Mangroves are both a genus of tropical and subtropical trees, the Rhizophora, and the family of plants to which the genus belongs, the Rhizophoracae.

The term most commonly refers to an assemblage of mangrove trees and other associated trees and shrubs that includes more than one hundred species. Such an assemblage may also be called a mangrove swamp, a mangrove forest, or a mangal.

Mangroves occur in shallow, protected coastal waters, such as flats in intertidal zones, bays, and estuaries in the tropics and subtropics. They cannot survive freezing or even consistently cold water. They are usually canopied forests up to about 10 meters tall, although in rare cases old-growth mangroves can reach 40 meters in height.

Mangroves survive in a difficult niche. They must be salt-tolerant plants (halophytes), and some can thrive in water of twice the salinity of seawater. Mangrove roots can filter salt from water intake, and those growing in the most saline areas excrete salt from their leaves. Mangrove forests shade from the most salt-tolerant species on their seaward side, to the least salt-tolerant species on their landward side, and they shade to conventional forests or freshwater plants (Lieth, 2008).

Mangrove mudflats tend to be oxygen poor (hypoxic), to contain toxic levels of sulfides, to be subject to periodic flooding, and to be very weak for holding trees. Consequently, mangroves have a maze of roots, some of which (pneumataphores) reach up out of the water to get oxygen, while some function as stilts to help brace the trees.

The mangrove niche between freshwater, dry land, and ocean is very small, perhaps only 0.1 percent of the Earth's surface. However, it is very important for three reasons: The maze of roots catches nutrient flows from the land and holds them for gradual release to the sea rather than short, polluting surges. Conversely, the roots protect the land from ordinary erosion and disasters such as hurricanes and tsunamis. Finally, the maze of roots holding leaves and nutrients from shore provides habitat and food for sea life. Young from as many as three-quarters of tropical deepwater fish species live in mangroves.

Theoretically, mangroves would benefit from global warming, because their tropical climate area would extend farther toward the poles. More mangroves would increase the capture (sequestration) of carbon from atmospheric carbon dioxide (CO2) and help reduce the greenhouse effect. Moreover, increased mangrove buffering of nutrient surges washing off the land would release a steadier gentle flow of dissolved organic compounds, allowing plankton and other marine plants to capture more CO2 from the air and to increase the oceans' reflectivity. Thus, mangroves would create a significant negative feedback retarding global warming.

In practice, however, the decline of mangroves due to human development may be a significant positive driver of global warming. The percentage of Earth's surface covered by mangroves has been cut in half (from 0.2 percent to 0.1 percent) in the last century, mostly as a result of human development (Hogarth, 2007). For instance, sand dredged from Biscayne Bay, Florida, buried a mangrove swamp and built a city--Miami. Recent increases in shrimp aquaculture have also caused major destruction of mangroves. Major warming would also raise sea levels, causing mangroves to retreat from the deeper water and attempt to colonize new inland swampy areas.

However, people would likely erect dikes to protect existing structures and agricultural land. Hence, the effective surface area available for mangroves would be constricted even further. This reduction in surface area would decrease the area of ocean fertility, the corresponding amount of carbon sequestration, and the buffering of the land against ocean disasters. Indeed, the damage to New Orleans caused by Hurricane Katrina in 2005 can be partly attributed to reduced swamplands around the city, which left the dikes more vulnerable (although the major causes of bayou losses in New Orleans were subsidence and navigation channels).

A number of groups have tried to replenish mangroves and even establish them in new areas. The best-known of these is the Manzanar Mangrove Initiative, which is backed by the Eritrean government on the East African coast. (It was largely sparked by Gordon Sato, who was interned during World War II at Manzanar, California.) The Manzanar Mangrove Initiative uses small amounts of fertilizer in bags for slow release to vastly increase the area of mangroves and resulting silage for livestock.

An extension of the Manzanar Mangrove Initiative would pump seawater into desert areas to vastly increase mangrove areas, food production, and capture of CO2 from the atmosphere. However, the required pumping would require major investments in either some form of alternate energy or fossil-fuel- fired power for the pumping. Moreover, skeptics of the Manzanar Mangrove Initiative worry that overzealous fertilization of the mangroves might damage coral reefs further out to sea.

References

1. Hogarth, Peter J. The Biology of Mangroves and Seagrasses. 2d ed. New York: Oxford University Press, 2007.

2. Lieth, Helmut, Maximo Garcia Sucre, and Brigitte Herzog, eds. Mangroves and Halophytes: Restoration and Utilisation. New York: Springer, 2008.

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