luxuriant forest, generally composed of broad-leaved trees and found in wet tropical uplands and lowlands around the Equator.

This section covers only the richest of rainforests—the tropical rainforests of the ever-wet tropics.

Tropical rainforests represent the oldest major vegetation type still present on the terrestrial Earth. Like all vegetation, however, that of the rainforest continues to evolve and change, so that modern tropical rainforests are not identical with rainforests of the geologic past.

The flowering plants (angiosperms (angiosperm)) first evolved and diversified during the Cretaceous Period about 100 million years ago, during which time global climatic conditions were warmer and wetter than those of the present. The vegetation types that evolved were the first tropical rainforests, which blanketed most of the Earth's land surfaces at that time. Only later—during the middle of the Cenozoic Era, about 40 million years ago—did cooler, drier climates develop, leading to the development across large areas of other vegetation types.

It is no surprise, therefore, to find the greatest diversity of flowering plants today in the tropical rainforests where they first evolved. Of particular interest is the fact that the majority of flowering plants displaying the most primitive characteristics are found in rainforests (especially tropical rainforests) in parts of the Southern Hemisphere, particularly South America, northern Australia and adjacent regions of Southeast Asia, and some larger South Pacific Islands. Of the 13 angiosperm families generally recognized as the most primitive, all but two— Magnoliaceae and Winteraceae—are overwhelmingly tropical in their present distribution. Three families—Illiciaceae, Magnoliaceae, and Schisandraceae—are found predominantly in Northern Hemisphere rainforests. Five families—Amborellaceae, Austrobaileyaceae, Degeneriaceae, Eupomatiaceae, and Himantandraceae—are restricted to rainforests in the tropical Australasian region. Members of the Winteraceae are shared between this latter region and South America, those of the Lactoridaceae grow only on the southeast Pacific islands of Juan Fernández, members of the Canellaceae are shared between South America and Africa, and two families— Annonaceae and Myristicaceae—generally occur in tropical regions. This has led some authorities to suggest that the original cradle of angiosperm evolution might lie in Gondwanaland (Gondwana), a supercontinent of the Southern Hemisphere thought to have existed in the Mesozoic Era (248 million to 65 million years ago) that consisted of Africa, South America, Australia, peninsular India, and Antarctica. An alternative explanation for this geographic pattern is that in the Southern Hemisphere, especially on islands, there are more refugia—i.e., isolated areas whose climates remained unaltered while those of the surrounding areas changed, enabling archaic life-forms to persist.

The first angiosperms are thought to have been massive, woody plants appropriate for a rainforest habitat. Most of the smaller, more delicate plants that are so widespread in the world today evolved later, ultimately from tropical rainforest ancestors. While it is possible that even earlier forms existed that await discovery, the oldest angiosperm fossils—leaves, wood, fruits, and flowers derived from trees—support the view that the earliest angiosperms were rainforest trees. Further evidence comes from the growth forms of the most primitive surviving angiosperms: all 13 of the most primitive angiosperm families consist of woody plants, most of which are large trees.

As the world climate cooled in the middle of the Cenozoic, it also became drier. This is because cooler temperatures led to a reduction in the rate of evaporation of water from, in particular, the surface of the oceans, which led in turn to less cloud formation and less precipitation. The entire hydrologic cycle slowed, and tropical rainforests—which depend on both warmth and consistently high rainfall—became increasingly restricted to equatorial latitudes. Within those regions rainforests were limited further to coastal and hilly areas where abundant rain still fell at all seasons. In the middle latitudes of both hemispheres, belts of atmospheric high pressure developed. Within these belts, especially in continental interiors, deserts formed (see desert: Origin (desert)). In regions lying between the wet tropics and the deserts, climatic zones developed in which rainfall adequate for luxuriant plant growth was experienced for only a part of the year. In these areas new plant forms evolved from tropical rainforest ancestors to cope with seasonally dry weather, forming tropical deciduous forests. In the drier and more fire-prone places, savannas (savanna) and tropical grasslands (grassland) developed.

