Tropical rainforests

Tropical rainforests are forest biomes with an average rainfall between 67 inches (170 centimeters) and 400 inches (1,016 centimeters). The World Wide Fund for Nature (WWF) defines them as a type of tropical wet forest, tropical moist broad-leaf forest, or lowland equatorial evergreen rainforest. Tropical rainforests are found in the equatorial zone between the Tropics of Cancer and Capricorn, roughly between latitudes 28 degrees north or south of the equator, where the direct angle of the sun rays causes high rates of evaporation. As water vapor evaporates from the forests, it rises, cools, and falls about an eighth-inch per day, giving these forests their name. Average humidity is usually around 77–88 percent. Mean monthly temperatures exceed 64 degrees F (18 degrees C) throughout the entire year. Tropical rainforests are found in five major geographical areas: Central and South America in the Amazon River Basin; Africa in the Zaire Basin; a small amount in West Africa and East Madagascar; in Indo-Malaysia on the west coast of India, Assam, southeast Asia; as well as New Guinea and Queensland, Australia.

There are four main types of rainforest; lowland equatorial evergreen rainforest, moist deciduous and semi-evergreen seasonal forests, montane rainforests, and flooded forests. Lowland equatorial evergreen rainforests have high rainfall throughout the year. They form a belt around the equator of the globe, covering the Amazon Basin of South America, the Congo Basin of central Africa, Indonesia, and New Guinea. Moist deciduous and semi-evergreen seasonal forests have high rainfall overall but have a distinct warm summer wet season and a cooler winter dry season. Some species of trees in this type of rainforest even drop their leaves during the winter dry season to conserve water. These forests make up parts of South America, Central America and the Caribbean, coastal west Africa, India, and Indochina. Montane rainforests are also sometimes called cloud forests because of their almost constant shroud of fog. These forests are found in cooler-climate mountain areas, depending on the latitude, between elevations of about 4,920 and 18,827 feet (1,500 and 3,300 meters). Flooded forests are caused by seasonal flooding of tropical rivers. There are seven types of flooded forests recognized in the Tambopata Reserve in Amazonian Peru alone.

It is estimated that more than half of all plant and animal species are found in tropical rainforests, and there are likely millions of species yet to be discovered. Many of these species are, or could be, important sources of food and medicine. Tropical rainforests produce about 40 percent of the Earth's oxygen and sequester carbon from the atmosphere, which helps to mediate climate change. However, due to heavy logging and clearing for agriculture throughout the twentieth century, tropical rainforests now cover only about 7 percent of the Earth's land surface. They are threatened globally by deforestation and fragmentation for agriculture and urbanization, mining, drilling, invasive species, and climate change.

Layers of the Tropical Rainforest

Tropical rainforests are typically made up of four layers or zones typified by the structure of the vegetation. These different zones form habitats for a variety of animal species. Though this stratification is not always clear-cut, these four layers are generally found at different heights in the forest: starting at the forest floor, the lower canopy, the upper canopy, and ending at the emergent layer.

The forest floor of the tropical rainforest is completely shaded from the dense forest canopy except for clearings, formed by fallen trees, where the sun can reach through the opening. Only about 1 percent of the sunlight that strikes the upper canopy reaches the forest floor. Except for dense growth near riverbanks, swamps, and clearings, most areas are so shaded that very little can grow, leaving a relatively open forest floor that allows for easy movement of large animals. Litter falls to the ground from the trees above and then is broken down by decomposers like termites, earthworms, and fungi. The constant heat and humidity accelerates the decomposition process. This leads to a very short nutrient cycle wherein organic matter is consumed almost as quickly as it is deposited, leaving only a thin layer of poor quality soil.

Also called the lower canopy, the understory is composed of 60-foot trees, the trunks of upper canopy trees, shrubs, and plants. Epiphytes, mosses, and lichens grow on the trunks and branches of trees. This area is dense and humid, with very little air movement. This area receives slightly more light than the forest floor, 5 percent of the light that strikes the upper canopy, but is mostly still in constant shade.

