Rainforests are forests characterized by a closed and continuous tree canopy, moisture-dependent vegetation, the presence of epiphytes and lianas and the absence of wildfire. Rainforests can generally be classified as tropical rainforests or temperate rainforests, but other types have been described.
Estimates vary from 40% to 75% of all biotic species being indigenous to the rainforests. There may be many millions of species of plants, insects and microorganisms still undiscovered in tropical rainforests. Tropical rainforests have been called the "jewels of the Earth" and the "world's largest pharmacy", because over one quarter of natural medicines have been discovered there.
Rainforests as well as endemic rainforest species are rapidly disappearing due to deforestation, the resulting habitat loss and pollution of the atmosphere.
Definition Rainforests are characterized by a closed and continuous tree canopy, high humidity, the presence of moisture-dependent vegetation, a moist layer of leaf litter, the presence of epiphytes and lianas and the absence of wildfire. The largest areas of rainforest are tropical or temperate rainforests, but other vegetation associations including subtropical rainforest, littoral rainforest, cloud forest, vine thicket and even dry rainforest have been described.
Tropical rainforest
Tropical rainforests are characterized by a warm and wet climate usually with no substantial dry season: typically found within 10 degrees north and south of the equator. Mean monthly temperatures exceed during all months of the year.Susan Woodward. Tropical broadleaf Evergreen Forest: The rainforest. Retrieved on 14 March 2008. Average annual rainfall is no less than and can exceed although it typically lies between and .Newman, Arnold. The Tropical Rainforest : A World Survey of Our Most Valuable Endangered Habitat : With a Blueprint for Its Survival. New York: Checkmark, 2002. Print.
Many of the world's tropical forests are associated with the location of the monsoon trough, also known as the Intertropical Convergence Zone.Hobgood (2008). Global Pattern of Surface Pressure and Wind. Ohio State University. Retrieved on 8 March 2009. The broader category of tropical moist forests are located in the equatorial zone between the Tropic of Cancer and Tropic of Capricorn. Tropical rainforests exist in Southeast Asia (from Myanmar (Burma)) to the Philippines, Malaysia, Indonesia, Papua New Guinea and Sri Lanka; also in Sub-Saharan Africa from the Cameroon to the Congo (Congo Rainforest), South America (e.g. the Amazon rainforest), Central America (e.g. Bosawás, the southern Yucatán Peninsula-El Peten-Belize-Calakmul), Australia, and on Pacific Islands (such as Hawaii). Tropical forests have been called the "Earth's lungs", although it is now known that rainforests contribute little net oxygen addition to the atmosphere through photosynthesis.Broecker, Wallace S. (2006). "Breathing easy: Et tu, O2." Columbia University Columbia.edu
Temperate rainforest
Tropical forests cover a large part of the globe, but temperate rainforests only occur in a few regions around the world. Temperate rainforests are rainforests in temperate regions. They occur in North America (in the Pacific Northwest in Alaska, British Columbia, Washington, Oregon as well as California and the Appalachian Mountains), in Europe (parts of the British Isles such as the coastal areas of Ireland and Scotland, southern Norway, parts of the western Balkans along the Adriatic coast, as well as in Galicia and coastal areas of the eastern Black Sea, including Georgia and coastal Turkey), in East Asia (in southern China, Highlands of Taiwan, much of Japan and Korea, and on Sakhalin Island and the adjacent Russian Far East coast), in South America (southern Chile) and also in Australia and New Zealand.
Dry rainforest Dry rainforests have a more open canopy layer than other rainforests, and are found in areas of lower rainfall (. Dry rainforests typically occur in the rain-shadow of mountainous areas. In Australia, dry rainforests have structurally complex species composition. These species can have various adaptations such as deciduousness or semi-deciduousness, reduced leaf size, thick leathery leaves (sclerophylly), sharp thorns or leaf margins and prickles. Dry rainforests containing a dense vine understory can be split up into different sub-types which include low microphyll vine forest, Araucarian vine forest, Semi-evergreen vine thicket, and deciduous vine thicket. These can further split up into either microphyll or notophyll depending on their leaf sizes.
Layers
A tropical rainforest typically has a number of layers, each with different plants and animals adapted for life in that particular area. Examples include the emergent, canopy, understory and forest floor layers.
Emergent layer The emergent layer contains a small number of very large trees called emergents, which grow above the general canopy, reaching heights of 45–55 m, although on occasion a few species will grow to 70–80 m tall. They need to be able to withstand the hot temperatures and strong winds that occur above the canopy in some areas. Eagles, butterflies, bats and certain monkeys inhabit this layer.