Retreat of the rainforests was particularly rapid during the period beginning 5,000,000 (Cenozoic Era) years ago leading up to and including the Pleistocene (Pleistocene Epoch) Ice Ages, or glacial intervals, that occurred between 1,600,000 and 10,000 years ago. Climates fluctuated throughout this time, forcing vegetation in all parts of the world to repeatedly migrate, by seed dispersal, to reach areas of suitable climate. Not all plants were able to do this equally well because some had less-effective means of seed dispersal than others. Many extinctions resulted. During the most extreme periods (the glacial maxima, when climates were at their coldest and, in most places, also driest), the range of tropical rainforests shrank to its smallest extent, becoming restricted to relatively small refugia. Alternating intervals of climatic amelioration led to repeated range expansion, most recently from the close of the last glacial period about 10,000 years ago. Today large areas of tropical rainforest, such as Amazonia, have developed as a result of this relatively recent expansion. Within them it is possible to recognize “hot spots” of plant and animal diversity that have been interpreted as glacial refugia.

Tropical rainforests today represent a treasure trove of biological heritage. They not only retain many primitive plant and animal species but also are communities that exhibit unparalleled biodiversity and a great variety of ecological interactions. The tropical rainforest of Africa was the habitat in which the ancestors of humans evolved, and it is where the nearest surviving human relatives—chimpanzees and gorillas—live still. Tropical rainforests supplied a rich variety of food and other resources to indigenous peoples, who, for the most part, exploited this bounty without degrading the vegetation or reducing its range to any significant degree. However, in some regions a long history of forest burning by the inhabitants is thought to have caused extensive replacement of tropical rainforest and tropical deciduous forest with savanna.

Not until the past century, however, has widespread destruction of tropical forests occurred. Regrettably, tropical rainforests and tropical deciduous forests are now being destroyed at a rapid rate in order to provide resources such as timber and to create land that can be used for other purposes, such as cattle grazing. Today tropical forests, more than any other ecosystem, are experiencing habitat alteration and species extinction on a greater scale and at a more rapid pace than at any time in their history—at least since the major extinction event at the end of the Cretaceous Period some 65 million years ago (see ).

The equatorial latitude of tropical rainforests and tropical deciduous forests keeps day length and mean temperature fairly constant throughout the year. The sun rises daily to a near-vertical position at noon, ensuring a high level of incoming radiant energy at all seasons. Although there is no cold season during which plants experience unfavourable temperatures that prohibit growth, there are many local variations in climate that result from topography, and these variations influence and restrict rainforest distribution within the tropics.

Tropical rainforests occur in regions of the tropics where temperatures are always high and where rainfall exceeds about 1,800 to 2,500 mm (about 70 to 100 inches) annually and occurs fairly evenly throughout the year. Similar hot climates in which annual rainfall lies between about 800 and 1,800 mm and in which a pronounced season of low rainfall occurs typically support tropical deciduous forests—i.e., rainforests in which up to about three-quarters of the trees lose their leaves in the dry season. The principal determining climatic factor for the distribution of rainforests in lowland regions of the tropics, therefore, is rainfall, both the total amount and the seasonal variation. Soil, human disturbance, and other factors also can be important controlling influences.

The climate is always hot and wet in most parts of the equatorial belt, but in regions to its north and south seasonal rainfall is experienced. During the summer months of the Northern Hemisphere—June to August—weather systems shift northward, bringing rain to regions in the northern parts of the tropics, as do the monsoon rains of India and Myanmar. Conversely, during the Southern Hemisphere's summer, weather systems move southward, bringing rain from December to February to places such as northern Australia. In these hot, seasonally wet areas grow tropical deciduous forests, such as the teak forests of Myanmar and Thailand. In other locations where conditions are similar but rainfall is not so reliable or burning has been a factor, savannas are found.

Topographic factors influence rainfall and consequently affect rainforest distribution within a region. For example, coastal regions where prevailing winds blow onshore are likely to have a wetter climate than coasts that experience primarily offshore winds. The west coasts of tropical Australia and South America south of the Equator experience offshore winds, and these dry regions can support rainforests only in very small areas. This contrasts with the more extensively rainforest-clad, east-facing coasts of these same continents at the same latitudes. The same phenomenon is apparent on a smaller scale where the orientation of coastlines is parallel to, rather than perpendicular to, wind direction. For example, in the Townsville area of northeastern Australia and in Benin in West Africa, gaps in otherwise fairly continuous tracts of tropical rainforest occur where the prevailing winds blow along the coast rather than across it.