The canopy, found above the understory, is also called the upper canopy and it is made up of 60- to 130-foot trees. The trees in this layer compete and grow taller to reach the sunlight and block light access to the layers below. They form the roof of the rainforest. Leaves on these tallest trees often have "drip spouts" that allow rainwater to run off them to keep them dry and light and to prevent mold growth. Most of the animals in the rainforest live in this layer, where there is lots of food available from fruiting trees. Many animals that live in the upper canopy never have to go to the ground, getting most of their water from their food or rain pooled in the trees.

The emergent layer is the only one truly unique to the tropical rainforest, as the other layers are found in temperate rainforests as well. This layer is made of widely spaced trees that grow above the upper canopy. These trees, called emergents, are 100 to 240 feet (30 to 73 meters) tall with umbrella-shaped crowns. Emergent trees are exposed to extreme drying winds and heat. Many have adapted small pointed leaves that they sometimes drop during the dry season in monsoon areas to conserve water. Due to the shallow soil, these giant trees have shallow roots and must grow large buttress structures that can grow up to 35 feet wide for support. The emergent layer is home to animals such as the crowned eagle (Stephanoaetus coronatus), king colobus (Colobus polykomos), and large flying fox (Pteropus vampyrus).

Disturbance and Succession

Succession is the ecological process that changes biotic community structure over time. This generally trends toward a more stable, diverse community structure after the initial disturbance. Disturbances can be a natural phenomenon or human-caused. Natural phenomena include hurricanes, volcanoes, river movements, or fallen trees. Such natural disturbances are well documented in the fossil record to encourage speciation and endemism. Fallen trees, often caused by windstorms, are especially important disturbances in tropical rainforests. They open clearings in the dense canopy, allowing light to reach the forest floor where seedlings are waiting to spring into growth to take advantage of the gaps. These saplings might otherwise never be able to establish themselves and mature. About 70 percent of seedlings depend on such gaps for germination or growth past seedling size.

Soil Ecology

As previously mentioned, soil quality in tropical rainforests is typically poor due to the short nutrient cycle of the ecosystem. High rates of precipitation also contribute to the leaching or loss of nutrients from the system. Soil types throughout rainforests are, however, variable and some areas are very fertile. Each area's soil type is influenced by that region's climate, vegetation, topographic position, parent material, and age. There are two main classes of soil: ultisols and oxisols. Ultisols are defined as well-weathered, acidic red clay soils. They have low levels of calcium and potassium. Their high clay content helps them to retain water. Oxisols are also acidic, reddish soils but are old and highly weathered and leached because of good drainage, leaving them nutrient-poor. The red color in both of these soil types originates from the high heat and moisture environment, which forms oxides of iron and aluminum. These oxides are then insoluble in water and not readily taken up by rainforest plants.

The soils of the eastern and central Amazon rainforests and those of southeast Asia are generally old and mineral-poor. The nutrient flood plains of the western Amazon rainforests in Ecuador and Peru and volcanic areas of Costa Rica, southeast Asia, Africa, and Central America have young, mineral-rich soil. These soil characteristics influence the ecological processes of the regions in question. For example, primary productivity, or wood production, is higher in the western Amazon region than in the eastern Amazon oxisols. Above-ground productivity is closely linked to soil types and mineral contents. While the physical properties of a forest, like disturbance regimes, control tree turnover rates, the chemical properties such as nitrogen and phosphorous levels control the growth rates of trees. Phosphorous, potassium, calcium, and magnesium are all needed by growing trees and generally come from the weathering of rocks. Increased rates of decomposition result from increased phosphorous content in the soils. Soils that are limited in phosphorous content limit the growth of trees and subsequently the ability of a forest to uptake and sequester carbon.

