Avicennia germinans (L.) L.
Black Mangrove (Avicennia germinans) has a broad tropical distribution. Avicennia includes about 15 species, mostly restricted to coastal tidal regions in the tropics (Elias 1980). Tree and shrub species are described as mangroves based on their ecology rather than evolutionary relatedness, so mangrove species represent numerous plant families.
Black Mangrove groves are virtually impenetrable because of the dense branches. The trees produce numerous upright, unbranched roots (pneumatophores) above water and around the edges of the trees to provide the extensive root system with air. These pneumatophores also trap detritus brought in by the tides. (Elias 1980)
An excellent resource for identifying the mangroves of Florida can be found at http://www.selby.org/
Black Mangrove has opposite, oblong to elliptical, evergreen leaves, 5 to 12 cm long and 2 to 4 cm wide, with smooth, slightly curled margins; the leaves are hairy below. The upper leaf surface is yellow-green and often shiny, the lower surface gray-green, often with scattered salt crystals apparent on both surfaces. The small 4-lobed white flowers, about 0.5 cm long and 1 cm across, are borne in terminal clusters up to 4 cm long. The fruit is a compressed (flattened) 2-valved and 1-seeded capsule 3 to 5 cm long that is yellow-green and finely hairy, with unequal sides. The bark of larger trees is dark reddish brown and scaly, with orange-red inner bark sometimes exposed between the scales. (Little and Wadsworth 1964; Brockman 1968; Elias 1980)
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)
In Florida and the adjacent Gulf Coast, Black Mangrove reaches about 10 meters, but in much of its broad range it may grow to more than twice this height (Brockman 1968; Elias 1980).
Ecologically, tropical mangrove swamp forests share many similarities with salt marshes to the north (although mangroves are woody and salt marshes are generally dominated by grasses and other herbaceous vegetation). Both mangrove swamps and salt marshes occur at the interface of land and sea, protect the coast from storm damage (especially hurricanes), and serve as important nurseries for fish and invertebrates. Mangrove leaves are an important source of energy for marine food webs: fallen leaves are colonized by bacteria, fungi, and protozoans, which are in turn fed upon by zooplankton, which in turn are consumed by juvenile fish and larval invertebrates. (Kricher 1988)
In southern Florida and the Caribbean, Black Mangrove forms dense thickets just inshore of Red Mangrove (Rhizophora mangle) (Brockman 1968). In one of the best studied mangrove regions, the Caribbean, Rhizophora mangle typically grows in a pure stand at the seaward forest edge. About 10 to 20 m from the water's edge, Laguncularia racemosa (White Mangrove) joins the canopy, forming a nearly even mixture with Rhizophora in the low intertidal. Avicennia germinans enters the canopy in the mid-intertidal, creating a mixed canopy of the three species, and it then gradually monopolizes most upper intertidal stands. Laguncularia often reappears in the canopy near the upland edge, growing as scattered individuals or small monospecific stands along the mangrove– forest ecotone. Although at one time this spatial distribution of the different mangrove species was presumed to be attributable to spatial gradients in factors such as salinity, a variety of experimental and other data have indicated that differences among species in their tolerance of different environmental conditions is insufficient to explain the observed zonation. (Sousa et al. 2007 and references therein)
Black Mangrove (Avicennia germinans) is very widely distributed along tropical silty seashores of Bermuda; throughout most of the West Indies; in the southeastern U.S. along both coasts of northern Florida to the Florida Keys, Mississippi, Louisiana, and Texas; along both coasts of Mexico south along Central America to Ecuador, northwestern Peru, the Galapagos Islands, and Brazil; and along the west coast of Africa. (Little and Wadsworth 1964)
The Black Mangrove (A. germinans) is distributed along the tropical and subtropical coasts of the American continent, the Caribbean islands, and West Africa. Three geographical units can be defined, including east Pacific (American Pacific), west Atlantic (American Atlantic and Caribbean), and east Atlantic (West Africa) (Nettel and Dodd 2007)
Avicennia germinans is a widespread mangrove species occupying the west coast of Africa and the Atlantic and Pacific coasts of the Americas from the Bahamas to Brazil and Baja California to Peru (Dodd et al. 2002).
Black Mangrove reaches its northern limit in the northern hemisphere in Florida, Louisiana, and Texas, where in recent decades it has been moving northward into temperate salt marshes typically dominated by the salt marsh grass Spartina alterniflora. In Louisiana marshes, Black Mangroves were historically restricted to the southernmost barrier islands and beaches by winter freeze events. However, in recent years freeze-free winters have facilitated a noticeable expansion of Black Mangrove northward into Spartina marshes. Nearly two decades of warm winter temperatures in coastal Louisiana have facilitated this northward expansion. (Perry and Mendelssohn 2009)
Black Mangrove seeds often germinate and split open the fruit while still on the parent tree (Elias 1980).
The seedlings of Black Mangrove are often subject to heavy predation by various species of mangrove crabs (McKee 1995; Lindquist et al. 2009 and references therein).
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)
Evolution and Systematics
Genetic studies of Black Mangrove have revealed closer similarities between populations of Atlantic South America and those of the east Atlantic (West Africa) than between Atlantic South America and Atlantic North America (Dodd et al. 2000). Levels of genetic diversity vary considerably among populations, but are generally higher in populations from the east Atlantic. Regional differentiation between the Pacific coast and Atlantic populations is greater than between east and west Atlantic populations, suggesting that the Central American Isthmus has had an important influence on population genetic structure in this species. The lower level of divergence of east Atlantic from west Atlantic populations and results from detailed genetic analyses are consistent with dispersal of propagules across the Atlantic Ocean during the Quaternary (Dodd et al. 2002; Nettel and Dodd 2007).
The fragrant white flowers of Black Mangrove are rich in nectar and honeybees make excellent honey from them (Elias 1980; Petrides 1988).
- Avicennia nitida Jacq. (synonym)
- Bontia germinans Linnaeus (synonym)