Canopy layer
The canopy layer contains the majority of the largest trees, typically to tall. The densest areas of biodiversity are found in the forest canopy, a more or less continuous cover of foliage formed by adjacent treetops. The canopy, by some estimates, is home to 50 percent of all plant species. Epiphytic plants attach to trunks and branches, and obtain water and minerals from rain and debris that collects on the supporting plants. The fauna is similar to that found in the emergent layer but more diverse. A quarter of all insect species are believed to exist in the rainforest canopy. Scientists have long suspected the richness of the canopy as a habitat, but have only recently developed practical methods of exploring it. As long ago as 1917, naturalist William Beebe declared that "another continent of life remains to be discovered, not upon the Earth, but one to two hundred feet above it, extending over thousands of square miles." A true exploration of this habitat only began in the 1980s, when scientists developed methods to reach the canopy, such as firing ropes into the trees using crossbows. Exploration of the canopy is still in its infancy, but other methods include the use of balloons and airships to float above the highest branches and the building of cranes and walkways planted on the forest floor. The science of accessing tropical forest canopy using airships or similar aerial platforms is called dendronautics.
Understory layer
The understory or understorey layer lies between the canopy and the forest floor. It is home to a number of birds, snakes and lizards, as well as predators such as jaguars, boa constrictors and leopards. The leaves are much larger at this level and insect life is abundant. Many seedlings that will grow to the canopy level are present in the understory. Only about 5% of the sunlight shining on the rainforest canopy reaches the understory. This layer can be called a shrub layer, although the shrub layer may also be considered a separate layer.
Forest floor
The forest floor, the bottom-most layer, receives only 2% of the sunlight. Only plants adapted to low light can grow in this region. Away from riverbanks, swamps and clearings, where dense undergrowth is found, the forest floor is relatively clear of vegetation because of the low sunlight penetration. It also contains decaying plant and animal matter, which disappears quickly, because the warm, humid conditions promote rapid decay. Many forms of fungi growing here help decay the animal and plant waste.
Flora and fauna
It is unknown how many species are found in the world's tropical rainforests, however a very large proportion of vascular plants are native to them. Rainforests support a very broad array of fauna, including mammals, reptiles, amphibians, birds and invertebrates. Mammals may include primates, felids and other families. Reptiles include snakes, turtles, chameleons and other families; while birds include such families as Vangidae and Cuculidae. Dozens of families of invertebrates are found in rainforests. Fungi are also very common in rainforest areas as they can feed on the decomposing remains of plants and animals.
The great diversity in rainforest species is in large part the result of diverse and numerous physical refuges, i.e. places in which plants are inaccessible to many herbivores, or in which animals can hide from predators. Having numerous refuges available also results in much higher total biomass than would otherwise be possible.
Some species of fauna show a trend towards declining populations in rainforests, for example, reptiles that feed on amphibians and reptiles. This trend requires close monitoring.Barquero-González, J.P., Stice, T.L., Gómez, G., & Monge-Nájera, J. (2020). Are tropical reptiles really declining? A six-year survey of snakes in a tropical coastal rainforest: role of prey and environment. Revista de Biología Tropical, 68(1), 336–343. The seasonality of rainforests affects the reproductive patterns of amphibians, and this in turn can directly affect the species of reptiles that feed on these groups,Oliveira, M.E., & Martins, M. (2001). When and where to find a pitviper: activity patterns and habitat use of the lancehead, Bothrops atrox, in central Amazonia, Brazil. Herpetological Natural History, 8(2), 101-110. particularly species with specialized feeding, since these are less likely to use alternative resources.Terborgh, J., & Winter, B. (1980). Some causes of extinction. Conservation Biology, 2, 119-133.
Soils
Despite the growth of vegetation in a tropical rainforest, soil quality is often quite poor. Rapid bacterial decay prevents the accumulation of humus. The concentration of iron and aluminium oxides by the laterization process gives the oxisols a bright red colour and sometimes produces mineral deposits such as bauxite. Most trees have roots near the surface because there are insufficient nutrients below the surface; most of the trees' minerals come from the top layer of decomposing leaves and animals. On younger substrates, especially of volcanic origin, tropical soils may be quite fertile. If rainforest trees are cleared, rain can accumulate on the exposed soil surfaces, creating run-off, and beginning a process of soil erosion. Eventually, streams and rivers form and flooding becomes possible. There are several reasons for the poor soil quality. First is that the soil is highly acidic. The roots of plants rely on an acidity difference between the roots and the soil in order to absorb nutrients. When the soil is acidic, there is little difference, and therefore little absorption of nutrients from the soil. Second, the type of clay particles present in tropical rainforest soil has a poor ability to trap nutrients and stop them from washing away. Even if humans artificially add nutrients to the soil, the nutrients mostly wash away and are not absorbed by the plants. Finally, these soils are poor due to the high volume of rain in tropical rainforests washing nutrients out of the soil more quickly than in other climates.
Effect on global climate