What are tropical rainforests?

By Rhett A. Butler
April 1, 2019

Part I:

TROPICAL RAINFORESTS OF THE WORLD

Tropical rainforests are unparalleled in their biological diversity and play a crucial role in the global ecosystem and human existence. They serve as vast reservoirs of genetic diversity, offering medicinal plants, high-yield foods, and numerous other valuable resources. These forests provide essential habitats for migratory species and support nearly 50 percent of Earth’s biodiversity, along with diverse and unique Indigenous cultures. Beyond their ecological significance, tropical rainforests help regulate global weather, sustain regular rainfall, and buffer against floods, droughts, and erosion. They also store immense amounts of carbon while producing a significant share of the world’s oxygen.

Despite their immense importance, tropical rainforests are confined to the narrow band between 23.5° North and 23.5° South of the equator—between the Tropic of Capricorn and the Tropic of Cancer. Given that most of Earth’s landmass lies north of this region, rainforests naturally occupy a limited area.

Like many other natural landscapes, tropical rainforests have become increasingly scarce in the 21st century. Just five generations ago, vast expanses of forest, swamp, desert, and savanna covered much of Earth’s surface. Today, more than two-thirds of the world's tropical rainforests exist as fragmented remnants. Currently, tropical forests cover about 13 percent of Earth’s land area—approximately 2 billion hectares (7.7 million square miles)—with a significant portion consisting of tropical rainforest. The largest continuous stretch of rainforest is found in the Amazon Basin of South America, with nearly two-thirds of this forest located in Brazil, which holds about one-third of the world’s remaining tropical rainforests. Indonesia and the Congo Basin account for another 20 percent, while the rest are scattered across tropical regions worldwide.

The global distribution of tropical rainforests can be categorized into four biogeographical realms, roughly corresponding to major forested continental regions: the Ethiopian (Afrotropical), Australasian (Australian), Oriental (Indomalayan/Asian), and Neotropical. More than half of the world’s rainforests are in the Neotropical realm, roughly a quarter are in Africa, and a fifth in Asia. The remaining five percent are scattered across Australia, New Guinea, and various Pacific islands.

Part II:

WHERE ARE TROPICAL RAINFORESTS LOCATED?

Most tropical rainforests are found within four biogeographic realms: the Afrotropical (mainland Africa, Madagascar, and surrounding islands), the Australian (Australia, New Guinea, and the Pacific Islands), the Indomalayan (India, Sri Lanka, mainland Asia, and Southeast Asia), and the Neotropical (South America, Central America, and the Caribbean).

AFROTROPICAL REALM

Most of Africa's tropical rainforests are concentrated in the Congo Basin, though a once-continuous forest stretched from Senegal on the Atlantic coast to the Rift Valley. West Africa has experienced extensive deforestation due to logging and agriculture, leaving only a fraction of its original forest cover intact. Since the 1990s, timber from Central Africa—particularly Gabon, Cameroon, and the Congo Basin—has increasingly filled the gap left by declining West African timber exports. As a result, deforestation in Central Africa has accelerated, driven by logging and large-scale agricultural expansion, including oil palm plantations.

At the turn of the century, West Africa had approximately 500,000 square kilometers (193,000 square miles) of coastal rainforest. However, much of this forest, primarily lowland formations accessible from the coast, has been depleted by commercial logging and agricultural conversion. According to the Food and Agriculture Organization of the United Nations, less than 20 percent of West Africa’s moist forests remain, and much of what remains is degraded. In countries like Nigeria, population pressures have intensified forest loss, while Côte d'Ivoire has experienced widespread deforestation due to commercial agriculture and logging. The full consequences of this forest loss remain unclear, but erosion has increased significantly, and interior countries like Mali and Niger have seen rising drought incidence, likely linked to reduced rainfall maintained by coastal forests.

Despite deforestation, Central Africa’s rainforests still cover a vast area. Approximately 75 percent of Africa’s remaining rainforests—about 1.4 million square kilometers (540,000 square miles)—are located in Central Africa, primarily in the Congo Basin. These forests face growing threats from logging, industrial agriculture, and subsistence activities, particularly fuelwood collection and small-scale farming.

