||Laguncularia racemosa (L.) Gaertn. f.
The white mangrove, Laguncularia
racemosa, is one of several species of trees known as mangroves
that occur along coastlines worldwide. There are approximately 55
species of true mangroves in 20 genera (Hogarth 2007), and another
60 or more species of mangrove associates. Most species occur throughout
the Indo-Pacific region. In the Indian River Lagoon, L. racemosa
is one of three true species of mangroves commonly occurring along
shorelines. The other two species are the red mangrove, Rhizophora
mangle, and the black mangrove, Avicennia
Laguncularia racemosa is
a medium-sized tree or shrub, covered in thick, scaly bark, often
reddish in color. The smooth, leathery leaves are up to 7 cm in
length, opposite, with a silvery to yellow-green cast. Oval in shape
and rounded at both apices, the leaves are often a distinguishing
characteristic, differentiating L. racemosa from other
mangrove species. White mangroves also exhibit unique glands called
extra-floral nectaries found on either side of the stem at the leaf
base. These structures excrete sugars which may attract ants that
protect the plant from herbivorous insects (Hogarth 2007). Flowers
are small and white, blooming at the leaf axils or branch tips.
Fruits are about 2 cm in length, greenish with longitudinal ribs.
HABITAT AND DISTRIBUTION
occurs in tropical and subtropical regions throughout the world.
Globally, the species ranges from Mexico, the West Indies to Brazil,
through Central America to Peru, South America to Ecuador, and West
Africa from Senegal to Angola (Exell 1958). In Florida, white mangroves
share similar geographical limits with the red mangrove, Rhizophora
mangle, having been reported as far north as Cedar Key on the
west coast (Rehm 1976), and as far north as Ponce de Leon Inlet
on the east coast (Teas 1977). Large populations can be found south
of Cape Canaveral on Florida's east coast and around Tarpon Springs
on the west coast (Odum & McIvor 1990).
White mangroves occur
throughout the Indian River Lagoon well above the high tide line,
generally upland of other mangroves and associated species. However,
they can be found intermingled with the black mangrove, Avicennia
germinans. Their distribution is often patchy and predominantly
occurs in the higher marsh areas (Ball 1980) along the lagoon, including
spoil islands, tidal creeks and mosquito impoundments.
LIFE HISTORY AND POPULATION BIOLOGY
Age, Size, Lifespan
Little is known regarding
typical age to maturation for mangroves in south Florida, though
the typical size of mature L. racemosa can reach or exceed
15 m in height (Odum & McIvor 1990).
is surpassed in abundance by both the red mangrove, R. mangle,
and the white mangrove, A. germinans in most areas of the
lagoon. However, large stands of white mangroves can be found in
patches, often in disturbed higher marsh areas.
White mangroves are
semiviviparous, with germination of seedlings starting while propagules
are still attached to the parent plant. The species is androdioecious,
with both hermaphroditic and male plants in a population (Tomlinson
1980, Landry & Rathcke 2007). Flowering occurs from May to December
in Florida, peaking in June and July (Tomlinson 1980). Male flowers
are typically open for one day, and hermaphroditic flowers remain
viable for two days (Landry & Rathcke 2007). Both flower types
produce nectar and are pollinated by a wide variety of insects.
However, hermaphroditic flowers have the ability to self-pollinate
(Rathcke et al. 2001, Landry 2005). Fruits, or propagules,
mature within a few months. Germination continues to completion
after the propagule drops from the parent tree and is dispersed
in the water.
Propagules of the white
mangrove are approximately 2 cm in length, flattened and lens-shaped.
Original coloration is pea-green, turning brown within days after
ripening and falling from the tree. Dispersal is facilitated by
the outer tissue layer, or pericarp, which acts as a float for the
propagule. To fully germinate, propagules must remain in the water
for a period of about eight days, and have a lifespan of approximately
35 days. Roots often begin to develop on floating propagules after
five days (Rabinowitz 1978).
The geographic range of L. racemosa is mostly tropical and subtropical, restricting
its distribution to areas where winter temperatures do not fall
below 20°C for extended periods of time (Waisel 1972). Temperature
stress can lead to changes in the size and structure of individual
plants and populations (Odum & McIvor 1990).
As facultative halophytes,
mangroves have the ability to thrive in waterlogged soils which
may have salinities ranging from 0 - 90 ppt. Mangroves do not require
salt, but flourish where other flora cannot thrive by utilizing
several different types of mechanisms for coping with highly salty
or hypersaline conditions. Unlike some other mangrove species, L.
racemosa takes in seawater through the roots, but then excretes
excess salt through pores, or salt glands, located on the surface
of leaves. In addition, alteration of the sodium/potassium levels
in the plant can help maintain osmotic balance, enhancing growth
in hypersaline waters (Sobrado & Ewe 2006). These adaptations
can allow salinity tolerance of L. racemosa to reach as
high as 105 ppt, depending on soil conditions (McMillan 1975).
