IELTS Academic Reading Practice Test 8: Passage 1

On many sheltered tropical coastlines, where fine mud can settle, forests stand with their roots in the sea. These are mangroves. The name covers not one kind of tree but several dozen species of trees and shrubs that share one remarkable ability: they can live between the high and low tide marks, where they are flooded by salt water once or twice a day. These species belong to a number of plant families that are not closely related, so what unites them is not their ancestry but their way of life. Most grow between the Tropic of Cancer and the Tropic of Capricorn, but a few hardy species survive much further from the equator, in parts of southern Australia and northern New Zealand.

Life in this zone poses two problems. The first is salt. Sea water contains enough salt to kill most land plants, because salty soil tends to draw water out of their roots instead of letting them absorb it. The red mangrove filters out the great majority of the salt at the surface of its roots, so the water travelling up to its leaves is nearly fresh. The black mangrove, by contrast, takes in more salt but then gets rid of it through special glands on its leaves; on a dry day, tiny crystals can be seen glittering on the leaf surface. Some species also move salt into older leaves, which then drop off.

The second problem lies underground. The waterlogged mud in which mangroves grow contains very little of the oxygen that roots need. The black mangrove therefore sends up hundreds of thin, upright roots called pneumatophores, which stick out of the mud around the tree like rows of pencils. These are covered in tiny openings known as lenticels, through which air enters whenever the tide is out. The red mangrove has a different solution: arching prop roots that grow out from its trunk and curve down into the mud. As well as taking in air, these hold the tree steady in soft ground that could not support an ordinary root system.

Mangroves also reproduce in an unusual way. Most plants' seeds lie inactive on the ground until conditions are suitable. In many mangroves, however, the seed starts to grow while it is still attached to the parent tree, a process known as vivipary. In the red mangrove, this produces a long, pencil-shaped seedling that keeps growing on the branch before it drops. If it falls at low tide, it may lodge in the mud and quickly put out roots. If it lands in water, it floats and can survive for many months while currents carry it along the coast or across the open sea. This is how mangroves have reached remote islands throughout the tropics.

For centuries, mangrove swamps were widely regarded as unhealthy wastelands, fit only to be cleared. Today they are recognised as some of the most productive ecosystems on Earth. The tangle of roots beneath the water shelters young fish, shrimps and crabs, many of which later move out to sea or to nearby coral reefs, so fishing communities some distance away may depend on mangroves without realising it. The Sundarbans, the world's largest continuous mangrove forest, which straddles the border between Bangladesh and India, is also one of the last strongholds of the Bengal tiger.

Mangroves also protect the coast itself. Their dense roots slow the movement of water, reducing the height of waves and trapping mud carried by rivers and tides, so that in some places the shoreline gradually extends outwards. They are remarkably good at storing carbon, too. Because their soils contain so little oxygen, fallen leaves and dead roots decay extremely slowly, and over centuries large amounts of carbon build up in the mud. A hectare of mangrove forest can hold several times as much carbon as a hectare of typical tropical forest on land, and most of it is stored below ground rather than in the trees.

Despite these benefits, mangroves have been cleared on a vast scale. In many countries, the main cause in recent decades has been the digging of ponds for farming shrimp, much of it for export, but land has also been taken for rice fields, salt production, ports and tourist resorts. When a forest is removed, much of the carbon in its soil may be released into the atmosphere, and coastal communities lose both a source of food and their natural defence against the sea.

Many governments and conservation groups have responded by planting mangroves, often with disappointing results. Seedlings were frequently placed on open mudflats lower down the shore than mangroves naturally grow, where they were under water for too long each day and soon died. Often, too, a single fast-growing species was used, creating plantations with far less wildlife than a natural forest. Specialists now argue that it is usually better to restore the conditions mangroves need, for example by reopening blocked channels so that tidal water can flow freely again. Where this is done, seeds carried in on the tide often take root without any planting.