Species Description: Mangrove swamps dominate much of the world's tropical and sub-tropical coastlines, and have a similar distribution pattern as coral reefs. There are approximately 35 species of true mangroves and another 60 or more species of mangrove associates. Most species occur throughout the Indo-Pacific region, with 3 commonly occurring in the Americas.
Avicennia germinans, the black mangrove, is a tropical/subtropical tree which colonizes coastal areas from the equator to 28 degrees north and south. Avicennia germinans along with Laguncularia racemosa (the white mangrove), are generally found at slightly higher tidal elevations than Rhizophora mangle, the red mangrove, which colonizes the intertidal zone.
Avicennia germinans is characterized by opposite leaves which are narrow and elliptical in shape, and often found encrusted with salt. Propagules are small (2-3 cm in diameter), flattened and bean-like. The root system of Avicennia germinans consists of long underground cable roots which produce hundreds of thin, upright pneumatophores on the ground around the tree. These structures have numerous pores which are thought to conduct oxygen to the underground portion of the root system.
Regional Occurrence: Avicennia germinans occurs worldwide in coastal and estuarine areas of the tropics and subtropics to about 28 degrees in both the northern and southern hemispheres.
IRL Distribution: In the Indian River Lagoon, it is a common landscape feature to approximately 28 degrees north, in the vicinity of Merritt Island, Florida. North of this location exists a transition zone where mangrove forests gradually give way to salt marshes. Frost stress north of the transition zone prevents mangroves from becoming well established.
Age, Size, Lifespan: Little is known regarding typical age to maturation in mangroves in south Florida, though it has been hypothesized that maturation age for mangroves in south Florida is in some way linked to the periodicity of hurricanes.
Abundance: The black mangrove is considered abundant to common in the Indian River Lagoon, as well as throughout much of its range.
Locomotion: Propagules of the black mangrove detach from the parent tree upon ripening and may float in salt water for approximately one year without rooting.
Reproduction: It is widely believed that the flowers of Avicennia germinans are pollinated by insects, principally bees. Black mangroves exhibit cryptovivipary, in which the embryo emerges from the seed coat, but remains in the fruit before abscission from the parent plant occurs. The seedlings, or propagules, eventually fall from the parent plant and are able, in the absence of suitable substrata, to float for an indefinite period in salt water without rooting.
Temperature: The range of A. germinans generally matches the 20°C isotherm in both the northern and southern hemispheres, and is similar to the range of coral reefs. Frost stress beyond 28 degrees north and south latitudes prevents most mangroves from becoming well established. Avicennia germinans, however, has a somewhat wider temperature range than other species. When subjected to cold stress, mangrove populations show differences in survival rate and amount of damage done per plant based on their geographic points of origin.
Salinity: As facultative halophytes, mangroves have the ability to thrive in waterlogged soils which may have salinities ranging from 0 - 90 ppt. Mangroves exhibit several different types of mechanisms for coping with highly saline conditions. The black mangrove (Avicennia germinans) and the white mangrove (Laguncularia racemosa) are able to take up seawater through their roots, but they excrete excess salt through pores, or salt glands, located on the surface of leaves. A different strategy is employed by Rhizophora mangle, the red mangrove, which excludes the salt in seawater at the root-substratum interface.
Other Physical Tolerances: Mangroves can experience reducing conditions conditions to at least -200 mV. One of the most visibly obvious adaptations to anoxia are root adaptations. The black mangrove utilizes upright pneumatophores which grow from the underground cable roots. These structures contain numerous pores which are thought to conduct oxygen to the underground portion of the root system. Fine lateral rootlets are able to accumulate in the substratum, and produce most of the underlying peat on which mangrove swamps are built.
Adaptations to extremes in pH have not been examined in the red mangrove, however, pH values between 5.3 and 7.8 have been reported.
Trophic Mode: Mangrove forests typically show a wide range of productivity, depending on factors such as hydrological regimes, nutrient supply, etc., and are considered to be vital sources of organic matter for estuarine systems.
Competitors: Ball (1980) suggested that competition among the 3 mangrove species may be partially responsible for the zonation observed in many mangrove areas. Direct consumers of mangrove propagules in Florida include the spotted mangrove crab (Goniosis cruentata), the mangrove land crab (Ucides cordatus), the coffee bean snail (Malapus coffeus) and the ladder horn snail (Cerithidea scalariformis). Consumers of mangrove leaves include the mangrove crab (Aratus pisonii), the spotted mangrove crab (G. cruentata), the blue land crab (Cardisoma guanhumi), and various types of insects. Wood boring isopods feed upon and damage prop roots.
Habitat: Propagules of A. germinans may float for an indefinite period without taking root. They generally take root upon coming to rest on a suitable substrate area consisting of sand, silt, mud or clay which offers some protection from waves. Propagules may root even while completely submerged; and mature trees, depending on type, tend not to be sensitive to hydroperiod; they may remain submerged anywhere from several hours to nearly permanently without showing adverse effects.
Mangroves form intertidal forests in which red mangrove prop roots, black mangrove pneumatophores, and their associated peat banks serve as the dominant substrata for other members of the mangrove community. Black mangroves (Avicennia germinans) and white mangroves (Laguncularia racemosa) are usually found in association with red mangroves. Segregation of the 3 species does occur, however; with red mangroves typically occupying the lowest intertidal position. Black and white mangroves occur at slightly higher tidal elevations.
Special Status: Habitat structure
Notes on Special Status: Mangrove communities support populations of invertebrates, birds and juvenile fishes. Birds utilize mangrove areas as important nesting habitat, while many species of commercially or recreationally important fish species utilize mangrove habitat as nursery grounds while juveniles.
Benefit in the IRL: Mangrove forest ecosystems are vital as sources of energy and provide nursery habitat for juvenile fish and invertebrates. They also provide roosting and nesting habitat for wading birds. In addition, mangrove communities provide a source for timber production and are important as buffers in decreasing storm impacts along coastlines.
Economic Importance: While black mangroves have little direct commercial importance, they support invertebrate communities, and populations of juvenile fishes. They additionally help buffer coastlines against the impacts of tropical storms and hurricanes.
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