A rainforest is an area of tall, mostly evergreen trees and a high amount of rainfall. Rainforests are Earth’s oldest living ecosystems, with some surviving in their present form for at least 70 million years. They are incredibly diverse and complex, home to more than half of the world’s plant and animal species—even though they cover just 6% of Earth’s surface. This makes rainforests astoundingly dense with flora and fauna; a 10-square-kilometer (4-square-mile) patch can contain as many as 1,500 flowering plants, 750 species of trees, 400 species of birds and 150 species of butterflies.
Distribution Of Rainforests
Rainforests usually occur in regions where there is a high annual rainfall of generally more than 1,800 mm (70 inches) and a hot and steamy climate. The trees found in these regions are evergreen. Rainforests may also be found in areas of the tropics in which a dry season occurs, such as the “dry rainforests” of northeastern Australia. In these regions annual rainfall is between 800 and 1,800 mm and as many as 75 percent of the trees are deciduous.
Tropical rainforests are found primarily in South and Central America, West and Central Africa, Indonesia, parts of Southeast Asia, and tropical Australia. The climate in these regions is one of relatively high humidity with no marked seasonal variation. Temperatures remain high, usually about 30 °C (86 °F) during the day and 20 °C (68 °F) at night. Where altitude increases along the borders of equatorial rainforests, the vegetation is replaced by montane forests, as in the highlands of New Guinea, the Gotel Mountains of Cameroon, and in the Ruwenzori mass of Central Africa. Tropical deciduous forests are located mainly in eastern Brazil, southeastern Africa, northern Australia, and parts of Southeast Asia.
The topography of rainforests varies considerably, from flat lowland plains marked by small rock hills to highland valleys criss-crossed by streams. Volcanoes that produce rich soils are fairly common in the humid tropical forests.
Soil conditions vary with location and climate, although most rainforest soils tend to be permanently moist and soggy. The presence of iron gives the soils a reddish or yellowish colour and develops them into two types of soils—extremely porous tropical red loams, which can be easily tilled, and lateritic soils, which occur in well-marked layers that are rich in different minerals.
Chemical weathering of rock and soil in the equatorial forests is intense, and in rainforests weathering produces soil mantles up to 100 metres (330 feet) deep. Although these soils are rich in aluminum, iron oxides, hydroxides, and kaolinite, other minerals are washed out of the soil by leaching and erosion. The soils are not very fertile, either, because the hot, humid weather causes organic matter to decompose rapidly and to be quickly absorbed by tree roots and fungi.
Classification Of Plants And Animals
Rainforests exhibit a highly vertical stratification in plant and animal development. The highest plant layer, or tree canopy, extends to heights between 30 and 50 metres. Most of the trees are dicotyledons, with thick leathery leaves and shallow root systems. The nutritive, food-gathering roots are usually no more than a few centimetres deep. Rain falling on the forests drips down from the leaves and trickles down tree trunks to the ground, although a great deal of water is lost to leaf transpiration.
Most of the herbaceous food for animals is found among the leaves and branches of the canopy, where a variety of animals have developed swinging, climbing, gliding, and leaping movements to seek food and escape predators. Monkeys, flying squirrels, and sharp-clawed woodpeckers are some of the animals that inhabit the treetops. They rarely need to come down to ground level.
The next lowest layer of the rainforest is filled with small trees, lianas, and epiphytes, such as orchids, bromeliads, and ferns. Some of these are parasitic, strangling their host’s trunks; others use the trees simply for support.
Above the ground surface the space is occupied by tree branches, twigs, and foliage. Many species of animals run, flutter, hop, and climb in the undergrowth. Most of these animals live on insects and fruit, although a few are carnivorous. They tend to communicate more by sound than by sight in this dense forest strata.
Contrary to popular belief, the rainforest floor is not impassable. The ground surface is bare, except for a thin layer of humus and fallen leaves. The animals inhabiting this strata, such as rhinoceroses, chimpanzees, gorillas, elephants, deer, leopards, and bears, are adapted to walking and climbing short distances. Below the soil surface, burrowing animals, such as armadillos and caecilians, are found, as are microorganisms that help decompose and free much of the organic litter accumulated by other plants and animals from all strata.
The climate of the ground layer is unusually stable. The upper stories of tree canopies and the lower branches filter sunlight and heat radiation, as well as reduce wind speeds, so that the temperatures remain fairly even throughout the day and night.
Virtually every group of animals except fishes is represented in the rainforest ecosystem. Many invertebrates are very large, such as giant snails and butterflies. The breeding seasons for most animals tend to be coordinated with the availability of food, which, although generally abundant, does vary seasonally from region to region. Climatic variations, however, are slight and thus affect animal behaviour very little. Those animals that do not have highly developed modes of quick locomotion are concealed from predators by camouflage or become nocturnal feeders.
Most rainforests are structured in four layers: emergent, canopy, understory, and forest floor. Each layer has unique characteristics based on differing levels of water, sunlight, and air circulation. While each layer is distinct, they exist in an interdependent system: processes and species in one layer influence those in another.
