is characterized by its opposite leaves which are narrow and elliptical in
shape; often found encrusted with salt. Propagules are small (2-3 cm in
diameter) and bean-like, flattened in shape. 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.
II. HABITAT AND
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.
In the Indian River Lagoon, it is a
common landscape feature to approximately 28 degrees North, around the vicinity
of Merritt Island, Florida. North of this location, there is 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.
III. LIFE HISTORY AND POPULATION BIOLOGY
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.
The black mangrove is considered
abundant to common in the Indian River Lagoon, as well as throughout much
of its range.
Propagules of the black mangrove detach
from the parent tree upon ripening and may float in salt water for approximately
one year without rooting.
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.
IV. PHYSICAL TOLERANCES
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.
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.
V. COMMUNITY ECOLOGY
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.
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.
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.
VI. SPECIAL STATUS
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.
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
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Smithsonian Miscellaneous Collections 52(UML
Report by: K. Hill,
Smithsonian Marine Station
with thanks to Gary Raulerson, LSU and C. Feller, NMNH
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