Mean temperatures (temperature) in tropical rainforest regions are between 20 and 29 °C (68 and 84° F), and in no month is the mean temperature below 18 °C (64 °F). Temperatures become critical with increasing altitude; in the wet tropics temperatures fall by about 0.5 °C (0.9 °F) for every 100 metres (328 feet) climbed. Vegetation change across altitudinal gradients tends to be gradual and variable and is interpreted variously by different authorities. For example, in Uganda tropical rainforest grows to an altitude of 1,100 to 1,300 metres and has been described as giving way, via a transition forest zone, to montane rainforest above 1,650 to 1,750 metres, which continues to 2,300 to 3,400 metres. In New Guinea, lowland tropical rainforest reaches 1,000 to 1,200 metres, above which montane rainforests extend, with altitudinal variation, to 3,900 metres. In Peru, lowland rainforest extends upward to 1,200 to 1,500 metres, with transitional forest giving way to montane rainforest above 1,800 to 2,000 metres, which continues to 3,400 to 4,000 metres. These limits are comparable and reflect the similarities of climate in all regions where tropical rainforests occur. Plant species, however, are often quite different among regions.

Although the climate supporting tropical rainforests is perpetually hot, temperatures never reach the high values regularly recorded in drier places to the north and south of the equatorial belt. This is partly due to high levels of cloud cover, which limit the mean number of sunshine hours per day to between four and six. In hilly areas where air masses rise and cool because of the topography, the hours of sunlight may be even fewer. Nevertheless, the heat may seem extreme owing to the high levels of atmospheric humidity, which usually exceed 50 percent by day and approach 100 percent at night. Exacerbating the discomfort is the fact that winds are usually light; mean wind speeds are generally less than 10 km (6.2 miles) per hour and less than 5 km per hour in many areas. Devastating hurricanes (cyclones and typhoons) occur periodically in some coastal regions toward the margins of the equatorial belt, such as in the West Indies and in parts of the western Pacific region. Although relatively infrequent, such storms have an important effect on forest structure and regeneration.

Even within the same area, however, there are likely to be signifiant variations in soil related to topographic position and to bedrock differences, and these variations are reflected in forest composition and structure. For example, as altitude increases—even within the same area and on the same bedrock—soil depth decreases markedly and its organic content increases in association with changes in forest composition and structure.

Only a minority of plant and animal species in tropical rainforests and tropical deciduous forests have been described formally and named. Therefore, only a rough estimate can be given of the total number of species contained in these ecosystems, as well as the number that are becoming extinct as a result of forest clearance. Nevertheless, it is quite clear that these vegetation types are the most diverse of all, containing more species than any other ecosystem. This is particularly so in regions in which tropical rainforests not only are widespread but also are separated into many small areas by geographic barriers, as in the island-studded Indonesian region (see biogeographic region: The distribution boundaries of flora and fauna (biogeographic region)). In this area different but related species often are found throughout various groups of islands, adding to the total regional diversity. Exceptionally large numbers of species also occur in areas of diverse habitat, such as in topographically or geologically complex regions and in places that are believed to have acted as refugia throughout the climatic fluctuations of the past few million years. According to some informed estimates, more than a hundred species of rainforest fauna and flora become extinct every week as a result of widespread clearing of forests by humans. Insects are believed to constitute the greatest percentage of disappearing species.

Tropical rainforests, which contain many different types of trees, seldom are dominated by a single species. A species can predominate, however, if particular soil conditions favour this occurrence or minimal disturbance occurs for several tree generations. Tropical deciduous forests are less diverse and often are dominated by only one or two tree species. The extensive deciduous forests of Myanmar, for example, cover wide areas and are dominated by only one or two tree species— teak (Tectona grandis) and the smaller leguminous tree Xylia xylocarpa. In Thailand and Indochina deciduous forests are dominated by members of the Dipterocarpaceae family, Dipterocarpus tuberculatus, Pentacme suavis, and Shorea obtusa.

