Mangrove Ecology

Healthy mangrove forests are key to a healthy marine ecology. Fallen leaves and branches from mangroves contribute to the forest detritus and provide nutrients for the marine environment. Intricate food webs of immense varieties of sea life are supported directly through this detritus.
Mangroves are a critical forest ecosystem, dominating coastlines in tropical and subtropical regions of the globe. There are 54-75 species of true mangroves, which are found only in the intertidal zones of coasts, and are taxonomically isolated from terrestrial counterparts. They are highly adapted to their environment, capable of excluding or expelling salt, allowing mangroves to thrive in highly saline waters and soils. Salinity can still limit the distribution of mangroves, however, as can other environmental factors such as climate, tidal fluctuation, and sediment and wave energy. Mangroves are found worldwide, but the greatest species diversity is in Southeast Asia, with only twelve species inhabiting New World countries, and only four of those are found in the United States along the southern coast.

Mangrove Habitat and Growth

Mangrove distribution is primarily determined by sea level and its fluctuations. Other secondary factors are: air temperature, salinity, ocean currents, storms, shore slope, and soil substrate. Most mangroves live on muddy soils, but they also can grow on sand, peat, and coral rock.
Zonation often characterizes mangrove forests. Certain species occupy particular areas, or niches, within the ecosystem. Some mangrove species occur close to shores, fringing islands, and sheltered bays; others are found further inland, in estuaries influenced by tidal action.

Mangroves vary in height according to species and environment, from mere shrubs to 40 meter (app. 131 feet) tall trees. The prop roots of some mangrove species, such as Rhizophora spp., or red mangrove, and the pneumataphores (unique breathing roots) of others, such as Avicennia spp., or black mangrove, contain many small “breathing” pores, called “lenticels.” These allow oxygen to diffuse into the plant and down to the underground roots by means of air space tissue in the cortex, called “aerenchyma.” The lenticels are inactive during high tide.

Lenticels in the exposed portions of mangrove roots are highly susceptible to clogging by crude oil and other pollutants, attacks by parasites, and prolonged flooding from artificial dikes or causeways. Over time, environmental stress can kill large numbers of mangrove trees.

Evolutionary adjustments to varying coastal marine environments have produced some astounding biological characteristics within mangrove plant communities. Certain species of mangroves exclude salt from their systems, others actually excrete the salt they take in via their leaves, roots, or branches. In species that exclude salt, the mangrove root system is so effective in filtering out salt that a thirsty traveler could drink fresh water from a cut root, though the tree itself stands in saline soil.

Mangrove Reproduction

Certain mangrove species can propagate successfully in a marine environment because of special adaptations. Embryo germination begins on the tree itself, a process called “viviparity.” The tree later drops its developed embryos, called propagules, which may take root in the soil beneath. Viviparity may have evolved as an adaptive mechanism to prepare the propagules for long-distance dispersal, and survival and growth within a harsh saline environment. During this viviparous development, the propagules are nourished on the parent tree, thus accumulating the carbohydrates and other compounds required for later autonomous growth.

Propagules may float for extended periods (depending on the species), up to a year, and still remain viable. Viviparity and the long-lived propagules allow mangrove species to disperse over wide areas.
Mangrove Ecology Workshop Manual (Feller & Sitnik editors, pdf 1.23 MB)

The Mangrove Ecosystem

Mangroves are a critical forest ecosystem, dominating coastlines in tropical and subtropical regions of the globe. There are 54-75 species of true mangroves, which are found only in the intertidal zones of coasts, and are taxonomically isolated from terrestrial counterparts. They are highly adapted to their environment, capable of excluding or expelling salt, allowing mangroves to thrive in highly saline waters and soils. Salinity can still limit the distribution of mangroves, however, as can other environmental factors such as climate, tidal fluctuation, and sediment and wave energy. Mangroves are found worldwide, but the greatest species diversity is in Southeast Asia, with only twelve species inhabiting New World countries, and only four of those are found in the United States along the southern coast.

Spatial variation, or zonation, is a common trait for mangrove forests both horizontally and vertically. Certain species are found in monospecific bands parallel to the shore or in mosaics; however, patterns of distribution vary with location, both locally and regionally. There are many hypotheses about how and why zonation occurs, but no consensus has been reached. Interspecific variation is also quite high; mangrove height ranges from only a few feet to over one hundred feet and species exhibit different adaptations to salinity.