Trees That Live in Water

BY AWAKE! WRITER IN AUSTRALIA

They provide refuge for many endangered species of mammals, birds, and reptiles. They also preserve the environment by filtering pollutants from the water. In south Florida, U.S.A., some 75 percent of game fish and 90 percent of fish caught for commercial purposes depend on them. And they create a barrier that protects coastal areas from storms and tides. What are they? Mangroves!

FOUND along more than half the world’s tropical seashores, mangroves are a type of tree, or shrub, that includes members of several families. They generally grow in the intertidal area between land and sea where the water is a mixture of seawater and freshwater. Although the water there is much saltier than most plants can stand, mangroves handle the conditions with ease. How? By using several fascinating methods​—sometimes in combination.

Surrounded by Salt

Some mangroves, known as salt-excluders, have filters that prevent the entry of salt through the root surface. They are so effective at excluding salt that a thirsty traveler can obtain freshwater by breaking open the root of such a mangrove. In other species, the mangrove lets the salt into its system and accumulates it, depositing the salt in old leaves or other parts of the plant, which then drop away.

Still others are salt-secreters, letting the salt into the plant but then quickly secreting it, usually through special salt glands on the leaves. If you were to lick the leaf of such a mangrove, it would taste very salty. But be careful which mangrove you choose! The latex from the leaves of the blind-your-eye mangrove can cause temporary blindness if it gets into your eye. However, the latex has medicinal properties and has been used to treat sores and stings.

How They Survive

In order to survive and thrive, most plants need well-aerated soil. Yet, the soil where mangroves live is generally waterlogged. The secret to their survival is their aerial roots, which develop above the ground and are thus able to take in air directly from the atmosphere. These roots come in a variety of shapes. Some, called knee roots, grow out of and then back into the soil, forming knobbly humps that look a little like bent knees.

Snorkel, peg, or pencil roots protrude vertically out of the soil. Prop roots, which later become stilt roots, branch off the mangrove’s lower trunk. Plank, or ribbon, roots radiate from the base of the tree in curving upright ridges, with the top portion above the soil. These different root systems not only allow the plants to breathe but also provide stability in the soft soil.

How They Reproduce

The cannonball mangrove has a large round fruit packed with irregularly shaped seeds. On ripening, the fruit explodes, dispersing its seeds into the water. Some float away with the tides, eventually finding an area in which to germinate.

Seeds of other mangroves germinate while still attached to the parent tree. This is something extremely unusual in the plant world. These mangroves bear seedlings that drop  from the tree into the water and may then drift for several months or even up to a year in search of a home.

The way the seedling floats maximizes its chances of settling in a brackish spot, which is its ideal environment. It floats horizontally in the more buoyant salty water, but when it enters brackish water, it floats vertically and is thus more likely to lodge in the mud.

A World Within a World

Mangroves form the basis of a complex food web. Leaf litter and disintegrating vegetation from the mangroves are a source of food for microorganisms, which, in turn, serve as food for other animals in the food chain. Many living things make the mangrove forest their home, their feeding ground, their breeding habitat, or their nursery.

For example, hundreds of species of birds use the mangrove habitat as nesting or feeding sites and as resting spots during migration. The country of Belize alone hosts more than 500 different bird species in its mangroves. Many fish either commence life in the mangroves or depend upon the mangrove ecosystem for food. Over 120 species of fish have been caught in the Sundarbans mangrove forest, between India and Bangladesh.

Plant life also thrives in the mangrove community. On the east coast of Australia, 105 different lichen species have been found growing on mangrove trees. Many ferns, orchids, mistletoes, and other plants also  prosper in the habitat. Indeed, the mangroves of the world provide an indispensable service for flora and fauna​—from lichens to tigers—​and also for humans.

A Myriad of Benefits to Man

Apart from helping to preserve the environment, mangroves are a direct or indirect source of many products, including firewood, charcoal, tannin, fodder, and medicine. The habitat also provides food delicacies, such as fish, crustaceans, mollusks, and honey. Some sailors, in fact, once thought that oysters grew on trees because they could be gathered from the mangroves easily when the roots were exposed at low tide.

Mangroves also provide products for such industries as paper, textile, leather, and construction. Other industries that benefit from them include fishing and tourism.

Though there is growing appreciation for their importance, mangrove forests are diminishing at an estimated rate of 400 square miles [100,000 ha] a year. Often they are destroyed to make way for seemingly more profitable projects, such as agricultural and housing developments. Many people view a mangrove swamp simply as a muddy, malodorous, and mosquito-ridden place that is better avoided.

The truth is, however, that mangroves serve valuable, even lifesaving, purposes. Their special adaptive aerial roots and salt-filtering tap roots have established rich and complex ecosystems. They are vital to inshore fisheries, wood-products industries, and wildlife. And they protect areas of coastline from erosion by absorbing the force of powerful hurricanes that might otherwise kill thousands of people. Surely we should be grateful for mangroves!

[Box/​Picture on page 24]

Hunting for Wild Honey in the Mangroves

The largest of the world’s mangrove forests is in the Sundarbans, part of the vast Ganges Delta, which straddles India and Bangladesh. Among the people living there are the Mowalis, who depend upon mangroves for their livelihood. Theirs is one of the highest-risk professions in the country.

Mowalis are honey hunters. In April and May each year, they venture into the shifting landscape of the mangroves to search for the honeycombs of the giant honeybee. The bees are large, growing to about an inch and a half in length. And they are aggressive, having been known to kill elephants!

So honey hunters carry torches made of mangrove vegetation, the smoke from which disperses the bees. Wise honey hunters leave part of the hive behind so that the bees can rebuild it, thereby sustaining the honey yield from year to year.

The bees are not the only threat to the honey hunters. Others include crocodiles and poisonous snakes, which inhabit the mangroves. Also, thieves may wait to ambush honey hunters as they leave the forest with their honey and wax. While these are a threat, the greatest peril is the Royal Bengal tiger. Every year, these animals kill between 15 and 20 honey hunters.

[Credit Line]

Zafer Kizilkaya/​Images & Stories

[Pictures on page 23]

Mangroves and their saplings thrive in an environment that would kill most other plants

[Credit Lines]

Top right: Zach Holmes Photography/​Photographers Direct; lower right: Martin Spragg Photography (www.spraggshots.com)/​Photographers Direct