Ferns, mosses, liverworts (liverwort), lichens (lichen), and algae are also abundant and diverse, although not as well studied and cataloged as the higher plants. Many are epiphytic and are found attached to the stems and sometimes the leaves of larger plants, especially in the wettest and most humid places. Fungi (fungus) and other saprophytic plants (vegetation growing on dead or decaying matter) are similarly diverse. Some perform a vital role in decomposing dead organic matter on the forest floor and thereby releasing mineral nutrients, which then become available to roots in the surface layers of the soil. Other fungi enter into symbiotic relationships with tree roots (mycorrhizae (mycorrhiza)).

Interacting with and dependent upon this vast array of plants are similarly numerous animals. Like the plants, most animal species are limited to only one or a few types of tropical rainforest within an area, with the result that the overall number of species is substantially greater than it is in a single forest type. For example, a study of insects in the canopy of four different types of tropical rainforest in Brazil revealed 1,080 species of beetle, of which 83 percent were found in only one forest type, 14 percent in two, and only 3 percent in three or four types. While the larger, more conspicuous vertebrates (mammals, birds, and to a lesser degree amphibians and reptiles) are well known, only a small minority of the far more diverse invertebrates (particularly insects) have ever been collected, let alone described and named.

As with the plants, some animal groups occur in all tropical rainforest regions. A variety of fruit-eating parrots, pigeons, and seed-eating weevil beetles, for example, can be expected to occur in any tropical rainforest. Other groups are more restricted. Monkeys, while typical of tropical rainforests in both the New and the Old Worlds, are entirely absent from New Guinea and areas to its east and south. Tree kangaroos inhabit tropical rainforest canopies only in Australia and New Guinea, and birds of paradise (bird-of-paradise) are restricted to the same areas.

Tropical rainforests are distinguished not only by a remarkable richness of biota but also by the complexity of the interrelationships of all the plant and animal inhabitants that have been evolving together throughout many millions of years. As in all ecosystems, but particularly in the complex tropical rainforest community, the removal of one species threatens the survival of others with which it interacts. Some interactions are mentioned below, but many have yet to be revealed.

Plants with similar stature and life-form can be grouped into categories called synusiae, which make up distinct layers of vegetation. In tropical rainforests the synusiae are more numerous than in other ecosystem types. They include not only mechanically independent forms, whose stems are self-supporting, and saprophytic plants but also mechanically dependent synusiae such as climbers, stranglers, epiphytes, and parasitic plants. An unusual mix of trees of different sizes is found in the tropical rainforest, and those trees form several canopies below the uppermost layer, although they are not always recognizably separate layers. The upper canopy of the tropical rainforest is typically greater than 40 metres above ground.

Gaps in the canopy of a tropical rainforest provide temporarily well-illuminated places at ground level and are vital to the regeneration of most of the forest's constituent plants. Few plants in the forest can successfully regenerate in the deep shade of an unbroken canopy; many tree species are represented there only as a population of slender, slow-growing seedlings or saplings that have no chance of growing to the well-lit canopy unless a gap forms. Other species are present, invisibly, as dormant seeds in the soil. When a gap is created, seedlings and saplings accelerate their growth in the increased light and are joined by new seedlings sprouting from seeds stored in the soil that have been stimulated to germinate by light or by temperature fluctuations resulting from the sun's shining directly on the soil surface. Other seeds arrive by various seed-dispersal processes (see below). A thicket of regrowth rapidly develops, with the fastest-growing shrubs and trees quickly shading out opportunistic, light-demanding, low-growing herbaceous plants and becoming festooned with lianas. Through it all slower-growing, more shade-tolerant but longer-lived trees eventually emerge and restore the full forest canopy. The trees that initially fill in the gap in the canopy live approximately one century, whereas the slower-growing trees that ultimately replace them may live for 200 to 500 years or, in extreme cases, even longer. Detailed mapping of the trees in a tropical rainforest can reveal the locations of previous gaps through identification of clumps of the quicker-growing, more light-demanding species, which have yet to be replaced by trees in the final stage of successional recovery. Local, natural disturbances of this sort are vital to the maintenance of the full biotic diversity of the tropical rainforest (see Rainforest Regeneration).

