Laguncularia racemosa (L.) Gaertn. f.
White Mangrove (Laguncularia racemosa) is found along the shores of Florida and the West Indies, along both coasts of the American tropics, and in tropical West Africa (Little and Wadsworth 1964). It often occurs together with other mangrove species, such as Red Mangrove (Rhizophora mangle) and Black Mangrove (Avicennia germinans).
In their large area of geographic overlap, White Mangrove (Laguncularia racemosa) and Black Mangrove (Avicennia germinans) both have erect breather roots (pneumatophores) protruding up out of the water, but those of White Mangrove are fewer in number, wider, and more often branched. White Mangrove grows landward of Red Mangrove (Rhizophora mangle), which has conspicuous prop roots, and Black Mangrove. It often occurs onshore with Buttonwood (Conocarpus erectus, like White Mangrove a member of the plant family Combretaceae), which may be easily distinguished from White Mangrove by the fact that Buttonwood has alternately arranged leaves and leafstalk glands that are less prominent than those of White Mangrove. (Petrides 1988) The position and appearance of the glands on both the petiole and on the leaf blade differ conspicuously between White Mangrove and Buttonwood (see images on this page and on the Buttonwood page).
An excellent resource for identifying the mangroves of Florida can be found at http://www.selby.org/
White Mangrove has opposite, leathery, slightly fleshy, elliptic leaves 4 to 10 cm long and 2 to 5 cm wide, rounded at both ends, dull yellow green on both sides and borne on reddish petioles (leaf stalks) with two raised gland dots near the apex. The numerous small, stalkless, bell-shaped whitish flowers are about 0.5 cm long and borne in terminal and lateral clusters 5 to 10 cm long. The clustered fruits are velvety gray-green and slightly pear-shaped, 1.5 to 2 cm long, flattened and ridged. (Little and Wadsworth 1964)
Gilbert et al. (2002) studied the possible role of salt excretion by mangroves as a defense against pathogenic fungi in a mangrove forest in Panama. Although presumably evolved for other reasons, salt excretion by leaves of some mangrove species may serve as an important defense against fungal attack, reducing the vulnerability of typically high-density, monospecific forest stands to severe disease pressure. In their study, Gilbert et al. found that Black Mangrove (Avicennia germinans) suffered much less fungal leaf damage from than did White Mangrove (Laguncularia racemosa) or Red Mangrove (Rhizophora mangle). Black Mangrove leaves also supported the least fungal growth on the leaf surface, the least endophytic colonization, and the lowest fungal diversity, followed by White Mangrove and Red Mangrove.
Host specificity of leaf-colonizing fungi was greater than expected at random. The fungal assemblage found on Black Mangrove appears to be a subset of the fungi that can grow on the leaves of Red and White Mangrove. The authors suggested that the different salt tolerance mechanisms in the three mangrove species may differentially regulate fungal colonization. The mangroves differ in their salt tolerance mechanisms such that Black Mangrove (which excretes salt through leaf glands) has the highest salinity of residual rain water on leaves, White Mangrove (which accumulates salt in the leaves) has the greatest bulk salt concentration, and Red Mangrove (which excludes salt at the roots) has little salt associated with leaves. The high salt concentrations associated with leaves of Black and White Mangrove, but not the low salinity of Red Mangrove, were sufficient to inhibit the germination of many fungi associated with mangrove forests. The authors suggest that efficient defenses against pathogens may be especially important in natural communities, such as mangrove forests, where host diversity is low and the density of individual hosts is high – ideal conditions for diseases to have strong impacts on plant populations.
Mangrove forests are unusual among tropical forests for their low tree species diversity and associated high population density of individual species. Mangrove species are unusual in their ability to grow in flooded, saline soils and for the array of mechanisms they have evolved to tolerate high salt concentrations. The work by Gilbert et al. suggests that some mangrove species may also be unusual in their escape from strong disease pressures, even when growing at high densities, through the inhibitory effects of
high foliar (leaf) salt concentration on fungal infection. (Gilbert et al. 2002)
The leaves of mangroves in general have abundant and diverse glands and their function and physiological and ecological importance appear to be poorly known. The glands on the petioles (leaf stalks) of White Mangrove and Buttonwood, for example are extrafloral nectaries (i.e., structures outside of flowers that secrete nectar), but the ecological significance of these structures seems not to have been investigated (these glands are not salt-secreting structures, as is sometimes claimed). The glands on the leaf blade of White Mangrove accumulate salt, but it is not secreted to the leaf surface, so in contrast to a Black Mangrove leaf, a White Mangrove leaf does not taste salty when licked. It is sometimes stated that the glands on the leaf blade of White Mangrove serve as domatia (special shelters) for beneficial mites. While this may be true, this appears to be another intriguing topic that remains unstudied.
