Mangrove swamps or forests are the tropical equivalent of temperate salt marshes. They grow in protected coastal embayments in tropical and subtropical areas around the world, and some scientists estimate that 60-75 percent of all tropical shores are populated by mangroves.
The term "mangrove" refers to individual trees or shrubs that are angiosperms (flowering plants) and belong to more than 80 species within 12 genera and five families. Though unrelated taxonomically, they share some common characteristics. Mangroves only grow in areas with minimal wave action, high salinity , and low soil oxygen. All of the trees have shallow roots, form pure stands, and have adapted to the harsh environment in which they grow. The mangrove swamp or forest community as a whole is called a mangal.
Mangroves typically grow in a sequence of zones from seaward to landward. This zonation is most highly pronounced in the Indo-Pacific regions, where 30-40 species of mangroves grow. Starting from the shore-line and moving inland, the sequence of genera there is Avicennia followed by Rhizophora, Bruguiera, and finally Ceriops. In the Caribbean, including Florida, only three species of trees normally grow: red mangroves (Rhizophora mangle ) represent the pioneer species growing on the water's edge, black mangroves (Avicennia germinans ) are next, and white mangroves (Laguncularia racemosa ) grow mostly inland. In addition, buttonwood (Conocarpus erectus ) often grows between the white mangroves and the terrestrial vegetation.
Mangrove trees have made special adaptations to live in this environment. Red mangroves form stilt-like prop roots that allow them to grow at the shoreline in water up to several feet deep. Like cacti, they have thick succulent leaves which store water and help prevent loss of moisture. They also produce seeds which germinate directly on the tree, then drop into the water, growing into a long, thin seedling known as a "sea pencil." These seedlings are denser at one end and thus float with the heavier hydrophilic (water-loving) end down. When the seedlings reach shore, they take root and grow. One acre of red mangroves can produce three tons of seeds per year, and the seeds can survive floating on the ocean for more than 12 months. Black mangroves produce straw-like roots called pneumatophores which protrude out of the sediment , thus enabling them to take oxygen out of the air instead of the anaerobic sediments. Both white and black mangroves have salt glands at the base of their leaves which help in the regulation of osmotic pressure.
Mangrove swamps are important to humans for several reasons. They provide water-resistant wood used in construction, charcoal, medicines, and dyes. The mass of prop roots at the shoreline also provides an important habitat for a rich assortment of organisms, such as snails, barnacles, oysters, crabs, periwinkles, jellyfish, tunicates, and many species of fish. One group of these fish, called mud skippers (Periophthalmus ), have large bulging eyes, seem to skip over the mud, and crawl up on the prop roots to catch insects and crabs. Birds such as egrets and herons feed in these productive waters and nest in the tree branches. Prop roots tend to trap sediment and can thus form new land with young mangroves. Scientists reported a growth rate of 656 feet (200 m) per year in one area near Java. These coastal forests can be helpful buffer zones to strong storms.
Despite their importance, mangrove swamps are fragile ecosystems whose ecological importance is commonly unrecognized. They are being adversely affected worldwide by increased pollution , use of herbicides, filling, dredging , channelizing, and logging .
[John Korstad ]
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man·grove / ˈmanˌgrōv; ˈmang-/ • n. a tree or shrub (families Rhizophoraceae and Verbenaceae or Avicenniaceae) that grows in muddy, chiefly tropical coastal swamps, typically having numerous tangled roots above ground that form dense thickets. ∎ (also mangrove swamp) a tidal swamp that is dominated by mangroves and associated vegetation.