Mangrove

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Mangroves (generally) are trees and shrubs that grow in saline coastal habitats. The word is used in at least three senses, (1) most broadly to refer to the habitat and entire plant assemblage or mangal ^[1], 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. Mangal is found in depositional coastal environments where fine sediments, often with high organic content, collect in areas protected from high energy wave action.

Contents 1 General description 2 Biology of mangroves 3 Species of mangroves 3.1 Major components 3.2 Minor components 4 Mangroves in geographical regions 4.1 Africa 4.2 Americas 4.3 Asia 4.4 Australasia 4.5 Pacific islands 5 Mangroves in other media 6 Notes 7 References and external links

[edit] General description

Plants in mangrove swamps are a diverse group which have been able to exploit a habitat (the intertidal zone) because they have developed a set of physiological adaptations to overcome the problems of anoxia, salinity and frequent tidal inundation. About 110 species have been identified as belonging to the mangal.^[2]. 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 due to variations in the range of environmental conditions across the intertidal zone. 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 also may be influenced by other factors such as predation of their seedlings by crabs.

Once established, the roots of the 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. It is usually the case that 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.

It is often stated that mangroves provide significant value in the coastal zone as a buffer against erosion, storm surge and tsunamis. While there is some attenuation of wave heights and energy as seawater passes through mangrove stands, it must be recognised that these trees typically inhabit areas of coastline where low wave energies are the norm. Therefore their capacity to ameliorate high energy events like storm surge and tsunamis is limited. Their long term impact on rates of erosion is also likely to be limited. Many river channels that wind through mangrove areas are actively eroding stands of mangroves on the outer sides of all the river bends, just as new stands of mangroves are appearing on the inner sides of these same bends where sediment is accreting.

They also provide habitats for wildlife, including several commercially important species of fish and crustacea and in at least some cases export of carbon fixed in mangroves is important in coastal foodwebs. 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 the world's mangroves have been lost.

[edit] Biology of mangroves

The word mangal describes the plant community and habitat where mangroves thrive^[2]; it is exclusively subtropical and tropical and tidal, and therefore having soil or sediment that is water-logged and saline or of variable salinity. Areas where mangal occurs includes estuaries and marine shorelines. 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^[1]. Evolutionary convergence 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 the mangal, with more than 20 species in a given area being uncommon.^[2] In higher latitudes, and in the Americas, this is particularly true. The greatest biodiversity occurs in the mangal of New Guinea, Indonesia and Malaysia.^[3]

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 30 centimeters, though some species have ones that reach over 3 meters high. 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 rather impermeable roots which are highly suberised, acting as an ultra-filtration mechanism to exclude sodium salts from the rest of the plant. Water inside the plant shows that 90%, and in some cases of high salinity, up to 97%, of the 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 (small pores on their leaf surfaces which exchange carbon dioxide gas and water vapour during photosynthesis) and also have the ability to vary the orientation of their leaves. By orienting their leaves to avoid the harsh midday sun, mangrove plants can reduce evaporation from their leaf surfaces. Anthony Calfo, a noted aquarium author, has observed, anecdotally, that a red mangrove in captivity will not grow unless its leaves are misted with fresh water several times a week, simulating the frequent rainstorms in the tropics.^[4]

Nutrient uptake

The biggest problem that mangroves face is nutrient uptake. Because the soil that mangroves live in is perpetually waterlogged, there is not much free oxygen available. At these low oxygen levels, anaerobic bacteria proceed to liberate nitrogen gas, soluble iron, inorganic phosphates, sulfides, and methane, which help contribute to a mangrove's particularly pungent odor and also make it a hostile environment to most plants. Since the soil is not particularly nutritious, mangroves have adapted by modifying their roots. Prop root systems allow mangroves to take up gasses directly from the atmosphere and various other nutrients, like iron, from the otherwise inhospitable soil. They quite often store gasses directly inside the roots so that they can be processed even when the roots are submerged during high tide.

Increasing survival of their offspring

In this harsh environment mangroves have evolved a special mechnanism to help their offspring to survive. All mangroves have buoyant seeds that are suited to dispersal in water. Unlike most plants, whose seeds germinate in the 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 what is called 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 a propagule does not root, it can alter its density so that it floats off again in search of more favorable conditions.

Mangrove ecosystems

Mangroves are excellent buffers between the violent ocean and the fragile coast, especially during hurricanes, which can bring powerful storm surges onto shores. The massive mangrove root system is quite efficient at dissipating wave energy. This same root system also helps prevent coastal erosion. As tidal water flows through the root system, it is slowed substantially enough so that it deposits its sediment as the tide comes in, and the return flow is kept slow as the tide goes out to prevent resuspension of some of the finer particles. As a result, mangroves can build their own environment. Because of the uniqueness of the mangrove ecosystems, they are frequently the object of conservation programs including national Biodiversity Action Plans.

Mangroves support unique ecosystems, especially on their intricate root systems. In areas where roots are permanently submerged, mangroves may be host to a wide variety of organisms, including algae, barnacles, oysters, sponges, and bryozoans, which all require a hard substratum for anchoring while they filter feed. Mangrove crabs improve the nutritional quality of the mangle muds for other bottom feeders by mulching the mangrove leaves. ^[5]

[edit] Species of mangroves

The following listing (modified from Tomlinson, 1986) gives the number of species of mangroves in each listed plant genus and family.