The Amazon River Basin contains more types of plant and animal life than any other biome. The second-most biodiverse rainforest is found in southeast Asia. African tropical rainforests have the lowest variety of all the rainforests. In areas of high diversity, tropical rainforests can harbor hundreds of species in a relatively small area.

Plant Biodiversity

It has been estimated that seventy percent of all plants in tropical rainforests are trees. There are more kinds of trees in this biome than in any other area. For example, one area in South America was found to have between 100 and 300 tree species in just two and a half acres (one hectare) of forest. Trees in the rainforest are often straight and unbranched due to the need to grow fast and tall to compete for light and space. They tend to have smooth, thin bark because they do not need protection from cold temperatures or to retain water. This smooth bark also deters parasitic epiphytic plants. Due to this similarity in adaptations of rainforest trees, it is often difficult to identify species by their bark and leaves and they can only be differentiated by their flowers, which are often highly specialized to different pollinators.

Many plants are adapted to help them deal with the intense water inputs to the system. They have developed structures like drip tips and grooved leaves with oily coatings that shed water so that it does not weigh them down, which might cause them to break. The trees give off water through their pores, called stomata, which further increases the humidity of these forests. This transpiration can account for as much as 50 percent of the precipitation in a rainforest. Some upper canopy trees have small, dark green, leathery leaves to reduce water loss in the direct sun but the same trees may produce large leaves lower in the canopy to funnel off water and increase their light-catching surface.

Competition for sunlight and space has also led to many plant adaptations. Plants and trees in the lower canopy have large leaves to absorb as much sunlight as possible. Some leaves even turn to track the sun as its angle changes throughout the day. There are plants that specialize in growing on tall trees in order to gain access to the light. These plants are called epiphytes and derive their water and nutrients from the air and rain as well as from debris that accumulates around them. Epiphytes include orchids and bromeliads.

There are over 2,500 species of vine that grow in the tropical rainforests. Lianas are long-stemmed, woody vines that start out as small shrubs and then send out tendrils to grab onto sapling trees. As the tree grows upward, the liana vine grows with it to reach the sunlight of the canopy. Lianas will even grow from one tree to another to reach the best source of light and can make up about 40 percent of the plant mass in the rainforest canopy. The rattan vine similarly has spikes on its underside that point backward to grab onto saplings. Strangler vines use trees as support and as they grow thicker they can actually strangle their host tree, which will die and rot away, leaving the "hollow" tree form of the vine.

Tall rainforest trees develop wide buttress roots or a system of stilt roots to support them in shallow soil, but these structures have other benefits for the trees as well. Buttress roots aid in the competition for and efficient uptake of nutrients and water. The roots help to collect leaf litter and funnel nutrient-rich rain water as it drains down the trunk and holds it in pools that can be absorbed as needed. The large root structures increase the surface area of the tree, which increases gas exchange. They also help to reduce soil erosion that would wash away needed nutrients.

Throughout tropical rainforests, tree species are not often found in monoculture, or large areas of only one species. This spacing is due to high seed dispersal from fruit-eating animals as well as the highly competitive nature of tree establishment in the forest. This well-spaced biodiversity ultimately helps to prevent the spread of infections among the trees and decreases mass die-offs. Many tree species also have staggered blooming and fruiting periods, which helps to provide a year-round source of food to rainforest animals.

Animal Biodiversity

Tropical rainforests are home to a huge diversity of animal life. Some forests support ungulates like okapi (Okapia johnstoni), tapir (Tapirus spp.), or Sumatran rhinoceros (Dicerorhinus sumatrensis). Others have apes like the western lowland gorilla (Gorilla gorilla). There are many predators in rainforests, such as leopards (Panthera pardus), piranha, poison dart frogs, ring-tailed coati (Nasua nasua), boa constrictor (Boa constrictor), and many insects, which make up the largest group of animal species in tropical rainforests. Many species are adapted for a specialist life in the treetops and have bright colors, sharp patterns, loud vocalizations, fruit-heavy diets, and structures to aid in climbing. For example, New World monkeys such as the spider monkey (Ateles spp.) all have prehensile tails that help them to climb in their canopy home. Other treetop specialists include the three-toed sloth (Bradypus spp.) which may only climb to the forest floor once a month to defecate, the kinkajou (Potos flavus), harpy eagle (Harpia harpyja), macaws, and other parrots.

