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Section I: WaterFlooded Forests
Although rainforest is defined as an area with almost constant rainfall, the amount of rainfall is not steady throughout the year. In most rainforest areas, there are dry seasons when it rains less and rainy or monsoon seasons when it rains much more heavily. As a result, river levels rise and fall markedly. The level of the Rio Negro at Manaus harbor, for example, can vary as much as 46 feet (14 m) between the lowest point in the dry season and the crest in the rainy season. Because of this extreme fluctuation, many tropical rivers overflow their banks and inundate large areas of forest. These flooded forests are quite different from forests on high ground that are never reached by floodwaters. The trees and shrubs of inundated forests have to stand with their roots and often their trunks in water for several months. Only some species have adapted to these stressful conditions, and so flooded forests tend to contain far fewer species of plants than do upland forests. The length of the flooding period also causes considerable variations in the inundated forests. Flooded forests occur in all parts of the tropics, such as in the Mekong River basin in Vietnam and along the Sepik River in New Guinea, but again by far the largest area is in the Amazon basin.
The inundated forests of Amazonia have been divided into various categories depending on the type of water. In Brazil, forest flooded periodically by white water is known as varzea. Since the varzea is flooded by water containing large amounts of sediment, and hence nutrients, it tends to be much richer and denser than forest flooded by black-water rivers. Unfortunately, this relatively good soil is also preferred by riverside dwellers, and therefore much of the virzea has already been destroyed to make way for small farms, cattle and buffalo ranches, and fields of jute and other crops such as beans.
The areas that are flooded by black and clear water are termed igapo, which is a native term for this dense and rather low type of vegetation with gnarled-looking trees, adapted to having their trunks submerged for several months each year. The black-water Rio Negro and the clear-water Tapajos River have good examples of igapo along their banks. They also have far fewer settlers.
The coastal lowlands of the Malay Peninsula, Borneo, and other areas have extensive swamp forests on peat. These domes of peat can be up to 56 feet (17 m) thick and in some ways are similar to the raised bogs of temperate regions. Like the black-water igapo of Amazonia, the peat swamps are extremely acidic (pH 3.5 to 4.5) and are low in nutrients, but they sustain a rich forest. The forest varies in composition according to its distance from the center of the raised dome. The outer portion of a swamp typically is characterized by tall evergreens that can reach as much as 165 feet (50 m) in height. The forest changes in both physiognomy and species composition toward the center of the swamp, and species diversity declines. The trees are much smaller, usually less than 65 feet (20 m) tall, but they are much more densely distributed. As many as 3,200 individual pole-sized trees have been recorded on a single acre (or 1,300 per hectare) of peat swamp forest. As the forest develops over time, the ground level rises above the water level and the forest is replaced by a scrubby woodland. In 1982-83, because of worldwide climatic patterns influenced by the El Nino effect, there was no real rainy season on Borneo. As a result, when local populations set fires to clear their land, the fires quickly spread into the surrounding peat swamp forest. Large areas where the peat was above the water level smoldered for many months, and some 15,500 square miles (40,000 km 2) of this type of rainforest were destroyed.
Scattered around the seashores of the tropics is a special kind of rainforest known as mangrove forest. This is forest that is adapted to grow in areas that are flooded daily by the salt water of the rising tide. Although mangrove occurs throughout the tropics, it is especially rich and abundant in Southeast Asia. The world's most extensive mangrove forest is that of the Sundarbans in the Ganges delta on the border of India and Bangladesh. This is the only sizable tract of rainforest remaining in the deforested country of Bangladesh. There are also large areas of mangrove forests on the Malay Peninsula, in Borneo, and in New Guinea. However, the mouths of the world's two greatest rainforest rivers, the Amazon and the Congo, support only relatively small areas of mangrove.
There are numerous challenges other than the salinity of the water for plants that grow beside the sea, and so the plants of mangrove forests exhibit a variety of fascinating adaptations. In many mangrove trees, such as those of the genus Rhizophora, the red mangroves, the trunk does not extend directly into the water but is supported by a tangled mass of arching prop roots that anchor the tree firmly into the soil and prevent it from being washed away by currents. As a result, mangroves play a vital ecological role in the stabilization of tropical coastlines. Many mangrove tree species, such as those of the genus Avicennia, the black mangroves, erect above the mud a series of periscope-like roots known as pneumatophores or breathing roots. In the waterlogged mud of a mangrove swamp, there is little oxygen, so the trees absorb it through their breathing roots from the air when the tide is low. Often the tidal movements deposit more and more mud around the roots of mangrove trees, and when this occurs the pneumatophores gradually extend to keep above the mud and so supply the roots with the gas that is vital for the life of the tree...