The top layer of the rainforest is the emergent layer. Here, trees as tall as 60 meters (200 feet) dominate the skyline. Foliage is often sparse on tree trunks, but spreads wide as the trees reach the sunny upper layer, where they photosynthesize the sun’s rays. Small, waxy leaves help trees in the emergent layer retain water during long droughts or dry seasons. Lightweight seeds are carried away from the parent plant by strong winds.
In the Amazon rainforest, the towering trees of the emergent layer include the Brazil nut tree and the kapok tree. The Brazil nut tree, a vulnerable species, can live up to 1,000 years in undisturbed rainforest habitats. Unlike many rainforest species, both the Brazil nut tree and the kapok tree are deciduous—they shed their leaves during the dry season.
Animals often maneuver through the emergent layer’s unstable topmost branches by flying or gliding. Animals that can’t fly or glide are usually quite small—they need to be light enough to be supported by a tree’s slender uppermost layers.
Beneath the emergent layer is the canopy, a deep layer of vegetation roughly 6 meters (20 feet) thick. The canopy’s dense network of leaves and branches forms a roof over the two remaining layers. The canopy blocks winds, rainfall, and sunlight, creating a humid, still, and dark environment below. Trees have adapted to this damp environment by producing glossy leaves with pointed tips that repel water.
While trees in the emergent layer rely on wind to scatter their seeds, many canopy plants, lacking wind, encase their seeds in fruit. Sweet fruit entices animals, which eat the fruit and deposit seeds on the forest floor as droppings. Fig trees, common throughout most of the world’s tropical rainforests, may be the most familiar fruit tree in the canopy.
Located several meters below the canopy, the understory is an even darker, stiller, and more humid environment. Plants here, such as palms and philodendrons, are much shorter and have larger leaves than plants that dominate the canopy. Understory plants’ large leaves catch the minimal sunlight reaching beyond the dense canopy.
Understory plants often produce flowers that are large and easy to see, such as Heliconia, native to the Americas and the South Pacific. Others have a strong smell, such as orchids. These features attract pollinators even in the understory’s low-light conditions. The fruit and seeds of many understory shrubs in temperate rainforests are edible. The temperate rainforests of North America, for example, bloom with berries.
Forest Floor Layer
The forest floor is the darkest of all rainforest layers, making it extremely difficult for plants to grow. Leaves that fall to the forest floor decay quickly. Decomposers, such as termites, slugs, scorpions, worms, and fungi, thrive on the forest floor. Organic matter falls from trees and plants, and these organisms break down the decaying material into nutrients. The shallow roots of rainforest trees absorb these nutrients, and dozens of predators consume the decomposers.
Types of Rainforests
Tropical rainforests are mainly located between the latitudes of 23.5°N (the Tropic of Cancer) and 23.5°S (the Tropic of Capricorn)—the tropics. Tropical rainforests are found in Central and South America, western and central Africa, western India, Southeast Asia, the island of New Guinea, and Australia.
Sunlight strikes the tropics almost straight on, producing intense solar energy that keeps temperatures high, between 21° and 30°C (70° and 85°F). High temperatures keep the air warm and wet, with an average humidity of between 77% and 88%. Such humid air produces extreme and frequent rainfall, ranging between 200-1000 centimeters (80-400 inches) per year. Tropical rainforests are so warm and moist that they produce as much as 75% of their own rain through evaporation and transpiration.
Temperate rainforests are located in the mid-latitudes, where temperatures are much more mild than the tropics. Temperate rainforests are found mostly in coastal, mountainous areas. These geographic conditions help create areas of high rainfall. Temperate rainforests can be found on the coasts of the Pacific Northwest in North America, Chile, the United Kingdom, Norway, Japan, New Zealand, and southern Australia.
As their name implies, temperate rainforests are much cooler than their tropical cousins, averaging between 10° and 21°C (50° and 70°F). They are also much less sunny and rainy, receiving anywhere between 150-500 centimeters (60-200 inches) of rain per year. Rainfall in these forests is produced by warm, moist air coming in from the coast and being trapped by nearby mountains.
Many individuals, communities, governments, intergovernmental organizations, and conservation groups are taking innovative approaches to protect threatened rainforest habitats. Many countries are supporting businesses and initiatives that promote the sustainable use of their rainforests. Costa Rica is a global pioneer in this field, investing in ecotourism projects that financially contribute to local economies and the forests they depend on. The country also signed an agreement with an American pharmaceutical company, Merck, which sets aside a portion of the proceeds from rainforest-derived pharmaceutical compounds to fund conservation projects.
Intergovernmental groups address rainforest conservation at a global scale. The United Nations’ REDD (Reducing Emissions from Deforestation and Forest Degradation) Program, for example, offers financial incentives for reducing carbon emissions created by deforestation to 58 member countries.
The Democratic Republic of the Congo used REDD funds to create an online National Forest Monitoring System that tracks and maps data on logging concessions, deforestation in protected areas, and national forestry sector measures. REDD funds were also used to investigate best practices in solving land disputes in Cambodia, which lacks proper forest zoning and boundary enforcement. Non profit organisations are tackling rainforest conservation through a variety of different approaches. The Rainforest Alliance is a non profit organization that helps business and consumers know that their products conserve rather than degrade rainforests.
-by Sanjana Pal (Content writer Intern)