Wild Florida: The Mangrove

The mangrove trees are an important part of Florida’s ecosystem–they provide habitat for many fish and other marine life, they form natural breakwaters which protect inland areas from winds and flooding during storms, and they colonize offshore areas to form new islands and new coastline.

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Red Mangrove prop roots

Worldwide, there are about 100 species known as “mangroves”. Biologically, these are not a single evolutionary group–trees from at least 20 different unrelated families, ranging from palms to hibiscus species, are known as “mangroves”, though the name is most properly applied to the trees of the family Rhizophoraceae, the “true mangroves”, with about 60 species. Through convergent evolution, all of the various versions of mangroves have developed the ability to live in the shallow coastal tropical marine environment, a place where few other trees can survive. These unique habitats are known as mangrove forests, and are found along much of the world’s tropical coastlines.

The greatest number of mangrove varieties are found in southeast Asia, particularly Indonesia, and it is likely that they first evolved there. Almost half of all the world’s mangroves are still found in Asia. Mangroves are very efficient at expanding their range, however, by making use of seawater as transportation, and they have now reached every tropical area in the world. About a dozen species are found in the western hemisphere.

All of the mangroves worldwide share a number of adaptations to allow them to survive in a coastal marine environment. The most visible of these characteristics are the distinctive “prop roots” which help hold the tree up in the mudflats and prevent it from being knocked over by wind or waves. In most mangroves, the props take the form of a network of arching stiltlike roots that grow outwards from the lower trunk and branch off into the mud: in some mangroves, the props take the form of flat boardlike projections from the trunk, giving it a sort of fluted appearance.  Another problem faced by mangrove trees is the thick black mud found along the coastal mudflats, which are poor in oxygen. (The subsurface is inhabited by anaerobic bacteria, and if you poke a stick into the mud you will release the distinctive aroma of hydrogen sulfide.) Many mangroves have therefore developed “pneumatophores”, long fingerlike projections from the roots which stick up above the mud, allowing the tree to draw oxygen from the air. Finally, mangroves must deal with the salt water that they live in. In most mangrove varieties, the roots have become adapted to extracting freshwater from the sea, excreting any excess salt from the leaves. Many species absorb fresh water during rainstorms and store it in thick fleshy leaves that look like small cactus pads, and which are coated with a waxy covering to prevent evaporation and help conserve fresh water.

The mangroves also depend on the sea to allow them to reproduce and spread by dropping seeds into the water. They can survive for as long as a year, carried off by the waves and currents, sometimes for hundreds of miles, until they encounter a bare patch of sand or mud and germinate. In many species, the fertilized seeds begin growing while still on the tree, forming a long pod known as a “propagule”. These grow at the ends of the branches until they fall off, where they float in the water. When the root end becomes stuck somewhere in shallow water, the tree begins growing, forming prop roots to hold it up. The props trap mud and sediment, anchoring the tree. Over time, as more mangroves join in, these sediments become deeper and higher, and form a tiny new island, which then grows to form new land area and shoreline.

Three species of mangrove form habitats in Florida: the Red Mangrove, the White Mangrove, and the Black Mangrove. Because they are very cold-sensitive, mangroves are rare in the US outside of Florida, though the more cold-tolerant Black Mangrove can be found in some parts of the Gulf Coast. In Hawaii there are no native mangroves, but introduced Red Mangroves have become established, where they are considered an invasive pest.

The Red Mangrove, Rhizophora mangle, tolerates flooding better than the others, and is often found forming small islets out in the shallow water with its arching prop roots. Unlike many mangroves, the Red Mangrove lacks pneumatophores, and instead can extract oxygen through the bark in its prop roots. It produces a large number of propagules and is often the pioneer that first arrives to form a new islet. The Black Mangrove, Avicennia germinans, often lives alongside the Red Mangrove, but since it lacks propagules it usually arrives later and inhabits slightly higher ground. The Black Mangrove produces numerous pneumatophores that allow it to breathe in the mudflats and when flooded by tides. The White Mangrove, Laguncularia racemosa, prefers to be a little higher in elevation than the other two species–it lacks both prop roots and pneumatophores, and prefers areas where the Red and Black Mangroves have already established themselves and built up a shoreline. The White Mangrove often forms a belt just behind the tidal mudflats where the other two species live. It has two distinctive salt glands at the base of each leaf.

The tangled jumble of mangrove roots in shallow waters provide a perfect habitat for small marine life. It is especially important for baby fish and larval crustaceans. A very large proportion of all coastal marine animals spend their youth among the protective root systems of the mangrove forests, where they are able to find food and escape from predators, and grow to a suitably large size before venturing out into the open sea. Mangrove root systems are also favored places for adult oysters to establish themselves, trapping more sediment and eventually extending outward to form long oyster bars.

Unfortunately, the coastal habitat used by mangroves is also a prime spot for human development. Mangrove forests have been under attack for decades, as humans clear them out for beachfront condos, hotels and marinas. In underdeveloped nations, mangroves are often replaced by commercial shrimp and fish farms. Worldwide, over half of the native mangrove habitat has been destroyed. In Florida, the percentage was even higher.

The effect everywhere was devastating. Fish and crab populations began to decline as the youngsters lost their habitats. With the loss of the protective belt provided by the mangrove forests, which had functioned as natural windbreaks and seawalls, tropical storms and hurricanes became worse, as winds and storm surge were stronger and able to penetrate further inland.

Today, efforts are being made around the world to protect and expand mangrove habitats. In many areas of Asia, mangrove tracts have been designated as protected national parks and conservation areas (the most famous of these is the Sundarbans National Park in India, which is prime habitat for the Bengal Tiger). There are many local grassroots programs, funded by environmental groups, to restore mangrove habitat by planting propagules and seedlings in suitable areas. In Florida, mangrove areas are now legally protected, to prevent them from being cleared by homeowners and developers.

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