[edit] 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

[edit] Minor components

Family	Genus, number of species
Acanthaceae	Acanthus, 1; Bravaisia, 2
Bombacaceae	Camptostemon, 2
Cyperaceae	Fimbristylis, 1
Euphorbiaceae	Excoecaria, 2
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	Heritiera, 3

[edit] Mangroves in geographical regions

Mangroves occur in numerous areas worldwide. See List of mangrove ecoregions for a full listing.

[edit] 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. 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.^[6].

[edit] Americas

Mangroves are found in many parts of coastal tropical and subtropical America. Brazil contains approximately 15% of the world's total of 172,000 km², or 26,000 km², an area larger than the state of New Jersey. Mangroves also occur on the west coast of Costa Rica, on both of Panama's coasts and on many Caribbean Islands, such as Antigua and St. Lucia. Significant mangals include the Marismas Nacionales-San Blas mangroves in Mexico and the Gulf of Guayaquil-Tumbes mangroves in Ecuador and Peru. Because of their sensitivity to sub-freezing temperatures, mangroves in the continental United States are limited to the coastal Florida Peninsula (see Florida mangroves).

[edit] Asia

Mangroves occur on the coasts of Pakistan, India, Bangladesh, and all the southeast Asian countries. The Sundarbans is the largest mangrove forest in the world, located in the Ganges delta in Bangladesh and West Bengal, India. Other major mangals in India include the Andaman and Nicobar Islands and the Gulf of Kutch in Gujarat.^[7] Other significant mangals include the Bhitarkanika Mangroves and Godavari-Krishna mangroves.

[edit] Australasia

Mangroves occur primarily on the eastern and northern coasts of Australia and round much of New Guinea. Australia has approximately 11,500km² of mangroves found primarily along the tropical northern coastlines but also as far south as Corner Inlet in Victoria and Barker Inlet in Adelaide, South Australia. ^[8]

[edit] 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.^[9] Mangroves are not native to Hawaii, but the Red mangrove, Rhizophora mangle, and Oriental mangrove, Bruguiera sexangula, have been introduced and are now naturalized.^[10] Both species are classified as "Pest Plants of Hawaiian Native Ecosystems" by the University of Hawaii Botany Department.^[11]

[edit] Mangroves in other media

• The mangrove is used as a symbol in Annie Dillard's essay Sojourner due to its significance as a self-sustaining biome.

[edit] Notes

[edit] References and external links

• Saenger, Peter (2002). Mangrove ecology, silviculture, and conservation. Kluwer Academic Publishers, Dordrecht. ISBN 1-4020-0686-1.
• Hogarth, Peter J. (1999). The Biology of Mangroves. Oxford University Press, Oxford. ISBN 0-19-850222-2.
• Thanikaimoni, Ganapathi (1986) Mangrove Palynology UNDP/UNESCO and the French Institute of Pondicherry, ISSN 0073-8336 (E).
• Tomlinson, Philip B. (1986). The Botany of Mangroves. Cambridge University Press, Cambridge, ISBN 0-521-25567-8.
• Teas, H. J. (1983). Biology and ecology of mangroves. W. Junk Publishers, The Hague. ISBN 90-6193-948-8.
• Plaziat, J.C., et al. (2001). History and biogeography of the mangrove ecosystem, based on a critical reassessment of the paleontological record. Wetlands Ecology and Management 9 (3): pp. 161-179.
• Jayatissa, L. P., Dahdouh-Guebas, F. & Koedam, N. (2002). A review of the floral composition and distribution of mangroves in Sri Lanka. Botanical Journal of the Linnean Society 138: 29-43.
• Warne.K, (February 2007). Forests of the Tide. National Geographic pp. 132-151
• Tsunami protection.
• The story of the UNESCO Mangrove Programme.
• Science and Management of Mangroves in the South China Sea and the Gulf of Thailand.
• WWF article about the mangrove biome.
• East African Mangroves, a website managed by the Universities of Brussels (Belgium) and Florence (Italy).
• Kenyan Mangroves.
• Large mangrove website.
• Borneo Mangroves.
• Sundarbans Tiger Project Research and Conservation of tigers in the largest remaining mangrove forest in the world.
• Coastal wetland and shoreline change mapping of Pichavaram, south east coast of India using Satellite data.
• Status of Indian Mangroves: Pollution Status of Pichavaram Mangrove, Southeast Coast of India.
• Mangrove Action Project. Advocacy group devoted to the conservation, restoration and sustainable management of mangroves and related coastal ecosystems.
• Global Mangrove database and Information System (GLOMIS).
• (Italian) Information and photo of Red Mangrove in aquarium - Rhizophora mangle.
• Mangroves of Rodrigues Island.
• Mangroves of Singapore.

Terrestrial biomes

Tropical and subtropical moist broadleaf forests · Tropical and subtropical dry broadleaf forests · Tropical and subtropical coniferous forests · Temperate broadleaf and mixed forests · Temperate coniferous forests · Mediterranean forests, woodlands, and shrub · Boreal forests/taiga · Mangrove · Tropical and subtropical grasslands, savannas, and shrublands · Temperate grasslands, savannas, and shrublands · Flooded grasslands and savannas · Montane grasslands and shrublands · Deserts and xeric shrublands · Tundra

Ecozones

Afrotropic · Antarctic · Australasia · Indomalaya · Nearctic · Neotropic · Oceania · Palearctic