The Amazon rain forest is the largest contiguous rain forest in the world. It is so large and so lush with tree growth that its transpiration is actually responsible, in part, for the wet climate of the region. Plant diversity is so great here that no comprehensive plant guide currently exists for many parts of the Amazon rain forest. Of tens of thousands of plant species, a large number have never been described.
This one-of-a-kind botanical treasure is being destroyed at a rapid pace of between 5,000 and 10,000 square miles (13,000-26,000 square kilometers) per year. The causes for this destruction are primarily logging, agriculture, and cattle ranching. A common practice for preparing an area for cattle ranching or farming has been to simply burn the forest, not even necessarily logging it first, and then to allow the grass and other vegetation or crops grow in its place. The soils of the rain forest are so poor, however, that this practice usually depletes the soil within a few years, and the land becomes a useless wasteland. Mining and oil drilling have also taken their toll.
The Amazon rain forest is an extremely complex biome. The main plant biomass is composed of trees, which form a closed canopy that prevents much of the sunlight from reaching the forest floor. Consequently, the forest floor has little herbaceous growth, and most smaller plants tend to grow as epiphytes on the branches and trunks of trees. Common epiphytes in the Amazon rain forest include orchids, bromeliads, and even some cacti. There is a large diversity of bromeliads, ranging from small, inconspicuous species to larger species, such as tank bromeliads that can collect significant amounts of water in their central whorl of leaves. The water in these plants can contain a whole miniature ecosystem, complete with mosquito larvae, aquatic insects, and frogs. Ferns are another significant member of the epiphyte community. Some larger species of ferns, often called tree ferns, also grow in the understory. Lianas, or vines, are a prominent component as well.
The trees that form the canopy are stratified into three fairly discrete levels. The lowest two levels are the most crowded, and the highest level comprises extremely tall trees, often referred to as emergent trees because they stand out randomly above the fairly continuous lower two layers. Many of the tallest trees are buttressed at the base, an adaptation that seems to give them greater stability. Beneath the canopy, there are some smaller palms, shrubs, and ferns, but they only occur densely where there is a break in the canopy that allows in greater light.
Some rain forest species of trees are well known, primarily because of their economic value. A favorite tree for use in furniture is the mahogany. Because its wood is highly prized, many species of mahogany are becoming rare or extinct. The South American rain forests are also the original source of rubber. Brazil had a monopoly on rubber until seeds were smuggled out and planted in Malaysia. Synthetic rubber has now replaced natural rubber for many applications. Another popular tree is the Brazil nut tree, an abundant food source that has been exploited by suppliers of mixed nuts. The native cacao tree produces fruits from which cacao beans are extracted, then processed to make chocolate.
Every year during the rainy season, the lowest elevation areas of the Amazon rain forest are flooded with several feet of water, which recedes after a few months. The trees flourish in this flooding cycle. A few even have unique adaptations, such as producing fruits that are eaten by fish, thus assuring the spread of their seeds. The flooding can be so extensive in some areas that the water reaches the lower parts of the canopy.
Coastal tropical rain forests also occur in northwestern and southeastern South America. There are a high number of endemic species in each of these forests. Some tree species are so rare that they may be found in an area of only a few square miles and nowhere else. Where the tropical rain forest meets the ocean, mangrove trees have become adapted to the tidal environment. Mangroves have prop roots, which make the trees look like they are growing on stilts. They also frequently have special root structures that extend above the water at high tide and allow the roots to breathe. Mangrove trees are also extremely salttolerant.
See also: Plants of the Mediterranean and Temperate Forest Biomes
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