Part III:

TYPES OF TROPICAL FORESTS

Rainforests exist across the globe, not only in tropical regions but also in temperate areas such as Canada, the United States, and parts of the former Soviet Union. Like their tropical counterparts, these forests receive abundant year-round rainfall, feature an enclosed canopy, and support high species diversity. However, they lack the constant warmth and sunlight characteristic of tropical rainforests. This section focuses exclusively on tropical rainforests.

Depending on altitude, latitude, soil composition, flooding patterns, and climate conditions, tropical rainforests transition into other types of forests. These variations create a mosaic of vegetation types, contributing to the immense biodiversity of the tropics.

EQUATORIAL EVERGREEN RAINFOREST VS. MOIST FOREST

Wet tropical forests can be categorized into two major types: equatorial evergreen rainforests and moist forests, which include monsoon forests and montane/cloud forests. Equatorial rainforests—often considered the “true rainforest”—receive more than 80 inches (2,000 mm) of rainfall annually, distributed evenly throughout the year. These forests have the highest biological diversity and feature a well-developed, multi-tiered canopy structure. Approximately two-thirds of the world's tropical wet forests fall into this category. They are located near the equator, where seasonal variation is minimal and daylight length remains constant year-round. The largest expanses of equatorial rainforest are found in the lowland Amazon, the Congo Basin, and the Southeast Asian islands of Indonesia and Papua New Guinea.

Tropical moist forests, by contrast, occur farther from the equator, where seasonal changes in rainfall and daylight length are more pronounced. These forests receive "only" 50 inches (1,270 mm) of rain annually and are distinct from equatorial rainforests due to the presence of a cooler dry season. During this period, many trees shed some or all of their leaves, temporarily reducing canopy cover and allowing more sunlight to reach the forest floor. This increased light supports vigorous understory vegetation, which is generally absent in lowland equatorial forests. Tropical moist forests are found in regions such as South America, the Caribbean, West Africa, and parts of Southeast Asia, including Thailand, Myanmar, Vietnam, and Sri Lanka.

PRIMARY VS. SECONDARY FOREST

The term "primary forest" appears frequently in discussions about tropical rainforests. Primary forest (data table) refers to untouched, pristine forest that remains in its original condition, largely unaffected by human activity. These forests are typically characterized by a continuous canopy ceiling and multiple layers of understory vegetation. The forest floor is generally sparse in vegetation because the dense canopy allows little sunlight to penetrate. When a canopy tree falls, it creates a temporary "light gap," enabling understory and ground-floor species to flourish. Primary forests are the most biologically diverse type of rainforest.

Secondary forest, by contrast, has been disturbed—either naturally or through human activity. Secondary forest (data table) can develop in various ways, from degraded forest recovering after selective logging to land cleared by slash-and-burn agriculture that has been left to regenerate. Depending on the extent of disturbance, secondary forests generally have a less developed canopy structure, smaller trees, and lower biodiversity. Due to gaps in the canopy, more sunlight reaches the forest floor, promoting dense ground vegetation. The term "jungle" is often used to describe secondary forests with thick undergrowth, though it is also applied to some tropical moist forests where seasonal conditions allow dense ground cover.

This data is from Global Forest Watch 2019 using a 30 percent tree cover thresh hold as the definition of "forest". All figures are hectares. The data includes tropical forest cover ranging from tropical dry forests to tropical rainforests.

LOWLAND VS. MONTANE FOREST

Lowland tropical rainforest refers to the vast majority of tropical rainforest—forests that grow on flatlands at elevations generally below 3,300 feet (1,000 meters), though this threshold can vary by region. Lowland primary forests, often characterized by more than five distinct canopy layers, tend to be taller and more diverse than montane forests. They contain a greater variety of fruiting trees, supporting a higher abundance of fruit-eating animals and large mammals. Lowland rainforests are also more threatened than montane forests due to their accessibility, fertile soils that make them desirable for agriculture, and valuable hardwoods sought for timber. In many countries, virtually all lowland primary forests have been lost, while montane forests remain relatively intact.

