Mangroves are trees in the family Rhizophoraceae, occurring in tropical and subtropical environments as swampy forests fringing muddy, tidal, estuarine, and oceanic shores. Mangrove forests are generally the first type of woody ecosystem that is encountered when a low-lying tropical shore is approached from the ocean.
Mangrove forests comprise a biome, that is, a distinctive ecosystem that occurs in appropriate habitats worldwide. Compared with other tropical forests, the mangrove ecosystem is rather poor in species. The richest mangrove forests occur closest to the equator, especially in the western Pacific Ocean. The number of mangrove species diminishes with increasing latitude in both hemispheres, with black mangrove (Avicennia spp.) generally being the last species to drop out, reaching about 32°N in Bermuda and 38°S in northern Australia.
Mangrove trees are well adapted to growing in saline water, having glands on their leaves for excreting their excess of absorbed salt, and evergreen foliage to aid in the retention of scarce nutrients. Some species have aerial roots that aid in transporting oxygen to their belowground tissues, and seeds that are specialized for establishing in tidal mud.
The family Rhizophoraceae contains about 100 species of woody plants, all of which are tropical or subtropical in distribution. The most important of the tree-sized species are in the genera Avicennia, Bruguiera, Ceriops, and Rhizophora.
The red mangrove (Rhizophora mangle ) is abundant in mangrove forests of south Florida, the Caribbean, and Central and South America. This species has distinctive round stilt roots, which emerge from aerial parts of the stem and then curve downwards to grow into the sediment. The red mangrove also retains ripe seeds on its branches, where they germinate aerially, extending a radicle up to 10 inches (25 cm) long. This sort of germination system is known as vivipary, and is analogous in some respects to the bearing of live young by animals. The germinated seedlings eventually detach from the parent tree, and may plop upright into the mud and establish a new plant, or they may float for a while until they become lodged in sediment after a longer-range dispersal from the parent. The established seedlings of red mangrove send out prop roots, which quickly become firmly anchored and help to accrete mud around the plant. Young plants of this sort are abundant along the leading edge of developing stands of red mangrove, with older, larger trees occurring further into the stand. The individual stands often occur as discrete mangrove “islands,” which may eventually coalesce as an extensive forest.
The black mangrove (Avicennia nitida ) is also abundant in mangrove forests of Florida, the Caribbean, and Latin America. This species has radially spreading underground roots from which emerge numerous vertically growing pneumatophores, or extensions of the roots that emerge from the mud. The pneumatophores are exposed to the atmosphere during
low tide, and are useful in conducting oxygen to the underwater tissues of the plant, which grow in an anaerobic environment. Black mangroves often do not reproduce well beneath their own closed canopy, and when their stands senesce and die back, the site may convert into a relatively open community dominated by plants of salt marshes and protected mudflats.
The mangrove environment is stressful to most plants, largely because of the high salt concentrations in water, which are physiologically difficult for most species to deal with. However, mangrove trees can tolerate this stress, and as a result they are able to assemble into forests under these environmental conditions, although these are relatively species-poor ecosystems in comparison with other types of tropical forests.
The mangrove ecosystem is periodically subject to catastrophic disturbance, usually associated with severe windstorms such as hurricanes. These can be energetic enough to uproot and kill mature trees, and to initiate ecological recovery through primary succession. Species of Rhizophora are often the primary mangrove colonist, followed by Avicennia and other secondary species as the ecosystem begins to stabilize and mature.
The patterns of successional dynamics of mangrove forests are related to the environmental tolerances of the species, and often result in distinct community zone types within this ecosystem. Usually the succession culminates in a mature forest of mangrove species. However, in some cases succession in the mangrove ecosystem can sufficiently reduce the influence of tidal waters to allow relatively freshwater conditions to develop. Under these circumstances succession can result in the development of a relatively species-rich forest that is lacking in mangrove species, because these are not very competitive under the less stressful conditions of fresh water.
Mangrove forests are very effective at binding coastal mud and helping to prevent erosion. This contributes to the development of a stable substrate that enhances the rate of ecosystem development and allows the forest to resist tidal and other disturbances and thereby form a relatively stable ecosystem.
Biome —A geographically extensive ecosystem, usually characterized by its dominant life forms.
Ecotourism —Ecology-based tourism, focused primarily on natural or cultural resources.
Mangrove —A coastal tropical wet forest growing on muddy substrate, dominated by species of mangrove trees in the family Rhizophoraceae.
Pneumatophores —Exposed roots of some marsh plants that are useful in conducting oxygen to the plant’s underwater tissues.
Radicle —Embryonic root.
Senescence —The state of being old.
Mangrove forests provides critical nursery habitat for various commercially important species of tropical fish and invertebrates, such as shrimp. These coastal wetlands also provide important habitat for a wide range of non-economic species of wildlife.
Mangrove forests can be commercially important. Lumber can be manufactured from all mangrove trees, but the most durable wood is that of Ceriops. Where it is abundant, Rhizophora may be harvested to manufacture lumber or pulp. In some places, mangroves trees are harvested and used to manufacture charcoal. The bark of mangroves is rich in tannins, and has been used for the commercial production of these chemicals, which are utilized to tan animal skins into leather. Mangrove forests are also commonly harvested for local use as firewood.
Ecotourism is a less consumptive use of the mangrove ecosystem. In large part, this recreational use is based on the fact that many species of large, colorful birds can be abundant in mangrove forests and their integrated, open-water wetlands and shores. These include species of herons, ibises, pelicans, gulls, terns, osprey, and shorebirds.
Mangrove forests in many parts are under intense pressure from various types of human stressors, partly from overly intensive harvesting of natural resources from these forests. In addition, many regions of mangrove forests are being lost to various types of coastal developments, which convert these natural ecosystems into agriculture, plantations, tourism developments, or aquaculture facilities, especially for the culturing of shrimp. In some places, mangrove forests are also being degraded through pollution associated with agricultural runoff, sewage dumping from residential areas, and aquatic industrial emissions of various types. Mangrove forests are rapidly being depleted in many regions, and in extensive areas they have virtually disappeared.
Stafford-Deitsch, J. Mangroves. London: Immel, 1994.