Epiphytes (epiphyte) are particularly diverse and include large plants such as orchids (orchid), aroids, bromeliads, and ferns in addition to smaller plants such as algae, mosses, and lichens. In tropical rainforests epiphytes are often so abundant that their weight fells trees. Epiphytes that grow near the upper canopy of the forest have access to bright sunlight but must survive without root contact with the soil. They depend on rain washing over them to provide water and mineral nutrients. During periods of drought, epiphytes undergo stress as water stored within their tissues becomes depleted. The diversity of epiphytes in tropical deciduous forests is much less than that of tropical rainforests because of the annual dry season (see ).

Parasitic (parasitism) flowering plants also occur. Hemiparasitic mistletoes (mistletoe) attached to tree branches extract water and minerals from their hosts but carry out their own photosynthesis. Plants that are completely parasitic also are found in tropical rainforests. Rafflesia (Rafflesiaceae), in Southeast Asia, parasitizes the roots of certain lianas and produces no aboveground parts until it flowers; its large orange and yellow blooms, nearly one metre in diameter, are the largest flowers of any plant.

Stranglers (strangler fig) make up a type of synusia virtually restricted to tropical rainforests. In this group are strangler figs (strangler fig) ( Ficus), which begin life as epiphytes, growing from seeds left on high tree branches by birds or fruit bats. As they grow, they develop long roots that descend along the trunk of the host tree, eventually reaching the ground and entering the soil. Several roots usually do this, and they become grafted together as they crisscross each other to form a lattice, ultimately creating a nearly complete sheath around the trunk. The host tree's canopy becomes shaded by the thick fig foliage, its trunk constricted by the surrounding root sheath and its own root system forced to compete with that of the strangling fig. The host tree is also much older than the strangler and eventually dies and rots away, leaving a giant fig “tree” whose apparent “trunk” is actually a cylinder of roots, full of large hollows that provide shelter and breeding sites for bats, birds, and other animals (see ). Stranglers may also develop roots from their branches, which, when they touch the ground, grow into the soil, thicken, and become additional “trunks.” In this way stranglers grow outward to become large patches of fig forest that consist of a single plant with many interconnected trunks.

Some of the tallest trees and lianas, and the epiphytes they support, bear flowers and fruits at the top of the rainforest canopy, where the air moves unfettered by vegetation. They are able to depend on the wind for dispersal of pollen from flower to flower, as well as for the spreading of fruits and seeds away from the immediate environment of the parent plant (see “Flying” Trees). Ferns, mosses, and other lower plants also exploit the wind to carry their minute spores. However, a great many flowering plants, including many that grow in the nearly windless environment of the understory, depend on animals to perform these functions. They are as dependent on animals for reproductive success as the animals are on them for food—one example of the mutual dependence between plants and animals (see ).

A variety of birds (bird) eat fleshy fruits also, voiding or regurgitating the unharmed seeds. Birds of different sizes are typically attracted to similarly scaled fruits, which are carried on stems of appropriate thickness and strength. For example, large pigeons in New Guinea feed preferentially on larger fruits borne on thicker stems that can bear not only the weight of the fruit but also the weight of the large bird; smaller pigeons tend to feed on smaller fruits borne on thinner twigs. In such a manner, the diverse plant community is matched by a similarly diverse animal community in interdependence.

Terrestrial mammals also help to disperse seeds. In many cases this has favoured the positioning of flowers and fruits beneath the canopy on the trunks of trees accessible to animals unable to climb or fly, an adaptation called cauliflory. In some cases fruits are grown in the canopy but drop as they ripen, opening only after they fall to attract ground-dwelling animals that will carry them away from the parent tree. The durian fruit Durio zibethinus of Southeast Asian rainforests is an example; its fruits are eaten and its seeds dispersed by a range of mammals, including pigs, elephants, and even tigers.

