What are Mangroves?


A mangrove is a shrub or small tree that grows in coastal saline or brackish water. The term is also used for tropical coastal vegetation consisting of such species.

Mangroves occur worldwide in the tropics and subtropics, mainly between latitudes 25° N and 25° S. The total mangrove forest area of the world in 2000 was 137,800 square kilometres (53,200 sq mi), spanning 118 countries and territories.

  • The mangrove biome  or mangal, is a distinct saline woodland or shrubland habitat characterized by depositional coastal environments, where fine sediments (often with high organic content) collect in areas protected from high-energy wave action. The saline conditions tolerated by various mangrove species range from brackish water, through pure seawater (3 to 4%), to water concentrated by evaporation to over twice the salinity of ocean seawater (up to 9%).
  • About 110 species are considered “mangroves”, in the sense of being a tree that grows in such a saline swamp, though only a few are from the mangrove plant.

Requirements For Development

  • They need average temperatures of the coldest month higher than 20°C. The seasonal temperature range should not exceed 5°C. They can tolerate temperatures of 5°C, but the development will be affected. They are not resistant to freezing.
  • The shores must be free of strong wave action and tidal current.
  • They need salt water. They are facultative halophytes.
  • They need a large tidal range. This causes limited erosion and deposition of sediments.

Functioning and Adaptations

  • The mangroves have several functions and adaptations to a life in an intertidal ecosystem. They need to conquer some problems to be resistant to the environment. The first problem is that mangrove trees are freshwater riverine trees and They grow in an environment whose salinity ranges between that of freshwater and seawater.They developed a mechanism to exclude salt by the roots or leaves. In this way, they are tolerant for saline conditions. Even with exclusion of most of the salts, concentration of chloride and sodium ions in the tissue is higher than other plants.
  • Several mangrove species deposit sodium and chloride in the bark of stems and roots. Other species deposit salt in senescent leaves, which later fall off the tree.
  • The most roots branch off from the stem underground. One type of roots is the prop root or rhizophore. This root diverges from the tree and anchors into the bottom to stabilize the tree in the soft, muddy substrate. The rhizophore has lenticels on the upper surface. Lenticels are large pores with a corky layer and enable the exchange of air. Seawater cannot get in the lenticels. The tissue of the prop roots consists of aerenchyma and is connected with the lenticels. Through this aerenchyma, air can be provided to the submerged parts of the tree. The root can periodically break the soil surface and submerges again. This forms a knee root.
  • Another type of roots is a shallow, horizontal root that radiates outwards. The vertical root is called a pneumatophore and can be as high as several decimeters. They also have lenticels and aerenchyma. This can create a huge network of vertical roots. The horizontal root is called the cable root.
  • At last, the buttress root is a root that covers the whole space between the upper part of the root and the bottom.
  • The pollination of the trees is by the wind or by organisms. All mangroves disperse their offspring by water. They produce unusually large propagating structures or propagules. The embryo initiates germination on the seed, still attached on the tree and further develops into a propagule. This phenomenon is known as vivipary.

Threats on Mangroves

  • Variations in river and surface run-off, that inhibit the tropical coastal deltas of fresh water and silt, cause losses of mangrove species diversity and organic production. This results in alternations in both the terrestrial and aquatic food web. This has an effect on the types of refugees available to consumers.
  • People will always be engaged in making projects. Soil reclamation for agriculture and aquaculture reduce the regional levels of biodiversity due to loss of mangrove habitats. The shrimp aquaculture for example has major effects on the biodiversity in the mangroves.
  • People are clear cutting the mangrove trees (deforestation, habitat loss) and are building dikes.
  • Another negative impact of humans on the mangrove habitat is the use of pesticides and fertilizers. The products that are used in the upstream agriculture end up in the water around the mangroves. This causes an increased nutrient concentration, especially nitrogen and phosphorus. These nutrients cause oxygen depletion in the water and promote the growth of algae. As a result, the ecosystems will be no longer in equilibrium.
  • Another problem is the cutting of the mangrove for their hard wood. This wood is an important export product for building constructions in areas with large concentrations of termites. This is because the wood is resistant against these termites. The wood can also be used as charcoal and fuelwood. The substrate will be no longer stable when the trees are cut away. The result of this unstable condition is erosion.
  • Other coastal development activities have an influence on the quality of the water. Industry, tourism and port development involve land reclamation and dredging. This causes resuspension of the sediment and makes the water turbid. Because of this, light can not penetrate enough in the water and causes in this way damage to mangroves.
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