions.
Species
The following listing (modified from Tomlinson, 1986) gives the number of species of mangroves in each listed plant genus and family.
Major components
| 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 |
Minor components
| Family | Genus, number of species |
|---|
| Acanthaceae | Acanthus, 1; Bravaisia, 2 | | Bombacaceae | Camptostemon, 2 | | Cyperaceae | Fimbristylis, 1 | | Euphorbiaceae | Excoecaria, 2 | | Lecythidaceae | Barringtonia, 6 | | Lythraceae | Pemphis, 1 | | Meliaceae | Xylocarpus, 2 | | Myrsinaceae | Aegiceras, 2 | | Myrtaceae | Osbornia, 1 | | Pellicieraceae | Pelliciera, 1 | | Plumbaginaceae | Aegialitis, 2 | | Pteridaceae | Acrostichum, 3 | | Rubiaceae | Scyphiphora, 1 | | Sterculiaceae | HeritieraHeritiera longipetiolata...
, 3 |
Geographical regions
Mangroves occur in numerous areas worldwide. See List of mangrove ecoregions.
Africa
There are important examples of mangrove swamps in Kenya and Madagascar, the latter even admixed at the coastal verge with the Madagascar dry deciduous forests. Nigeria has the largest concentration of mangroves in Africa, spanning an area of 36,000 km2. Many of Nigeria's mangroves have been destroyed in the last fifty years due to oil spills and leaks, destroying local fishing economy and water quality. Along the coast of the Red Sea both on the Egyptian side and in the Gulf of Aqaba, there are mangroves composed primarily of the two species Avicennia marina and Rhyzophora mucronata in about 28 stands covering about 525 hectars. Almost all of mangrove stands in Egypt are now protected areas to ensure their long term conservation.
Americas
Mangroves are found in many parts of the tropical and subtropical coastal parts of the Americas. Mississippi, Florida and other southern states the US contain Mangroves.
Continental United States
Because of their sensitivity to sub-freezing temperatures, mangroves in the continental United States are limited to the coastal Florida peninsula (see Florida mangroves) and isolated areas of the gulf coast of southern Louisiana and south Texas
Central America & Caribbean
Mangroves also 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 Antigua 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.
South America
Brazil contains approximately 26,000 km2 of mangals, which is 15% of the world's total of 172,000 km2.
Ecuador and Peru also have significant areas of mangroves mainly in the Gulf of Guayaquil-Tumbes mangroves.
Venezuela's northern Caribbean island, Margarita, also possesses mangrove forest in the Parque Nacional la Restinga.
Colombia also possesses large mangrove forests on both the Caribbean and Pacific coasts.
Asia
Mangroves occur on the south coast of Asia, throughout the Indian subcontinent, in all the southeast Asian countries, and on islands in the Indian Ocean, Arabian Sea, Bay of Bengal, South China Sea and the Pacific.Pakistan and Bangladesh once boosted one of the biggest mangrove forests in the world. The mangal is particularly prevalent in the deltas of large Asian rivers.
The Sundarbans is the largest mangrove forest in the world, located in the Ganges delta in Bangladesh and West Bengal, India. There are major mangals in the Andaman and Nicobar Islands and the Gulf of Kutch in Gujarat. Other significant mangals include the Bhitarkanika Mangroves and Godavari-Krishna mangroves.
The Pichavaram Mangrove Forest near Chidambaram, South India is the second largest mangrove forest in the world. It is home to a large variety of birds—local resident, migratory resident and the pure migratory birds—and is separated from the Bay of Bengal by a lovely beach. It is one of those rare mangrove forests which has actually increased by 90% between 1986 and 2002.
Pakistani mangroves are located mainly on the Indus delta, major mangrove forests are also found on the coastal line of provinces Sindh and Balochistan.
There are large areas of mangroves in Oman near Muscat, 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.
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.
, Taiwan]]
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 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.
Australasia
In Australasia, mangroves occur around much of New Guinea, Sulawesi and the surrounding islands. Australia has mangals primarily on the northern and eastern coasts of the continent. It has approximately 11,500 km2 of mangroves with occurrences as far south as Corner Inlet in Victoria (37°45'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).
Pacific islands
Twenty-five species of mangrove are found on various Pacific islands, with extensive mangals on some islands. Mangals on Guam, Palau, Kosrae and Yap have been badly affected by development. 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 "Pest Plants of Hawaiian Native Ecosystems" by the University of Hawaii Botany Department.
Growing mangroves
Red mangroves are the most commonly grown of all species, used particularly in marine aquariums in a sump to reduce proteins and other minerals in the water. People also may grow them just for their unusual appearance, either in aquariums, or as ornamental plants, such as in Japan. In Hawaii, these plants are considered pests, while in Florida they are heavily protected.
Destruction
The United Nations Environment Program has estimated that a quarter of the destruction of mangrove forests stems from shrimp farming.
Grassroots efforts to save mangroves from development are becoming more popular as the benefits of mangroves are becoming 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 popular media
- The mangrove is used as a symbol in Annie Dillard's essay Sojourner due to its significance as a self-sustaining biome.
- The manga series One Piece has a forest of giant mangroves forming the Sabaody Archipelago, notable for creating a resin combined with the oxygen breathed out of the trees to create large bubbles used and manipulated by the local population for everything from transport to hotels.
See also
External links
|
| |
|
|