Tropical rainforests have existed for hundreds of millions of years. Most that remain today are fragments of forest that covered the Mesozoic era Gondwana supercontinent. When this land mass separated, it resulted in a great loss of amphibian diversity but the consequent drier climate led to the diversification of reptiles. Many amphibians (frogs, toads, salamanders, newts, and wormlike caecilians) and reptiles (snakes, lizards, turtles, tortoises, and crocodiles) are found in current-day tropical rainforests. They form an important part of the rainforest ecosystem, helping to cycle nutrients through the food web. The most common types of amphibian in the rainforest are frogs. More than 1,000 known species of frog are found in the Amazon Basin alone. Many rainforest frogs are quite different from temperate frogs. The high humidity of the rainforest allows them to stay moist outside bodies of water and many are adapted to live high in the trees. These species also often lay their eggs in moist leaf litter or puddles of water formed in bromeliads high in the canopy.

Theories for the Origin of Biodiversity

There are two main theories for the origin of the great biodiversity of the tropical rainforests: the interspecific competition hypothesis and the Pleistocene refugia hypothesis. The interspecific competition hypothesis states that in tropical rainforests there is a high density of species with similar ecological niches, or the way that they respond to distributions of resources and competitors within an ecosystem. Since there are limited resources available, if two species share a niche and are therefore directly competing for resources, then one of those species must eventually lose the competition and find a new niche or go extinct. This pressure to find a new niche is called niche partitioning. Niche partitioning can involve utilizing a different habitat, food source, or behavior, such as eating the same food but at a different time of day.

The Pleistocene refugia hypothesis was formulated by Jurgen Haffer in 1969. He theorized that the present diversity was a product of rainforest patches being separated by stretches on nonrainforest vegetation during the last glacial period. These remaining rainforest patches served as refuges for the species of the rainforest and over time the species in each of these patches changed, causing speciation, or the creation of new species. At the end of the glacial period when humidity increased and the rainforest patches reconnected, the new rainforest stretch was populated by a larger diversity of species. This theory has been met with much debate and skepticism based on evidence that places rainforest speciation before the Pleistocene glacial period.

Importance to Humans: Carbon Sequestration, Medicine, and Food

Tropical rainforests provide many important services to humans, including oxygen production, carbon sequestration, and sources of food and medicine. It is estimated that more than half of all plant and animal species are found in tropical rainforests and there are likely millions of species yet to be discovered. Many of these species are, or could be, important sources of food and medicine. Tropical rainforests produce about 40 percent of the Earth's oxygen and sequester carbon from the atmosphere, which helps to mediate climate change.

Carbon flux is the exchange of carbon dioxide between the atmosphere and a reservoir, also known as a carbon sink. The net primary productivity is the amount of carbon that is retained in plant biomass over time. It is also the gross primary productivity minus carbon that is released through autotrophic respiration. This metric is used to measure the amount of carbon in a tropical rainforest. Tropical rainforests are important as a carbon sink because of their above- and below-ground biomass, the amount of carbon they store, and the rate of fixation by photosynthesis. Tropical rainforests absorb about 2.2 billion tons of carbon dioxide per year.

One-quarter of all medicines originate from tropical rainforest plants. For example, curare is made from a tropical vine (Chondrodendron tomentosum) that is used as an anesthetic and muscle relaxer, and quinine comes from the chinchona tree (Cinchona spp.) and is used to treat malaria. More than 1,400 plants are thought to contain properties that could potentially cure cancer. Rainforest frogs have been especially important for pharmaceutical discoveries because of the unique chemicals found in their skin. These chemicals, which the frogs use for defense or healing, have been found to act as powerful painkillers, muscle relaxants, and offer possible cures for cancer.

