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 Rainforests are living emeralds which adorn our world with rare beauty and natural wonders. They are a product of planetary processes and are - in turn - contributors to the water and carbon cycles on which all life depends. Tropical rainforests are the result of the unique climatic conditions found between the Tropics of Cancer and Capricorn, in the regions north and south of Earth’s equator. Here, constant heat and humidity allow trees and plants to grow year round, without seasonal time-outs. Vast columns of hot air rise and condense out as rain, resulting in annual rainfall of 80 to 400 inches: annual temperatures average over 80 degrees Fahrenheit. And so the tropics are garlanded with trees,plants, animals and insects, uniquely adapted to these conditions.
The earliest written description of rainforests in Western literature seems to be that of Christopher Columbus, but the term “Regenwald” (rain forest) was first used by a German naturalist, A. F. W. Schimper, many centuries later, in 1898. Exploring rainforests had a profound impact on the thinking of some of the greatest of 19th century biologists and naturalists—Alexander von Humboldt, Alfred Russell Wallace and Charles Darwin. Darwin’s and Wallace’s experiences in rainforests shaped their theories of evolution through natural selection, which in turn inspired 20th century biology, leading to recent advances in molecular biology and genetic engineering.
The last few decades of this century have also given us a new understanding of, and appreciation for, rainforests: new techniques have allowed researchers to explore the canopy for the first time and to census its otherwise unseen inhabitants.
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Buttress
Soils of the typical lowland rainforest are often shallow. In much of Amazonia, there’s only a few inches of soil above reddish clay. Many roots stretch out over the surface, rather than burrowing underground. Here many trees use wooden buttresses (like the buttresses which support the giant Gothic cathedrals of Europe), organic flanges which grow out from the base of the tree on all sides to balance it. Some biologists say this shape also allows the tree to gather nutrients from a greater surface area of soil since lots of small rootlets extend down from the bottom of the buttresses. |
| Still Roots In other places, so-called “prop” or “stilt” roots emerge like slanting rods from the main trunk 1 to 2 meters above the ground to help support the trunk. (The LFRF Guide provides a way to explore these engineering feats of Nature in Activity 1.2.). This particular kind of root is often found in flooded or mangrove forests, where it also protects the tree against waves and currents. |
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Drip tips Rainforest leaves also have distinctive features. Many have “drip tips”—a pointed shape which helps drain excess water from the leaf and reduces vulnerability to mold and predation. Researchers often speak of how different plants try to prevent ”herbivory”—the eating away of vegetation by insects or other parasites—and minimizing moisture with drip tips is one such strategy. When it fails, leaves become a lacy net of holes and fibers. |
Mutualistic relationships
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With the constant fight for light and water, nutrients and energy, many rainforest species come to rely on each other, and develop intimate and exclusive relationships. Some plants, for example,provide “ant houses,” home to a particular species of ant whose soldiers defend the leaves against other would-be insect predators. Other plants provide leaves with tiny feeding troughs,pools of sugar-solution, enticing ant patrols with sweet rewards. (You can find more information on these “ant-defended plants” in the LFRF Teacher Resource video which is part of the Multimedia Kit.)In addition, however, to listing and marveling at such distinctive aspects of rainforests, we should also try to comprehend the myriad relationships and interactions to be found in them. As Alexander von Humboldt wrote, “...more beautiful still than all the wonders individually is the impression conveyed by the whole... in its entirety.”(For more on rainforest relationships, please explore “ECOsystem” online, and if you’re a teacher or youth leader, consider implementing the Tropical Rainforest Food Web Game [Activity 2.3] which helps bring these intricate interactions to life.)". |
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 Rainforests are defined—as you would expect—by rainfall, and in fact they are literally created by it. (“Rain forest”—two words—is the older usage: both are accepted, but most modern authors and researchers combine the two, as does PTK, as in “Rainforest.”) They can be found where rain exceeds 80 inches per year, and can appear in temperate as well as tropical zones, so long as the rainfall is sufficiently plentiful.
Tropical rainforests often have from 160 to 400 inches of rain a year. But they aren’t the wettest or even the hottest places on Earth. (The wettest is Mount Waialeale, in Hawaii, USA, and the hottest is Libya in North Africa.) But just as important as the amount of rain in shaping the unique character of rainforests is the constant humidity and high average temperature. In the Amazon basin you can expect at least 130 days of rain a year and, in many places, up to 250 days. The relative humidity never falls below 80%, and temperatures vary little between daytime averages of 31 degrees Centigrade (88 Fahrenheit) and night-time lows of 22 degrees C (72 F).
 Sometimes this constancy of temperature and humidity leads people to argue that rainforests have no seasons, but in the tropics this is only partially correct. There may not be a cold winter and a hot summer, but there are DRY seasons and WET seasons. Plants and trees flower at these different times of year, profoundly influencing the lives of the creatures who inhabit them. And our contemporary understanding of rainforests (see ECOsystem) quickly dispels the misconception that this is a changeless Eden, where Nature’s endless bounty means things are always the same. In fact there’s a constant fight for light, water and nutrients, one of the reasons natural selection has had such a powerful effect in creating the great numbers of species which make tropical rainforests the richest places for biodiversity on Earth.
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 Emergent trees
Rainforests have a distinctive structure. Above a sea of green, which is called the "upper canopy", tower just a few “emergent trees” per acre, sometimes as tall as 40 meters (over 130 feet.) In all the forest they alone enjoy unfettered access to the sun and sky. The ways in which they disperse their seeds—by wind—and the creatures who inhabit them—birds like the harpy eagle, and the toucan—are very different from those found below.
Canopy
Most photosynthesis occurs in the canopy—that vast carpet of green which Alexander von Humboldt called a “forest above a forest”—which absorbs as much as 98% of the sunlight falling on the forest, darkening the lower regions. Here, 20-30 meters up (65-100 feet), live butterflies, and mammals like the three-toed sloth, moving slowly but efficiently in search of vegetation, descending only once a week to the ground to defecate. (See ECOsystem.) Temperatures here reach 32 degrees C (96 F) but the humidity is only 60% (compared to 98% down below.) Since this is where photosynthesis occurs, this is also where productivity is greatest: each year a tropical rainforest produces about 25-30 tonnes of new growth per acre (10-12 tons), twice as much as a temperate oak forest.
Understory
In virgin rainforest, the understory is not the “jungle” of tangled vines seen in old movies, or observed from boats on one of the thousands of rivers and tributaries also nourished by the heavy rains. Undisturbed rainforest is surprisingly clear of vegetation close to the ground in part because so little light filters down through the canopy, sometimes only 1 to 2 per cent of the sun’s original intensity. But even here life’s struggles are intense, and insects, fungi and roots all fight for access to energy the raw materials of existence. Here temperatures are cooler, averaging about 28 degrees C (82 F) but humidity is higher, about 98%. |
 
In addition to Brazil, Peru, Columbia and Venezuela have large regions of rainforest. Almost one third of Columbia is forested, as is much of southern Venezuela. The forested area of Guyana is largely untouched, since the population lives mainly on the coast, and in Suriname much of the nation is park or preserve.
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