Geography and distribution
Rhizophora mangle occurs worldwide between 28 degrees north and south of the equator. Temperature appears to be the factor limiting its distribution outside of this latitudinal range; R. mangle is killed by frost and cannot survive long periods of near-freezing temperatures.
Red mangroves are native to the western coast of Africa, and the eastern and western coasts of subtropical and tropical America. They have also been introduced to Hawaii, USA, and Queensland, Australia.
A development series to illustrate the fruit through to the young seedling of red mangrove from the island of Anegada in the British Virgin Islands (Photo: Angela Easterling)
Rhizophora mangle is an evergreen small shrub to medium-sized tree, growing up to 20 m in favourable conditions.
Its most prominent feature is an array of aerial stilt roots (props) arising from the main trunk at 2.0–4.5 m height. These roots not only anchor and stabilize the plant in the shifting intertidal zones (substrates) but also play an important role in gaseous exchange. Abundant pores, called lenticels, on root surfaces connect to spongy internal tissues that can hold large volumes of air. On tidal submergence, the lenticels close, oxygen is used by the plant and carbon dioxide is absorbed into the seawater. This results in lower pressure within the root system. When the tide falls and the lenticels re-open, air is sucked into the roots to rapidly replenish vital oxygen supplies.
Its bark is grey-brown and smooth when young, becoming more furrowed when mature. The thick, leathery leaves are arranged opposite each other and have a simple, elliptic (oval) shape with smooth margins (edges). They are glossy dark green above with a lighter, yellow-green underside, and are usually 6–12 cm in length and 2.5-6.0 cm wide.
Red mangroves produce hermaphroditic flowers (male and female sex organs in the same flower) throughout the year. Flower production peaks in the wet season in equatorial regions and spring to early summer in subtropical regions. The 2 cm long, bell-shaped, yellow-green flowers arise on short, forked stalks from the leaf axils and are wind- or self-pollinated. The fruit is a reddish-brown berry around 25 cm long and 12 mm in diameter.
Rhizophora mangle is a viviparous species: its seeds readily germinate while still attached to the mother plant, producing seedlings 20–25 cm long. As soon as seedlings (propagules) are ready they detach and fall to the ground. If they fall into the sea or brackish water, they can float for up to a year before settling in a suitable location, where they promptly root. Red mangrove propagules are often distributed over long distances by water currents – an important dispersal mechanism for this species.
Rhizophora samoensis occurs in the eastern tropical Pacific. It is closely related to R. mangle and the two were previously considered to be the same species. They can only be distinguished by minor anatomical differences in the flower’s calyx, that is the sepals that enclose the flower. R. samoensis has bracts (modified leaves) twice as wide as the pedicel (stalk within the inflorescence) and R. mangle has small or no bracts at all.
A 'living armour'
Roots create a 'living armour'.
Of the numerous mangrove species, red mangrove grows closest to the sea and has stilt (or prop) roots adapted to withstand total submersion in salt water and the action of tidal waves. These stilt root thickets form important habitats for a variety of marine life, particularly as nursery grounds for young fish. They also act as ‘living armour’, protecting tropical coastlines by dissipating the energy contained in storm waves breaking on the shore.
However, there is only a certain energy level up to which mangroves can dissipate energy and mangroves are destroyed by large waves and high winds. The jury is out on whether or not mangroves do really dissipate tsunami waves – and these long waves are different in character from shorter, steeper storm waves.
Threats and conservation
Red mangrove is regarded as an invasive alien species in some parts of the world where it has been introduced for coastal protection and stabilization of muddy foreshores (Hawaii and Queensland, Australia). Here it has begun to outcompete native coastal vegetation through the formation of dense thickets. However, mangrove habitats are globally in decline for various reasons: agriculture, urban development, land and water pollution, and the spread of tourist facilities and shrimp farms.
Red mangrove provides a number of useful products. Its timber is used for traditional house building (for poles, beams, rafters and flooring) and underground mine supports. Its wood is also used for making canoes, fences and fishing spears. It is burnt as fuel (either as firewood or charcoal). The bark is used to make a reddish-brown or black dye and contains tannins used for tanning leather. The bark fibres are pounded to make cordage.
Red mangrove has a number of traditional medicinal uses. For example, the bark is used for treating angina, boils and fungal infections, whereas leaves and bark are antiseptic and used to treat diarrhoea and dysentery.
Mangroves are important for erosion control, land reclamation and soil stabilisation around tropical coastlines. They provide natural 'nursery grounds' for many species of fish and other marine life, thereby supporting many commercial fisheries. By trapping sediments, mangroves reduce the turbidity (muddiness) of coastal waters, allowing development of healthy off-shore coral reefs, well known for their spectacular biological diversity – the marine equivalent of tropical rain forest. Such ‘biofiltration’ can also remove excess nutrients and other pollutants, most notably from sewage discharges and aquaculture operations.
Finally, high rates of productivity of mangrove forests, and accumulation of peaty sediments, may help to reduce carbon emissions and provide an important sink for carbon.
Red mangrove foliage (Photo: Magdalena Firganek)
Mangroves are relatively easy to cultivate in Kew’s glasshouses at a constant temperature above 20°C with high air humidity. The preferred compost is loam, which should be kept constantly damp or even submerged in water. Mangroves need high levels of light and as much exposure to direct sun as possible. This is especially important for young saplings, which benefit from supplementary artificial lighting over the winter months, until they reach a good size and can cope with the low light levels of the British winter.
Remarkably, salt or sea water is not necessary at all. Although mangroves can cope with the stress of salinity, it is not a requirement for them. They benefit greatly from a daily shower of pressurized water, which will detach any leaves about to drop, wash out pests such as mealy bugs and scale insects, and lubricate newly developing buds so that they unfurl with ease.
This species at Kew
A specimen of Rhizophora mangle is held in Kew’s Herbarium, where it is available to researchers from around the world by appointment. The details of this specimen can be seen online in the Herbarium Catalogue.
Specimens of Rhizophora mangle wood, bark and resin are held in the Economic Botany Collection in the Sir Joseph Banks Building and are available to researchers by appointment.