White Mangrove commonly reaches 12 meters in height, sometimes more (Little and Wadsworth 1964).
White Mangrove occurs along the shores of central and southern Florida, including the Florida Keys; in Bermuda and most of the West Indies; on both coasts of continental tropical America from Mexico south to Ecuador, northwestern Peru, and Brazil; and in western tropical Africa (Little and Wadsworth 1964)
The slightly flesht fruit (a drupe) of White Mangrove floats and is dispersed by water. The fruit contains a single large seed which starts to enlarge (and sometimes to germinate) within the fruit while still on the tree or floating in the water. White Mangrove grows rapidly and may flower and fruit when less than 2 years old. (Little and Wadsworth 1964)
Some White Mangrove populations are androdioecious (i.e., with separate male and hermaphrodite individuals), while others lack male plants. Landry et al. (2009) surveyed 65 populations in Florida and the Bahamas. Because White Mangrove fruits are water-dispersed, the observed distribution of breeding systems was compared to local and regional water currents in order to determine whether dispersal could be important to the maintenance of male plants in androdioecious populations. Twenty-two of the 36 populations surveyed in Florida were androdioecious, with male frequencies that ranged from 1 to 68%. On the east coast of Florida, all populations north of latitude 26 degrees 30' N lacked males, while all populations south of this latitude were androdioecious, suggesting that northern populations may lack males due to dispersal limitation. The pattern of distribution on the west coast of Florida suggests that males may be maintained in some populations via dispersal. Nine islands in the north-central Bahamas were surveyed. and androdioecious populations were found only on San Salvador island, where male frequencies ranged from 5 to 28%. Landry et al. discuss the possible roles of dispersal, fragmentation, and selection in explaining the observed pattern of distribution of androdioecious and pure hermaphrodite populations.
Gilbert and Sousa (2002) studied the host-associations of wood-decaying basidiomycete polypore fungi on three mangrove species (Rhizophora mangle, Avicennia germinans, and Laguncularia racemosa) in a Panamanian mangrove forest. They note that the pattern typically observed for these fungi in diverse tropical forests is that there are a large number of rare species, with the smaller number of common species necessarily being nonspecialists due to the challenge of host rarity. In contrast, the authors found that in the tropical mangrove forest they studied, the polypore assemblage was strongly dominated by a few host-specialized species. Three fungal species, each with a strong preference for a different mangrove host species, comprised 88 percent of all fungi collected (the authors note, however, that these fungi are all reported from other hosts outside of mangrove forests as well). At least for polypore fungi within tropical mangrove forests, where host diversity is low and the abundance of individual host species is high, the restriction against host specialization typically imposed by host rarity in tropical forests may be relaxed, resulting in a polypore community dominated by a few common host-specialist species. (Gilbert and Sousa 2002) One polypore found to be locally specialized on White Mangrove in the study by Gilbert and Sousa (2002), Datronia caperata, was found only on this host in several other Panamanian mangrove forests as well (Parrent et al. 2004).
Smith et al. (2009) studied the effects of fiddler crabs (Uca rapax and Uca pugilator) on the growth of White Mangroves in a restored Florida marsh. They found that the presence of crab burrowing increased final tree height by 27%, final basal trunk diameter by 25%, and final leaf production by 15% over mangroves growing where crabs were removed and excluded. They also observed significant positive associations between mangrove production and crab burrow density. Crab burrows accounted for 24, 29, and 16% of the variation in mangrove height, trunk diameter, and leaf production, respectively.
Bees make good honey from the flowers of White Mangrove (Elias 1980).
- Conocarpus racemosa L. (synonym)