Many cultivated foods and spices originate from the tropical rainforest biome. Hundreds of types of fruit have been harvested from the rainforests, including banana, mango, and papaya as well as yam, coffee, chocolate, macadamia, and sugarcane. In New Guinea alone, over 250 edible fruit trees have been identified, many of them relatively recently. Valuable crops may be mostly grown on plantations, but much of their genetic diversity is maintained through pollination from wild stock. This introduction of new genes helps the crops to avoid disease and pest damage.

Threats to Tropical Rainforests

Most of the original extant of tropical rainforests has been lost due to clearing of forests for logging, mining, drilling, and agriculture. The ecosystem of the forest biome has also been affected by poaching by humans, the introduction of invasive species and diseases, and climate change.

Deposits of precious metals such as gold and diamonds and fossil fuels such as oil and natural gas have instigated forest clearing to allow access to these resources. Such mining and drilling is often given priority over environmental concerns to ensure economic growth in developing countries. Mining and drilling activities are also often associated with air and water pollution that contaminate rainforest ecosystems.

Much of the tropical rainforests worldwide have been converted to open farm land. Then thin, infertile soil is quickly leached of all minerals from farming and is quickly eroded by heavy rains. This poor productivity has led to a technique of slash-and-burn agriculture. Once one patch of land has been used up, farmers push farther in to the forest, which is cleared and then burned to add the organic matter to the soil.

The abandoned patch of forest is often completely eroded of soil, leaving only hard red clay behind, making it difficult for new trees to grow. However, this fallow period after agriculture can allow secondary forest to regrow and slowly replenish the soil layer. Fertilizers and pesticides used by farmers to boost the nutrient content of the rainforest soil often run off and contaminate soil and rivers.

The delicate ecosystem of the tropical rainforest biome can be affected by many anthropogenic activities other than forest clearing. Poaching can remove species that are important to the system's functioning. Staggering amounts of wildlife are taken from rainforests every year for food and medicinal uses. In the Brazilian Amazon alone, millions of mammals, birds, and reptiles are harvested annually. It is estimated that this number might be as much as six times higher in Africa due to the demand for meat in poor and rural areas.

Invasive species are plants and animals that, when introduced to a new region, adversely affect the ecosystems therein. Invasive species compete with native species and can bring with them new parasites and diseases. For example, the brown tree snake (Boiga irregularis) was accidentally brought to Guam in a ship's cargo and has since extirpated, or caused the local extinction of, most of the native forest vertebrate species. Invasive diseases can be detrimental to tropical rainforest ecosystems as well. Such a disease is caused by the chytrid fungus (Batrachochytriumdendrobatidis), which is thought to have caused the extinction of over one hundred species of frogs in the past few decades. Frog species in the tropical rainforests are especially hard hit because of the constant moisture and temperatures.

Tropical rainforest ecology is very dependent on climate, with many of its processes driven by the characteristic constant high heat, humidity, and rainfall. Therefore, this biome is very sensitive to changes in precipitation and temperature regimes such as those that might accompany global climate change. The climate in these equatorial forests is controlled by bands of clouds called the intertropical convergence zone (ITCZ) that forms near the equator. This zone is created by the convergence of trade winds from the Northern and Southern Hemispheres. Changes in the position of this convergence zone throughout the year cause slight seasonality in tropical rainforests, leading to wet and dry seasons in some forests.

Some regions have already experienced strong warming trends with increases in average temperature. These regions have also experienced a decrease in rates of precipitation. For example, tropical rainforests in Asia have suffered from increased dry season intensity, with longer periods of drought conditions. The El Niño Southern Oscillation also drives rainforest climate variability between years, and these events are likely to increase in duration and intensity with anthropogenic warming. These altered climate regimes may cause stress and increased mortality of trees within the tropical rainforest biome.

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