Tropical Rain Forest

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The richest and most productive biological communities in the world are in the tropical forests. These forests have been reduced to less than half of their former extent by human activities and now cover only about 7% of Earth's land area. In this limited area, however, is about two-thirds of the vegetation mass and about half of all living species in the world.

The largest, lushest, and most biologically diverse of the remaining tropical moist forests are in the Amazon Basin of South America, the Congo River basin of central Africa, and the large islands of southeast Asia (Sumatra, Borneo, and Papua, New Guinea). Whereas the forests of mainland southeast Asia, western Africa, and Central America are strongly seasonal, with wet and dry seasons, the South American and central African forests are true rain forests. Rainfall is generally over 160 in (406 cm) per year and falls at a relatively even rate throughout the year. It is said that such rainforests "make their own rain," because about half the rain that falls in the forests comes from condensation of water vapor released by transpiration from the trees themselves. Rain forests at lower elevations are hot and humid year-round. At higher elevations, tropical mountains intercept moisture-laden clouds, so the forests that blanket their slopes are cool, wet, and fog-shrouded. They are aptly and poetically called "cloud forests." Tropical forests are generally very old. Unlike temperate rain forests, they have not been disturbed by glaciation or mountain-building for hundreds of millions of years. This long period of evolution under conditions of ample moisture and stable temperatures has created an incredible diversity of organisms of amazing shapes, colors, sizes, habits, and specialized adaptations.

Habitats in a tropical rain forest are stratified into three to five distinct layers from ground level to the tops of the tallest trees. Hundreds of tree species grow together in lush profusion, their crowns interlocking to form a dense, dappled canopy about 120 ft (37 m) above the forest floor. These unusually tall trees are supported by relatively thin trunks reinforced by wedge-shaped buttresses that attach to a thick mat of roots just under the soil surface. A few emergent trees rise above the seemingly solid canopy into a world of sunlight, wind, and open space. Numerous species of birds, insects, reptiles, and small mammals live exclusively in the forest canopy, never descending below the crowns of the trees.

The forest understory is composed of small trees and shrubs growing between the trunks of the major trees, as well as climbing woody vines (lianas) and many epiphytes—mainly orchids, bromeliads, and arboreal ferns—that attach themselves to the trees. Some of the larger trees may support 50–100 different species of epiphytes and an even larger population of animals that are specialized to live in the many habitats they create. These understory layers are a world of bright but filtered light abuzz with animal activity.

By contrast, the forest floor is generally dark, humid, quiet, and rather open. Few herbaceous plants can survive in the deep shade created by the layered canopy of the forest trees and their epiphytes. The most numerous animals are ants and termites that scavenge on the detritus raining down from above. A few rodent species gather fallen fruits and nuts. Rare predators such as leopards, jaguars, smaller cats, and large snakes hunt both on the ground and in the understory.

A tropical rain forest may produce as much as 90 tons
(81.6 metric tons) of biomass per acre (0.4 ha) per year, and one might think that the soil that supports this incredible growth is rich and fertile. However, the soil is old, acidic, and nutrient-poor. Ages of incessant tropical rains and high temperatures have depleted minerals, leaving an iron- andaluminum-rich podzol. Tropical forests have only about 10% of their organic material and nutrients in the soil, compared to boreal forests, which may have 90% of their organic material in litter and sediments.

The interactions of decomposers and living plant roots in the soil are, literally, the critical base that maintains the rain forest ecosystem. Tropical rain forests are able to maintain high productivity only through rapid recycling of nutrients. The constant rain of detritus and litter that falls to the ground is quickly decomposed by populations of fungi and bacteria that flourish in the warm, moist environment. Some of these decomposers have symbiotic relationships with the roots of specific trees. Trees have broad, shallow root systems to capitalize on this surface nutrient source; an individual tree might create a dense mat of superficial roots 328 ft (100 m) in diameter and 3 ft (0.9 m) thick. In this way, nutrients are absorbed quickly and almost entirely and are reused almost immediately to build fresh plant growth, the necessary base to the trophic pyramid of this incredible ecosystem.

Biodiversity

Caufield, C. In the Rainforest. New York: Knopf, 1985.

Hecht, S., and A. Cockburn. Fate of the Forest. New York: Harper Collins, 1991.

Myers, N. The Primary Source: Tropical Forests and Our Future. New York: Norton, 1992.

Revkin, A. The Burning Season: The Murder of Chico Mendes and the Fight for the Amazon Rain Forest. New York: Houghton Mifflin, 1991.

Repetto, R. "Deforestation in the Tropics." Scientific American 262 (April 1990): 36–42.

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