Many other animals, from ants to apes, are involved in seed dispersal. In the Amazon basin of Brazil, where large areas of tropical rainforest are seasonally flooded, many trees produce fruit attractive to fish, which swallow them whole and void the seeds (see ). Squirrels are also important seed dispersers in parts of South America. In the tropical rainforests of northeastern Australia, cassowaries (cassowary) are responsible for generating mixed clumps of tree seedlings of several species that grow from their dung sites.

It is important for seeds to be spread away from parent plants, both to allow seedlings to escape competition with the parent and to expand the range of the species. Another capacity important to seed survival, particularly in the diverse tropical rainforest community, involves the evasion of seed predators. Many different beetles and other insects are specialized to feed on particular types of seed. Seeds concentrated beneath a parent plant are easy for seed predators to locate. Seeds that are carried away to areas occupied by different plant species—and different seed predators—are more likely to survive (see ).

In addition to dispersing seeds, animals are vital to tropical rainforest reproduction through flower pollination. Many insects such as bees, moths, flies, and beetles as well as birds and bats carry out this activity. Birds such as the hummingbirds (hummingbird) of South and Central America and the flower-peckers (flowerpecker) of Asia have adaptations that allow them to sip nectar from flowers. In the process they inadvertently become dusted with pollen, which they subsequently transport to other flowers, pollinating them. The plants involved also show special adaptations in flower structure and colour. Most flowers pollinated by birds are red, a colour highly visible to these animals, whereas flowers pollinated by night-flying moths are white or pink, and those pollinated by insects that fly during the day are often yellow or orange. Bats are important pollinators of certain pale, fragrant flowers that open in the evening in Asian rainforests (see also Chiropterophilous Plants).

Of all vegetation types, tropical rainforests grow in climatic conditions that are least limiting to plant growth. It is to be expected that the growth and productivity (total amount of organic matter produced per unit area per unit time) of tropical rainforests would be higher than that of other vegetation, provided that other factors such as soil fertility or consumption by herbivorous animals are not extremely low or high.

Various methods are employed to assess productivity. Gross primary productivity is the amount of carbon fixed during photosynthesis by all producers in the ecosystem. However, a large part of the harnessed energy is used up by the metabolic processes of the producers ( respiration). The amount of fixed carbon not used by plants is called net primary productivity, and it is this remainder that is available to various consumers in the ecosystem—e.g., the herbivores, decomposers, and carnivores. Of course, in any stable ecosystem there is neither an accumulation nor a diminution in the total amount of organic matter present, so that overall there is a balance between the gross primary productivity and the total consumption. The amount of organic matter in the system at any point in time, the total mass of all the organisms present, is called the biomass. (For further discussion of productivity, see biosphere: Resources of the biosphere (biosphere).)

The biomass of tropical rainforests is larger than that of other vegetation. It is not an easy quantity to measure, involving the destructive sampling of all the plants in an area (including their underground parts), with estimates made of the mass of other organisms belonging to the ecosystem such as animals. Measurements show that tropical rainforests typically have biomass values on the order of 400 to 700 metric tons per hectare, greater than most temperate forests and substantially more than other vegetation with fewer or no trees. A measurement of biomass in a tropical deciduous forest in Thailand yielded a value of about 340 metric tons per hectare.

Increase in biomass over the period of a year at one rainforest site in Malaysia was estimated at 7 metric tons per hectare, while total litter fall was 14 metric tons, estimated mass of sloughed roots was 4 metric tons, and total live plant matter eaten by herbivorous animals (both invertebrate and vertebrate) was about 5 metric tons per hectare per year. These values add up to a total net production of 30 metric tons per hectare per year. Respiration by the vegetation itself was estimated at 50 metric tons, so that gross primary productivity was about 80 metric tons per hectare per year. Compared with temperate forests, these values are approximately 2.5 times higher for net productivity and 4 times higher for gross productivity, the difference being that the respiration rate at the tropical site was 5 times that of temperate forests.

Despite the overall high rates of productivity and biomass in tropical rainforests, the growth rates of their timber trees are not unusually fast; in fact, some temperate trees and many smaller herbaceous plants grow more rapidly. The high productivity of tropical rainforests instead results in their high biomass and year-round growth. They also have particularly high levels of consumption by herbivores (see ), litter production, and especially plant respiration.