Anoxia: Another major physiological
adaptation which increases success in mangroves is their ability
to thrive in anoxic soils. Like other mangrove species, L. racemosa
has hydrophobic pores on the surface of the trunk and above-ground
roots, called lenticels, which allow for the intake of oxygen into
porous tissue called arenchyma. These structures supply the plant
with oxygen to compensate for the roots that remain submerged in
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. An
average of 2 to 3 g dry weight of leaf litter is produced by mature
mangrove forests each day (Odum et al. 1982). This litter,
consisting of twigs, leaves, bark, fruit and flowers, is broken
down by bacteria and consumed by a wide variety of fauna inhabiting
mangrove ecosystems. Litter fall occurs throughout the year in Florida,
peaking at the beginning of the summer wet season and after periods
of stress (Heald 1969, Pool et al. 1975, Twilley et
In addition to propagule
dispersal, Ball (1980) suggested that competition among the three
mangrove species may be partially responsible for the zonation observed
in many mangrove areas. White mangroves thrive throughout intertidal
areas in the absence of large numbers of red and black mangroves.
However, white mangroves appear to dominate in higher areas because
of some competitive advantage over red mangroves.
Direct consumers of mangrove propagules
in Florida include the mangrove root crab (Goniopsis cruentata),
the swamp ghost crab (Ucides cordatus), the coffee bean
snail (Melampus coffeus) and the ladder hornsnail (Cerithidea
scalariformis). Consumers of mangrove leaves include G.
cruentata, the mangrove tree crab (Aratus pisonii),
the blue land crab (Cardisoma guanhumi)
and various types of insects.
Mangroves form intertidal
forests in which red mangrove prop roots, black mangrove pneumatophores,
and their associated peat banks serve as the dominant intertidal
substrata for other members of the mangrove community. All three
species are commonly found in association with one another. However,
segregation of the species does occur, with red mangroves typically
occupying the lowest intertidal position. Black and white mangroves
occur at slightly higher tidal elevations. White mangroves can be
distinguished from the other species by leaf shape, the presence
of extra-floral nectaries and the lack of either pneumatophores
or prop roots that occur in black and red mangroves, respectively.
In addition to other mangrove species, the buttonwood, Conocarpus
erecta, can be found in the landward edge of L. racemosa
stands. Several species of flora and fauna, including epiphytic
plants, insects, birds, reptiles and mammals occur in and around
Benefit to the IRL
Mangrove forest ecosystems
are vital as sources of energy, provide nursery habitat for juvenile
fishes and invertebrates, and are important as buffers in decreasing
storm impacts along coastlines. Additionally, they provide roosting
and nesting habitat for wading birds and serve as a source for timber
Because of their
vital role in providing nursery habitat for juvenile fishes, many
of which are commercially or recreationally important, mangroves
contribute significantly to the continuing success of Florida's
tourism and fishing industries.
Ball, MC. 1980. Patterns of secondary
succession in a mangrove forest of southern Florida. Oecologia
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Ann. Miss. Bot. Gdn. 45: 143-164.
Heald, EJ. 1969. The production of organic
detritus in a south Florida estuary. Ph.D. Thesis, Univ. of Miami.
Coral Gables, Florida, USA.
Hogarth, PJ. 2007. The biology of mangroves
and seagrasses. 2nd edition. Oxford University Press. New York,
USA: 273 pp.
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(Laguncularia racemosa) maintenance of a rare breeding
system through plant-pollinator interactions. Ph.D. Thesis. Ann
Arbor, MI, USA: University of Michigan.
Landry, CL & BJ Rathcke. 2007. Do inbreeding
depression and relative male fitness explain the maintenance of
androdioecy in white mangrove, Laguncularia racemosa (Combretaceae)?
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with salinity tolerances of black mangrove (Avicennia)
and white mangrove (Laguncularia) from North America.
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of the international symposium on biology and management of mangroves.
Honolulu, HI: East-West Center, 561-566.
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UCF Press. Orlando, FL, USA: 517-548.
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Pool, DJ, Lugo, AE & SC Snedaker. 1975.
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of mangrove propagules. Biotropica 10: 47-57.
Rathcke, BJ, Landry, CL & LB Kass. 2001.
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C & G Smith, eds. Proceedings of the eighth symposium on
the natural history of the Bahamas. San Salvador Island, Bahamas:
Gerace Research Center, 89-96.
Rehm, AE. 1976. The effects of the wood-boring
isopod, Sphaeroma terebrans, on the mangrove communities
of Florida. Environ. Conserv. 3: 47-57.
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characteristics of Avicennia germinans and Laguncularia
racemosa coexisting in a scrub mangrove forest at the Indian
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Report by: LH Sweat,
Smithsonian Marine Station at Fort Pierce
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Page last updated: 6 May 2009