Species Description: 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 germinans.
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.
Regional Occurrence: Laguncularia racemosa 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).
IRL Distribution: 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.
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).
Abundance: Laguncularia racemosa 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.
Reproduction: 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.
Dispersal: 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).
Temperature: 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).
Salinity: 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 anoxic soils.
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. 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 al. 1986).
Competitors: 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.
Associated Species: 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 white mangroves.
Special Status: Habitat structure
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 production.
Economic Importance: 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.
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