Marine Biology

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.