Tropical montane rainforests grow at higher elevations, typically above 3,300 feet (1,000 meters). High montane forests—those found between 6,600 and 10,000 feet (2,500–3,000 meters)—often take the form of "cloud forests," which receive much of their moisture from mist and fog rising from the humid lowlands below. Cloud forest trees are typically shorter than those in lowland forests, resulting in a less-developed canopy. However, the constant moisture supports an abundance of epiphytes—plants that grow on other plants rather than in soil—such as mosses, ferns, and orchids. These forests, found in places such as the lower elevations of the Andes in Ecuador, Peru, Colombia, and Venezuela; Central America (particularly Monteverde in Costa Rica); Borneo (Mount Kinabalu); and Africa (Ethiopia, Kenya, Rwanda, the Democratic Republic of Congo, and Uganda), are often blanketed in dense moss and host a remarkable diversity of rare and beautiful orchids.

LOWLAND VS. MONTANE FOREST

Cloud forests often support many endemic species due to their isolation by valleys and ridges. These natural barriers prevent species from migrating to other forest areas, as lowland forests below and steep cliffs above create further obstacles. Cloud forests are rich in hummingbirds, frogs, and epiphytes such as orchids, bromeliads, and mosses. Many species are found only in a single location, like the Golden Toad of Monteverde, Costa Rica, which is now believed to be extinct. Cloud forests generally lack large-bodied mammals due to the limited number of fruiting trees.

Tropical montane forests are particularly prominent in the South American Andes, where much of the forest has been cleared for agriculture. A disproportionately high number of South America’s endangered species are found in yungas, the local name for tropical montane forests in the Andes. These forests remain poorly studied despite their ecological significance.

Above 10,000 feet (3,300 meters), cloud forests give way to sub-alpine and alpine forests. These higher-elevation habitats receive less rainfall, have fewer trees, and support lower biodiversity compared to forests at lower elevations.

OTHER TYPES OF FOREST

SEASONAL OR MONSOON FOREST
Monsoon forests are tropical moist or seasonal rainforests primarily found in Asia (India, Sri Lanka, and China), West and East Africa, northern Australia, and eastern Brazil. These forests experience distinct wet and dry seasons. Compared to equatorial rainforests, monsoon forests tend to be less diverse and have smaller trees.

Monsoon forests face significant threats from agricultural expansion, particularly in West Africa, where over 90 percent of coastal and monsoon forests have been cleared.

IGAPÒ FOREST
Igapò forests are rainforests that are regularly inundated during seasonal flooding, sometimes remaining underwater for extended periods. The most well-known examples are found in the Amazon Basin, where they account for approximately 2 percent of the total rainforest area. Trees in igapò forests are generally shorter than those in non-flooded forests due to the instability of waterlogged, poorly drained soils—hence the alternative name "swamp forest." These forests are dominated by tree species such as Cecropia, Ceiba, and Mauritia palms (aguaje palm). Many igapò trees have stilt roots and buttresses for structural support. These forests are flooded for 4 to 10 months annually in a predictable cycle, and fish play a key role in seed dispersal within this ecosystem.

VÀRZEA FOREST
Vàrzea forests are seasonally flooded floodplain forests, distinct from swamp forests in that they have relatively nutrient-rich soils due to the annual replenishment of sediments from whitewater rivers. These forests are more suitable for agriculture than typical rainforests, making them among the most threatened. Even in the Amazon, where most of the world's vàrzea forests are located, they are rapidly disappearing due to development.

Floodplain forests, especially those along riverbanks and islands, are often short-lived due to the meandering nature of tropical lowland rivers, which erode their bases. Research from Amazon Headwaters by Michael Goulding and colleagues suggests that most floodplain forests are rarely older than 200 years and have high turnover rates, with some forests experiencing tree replacement cycles averaging 63 years. As these forests mature, pioneer species like Cecropia are replaced by larger species such as Kapok (Ceiba) and fig trees.

HEATH FOREST
Heath forests grow on well-drained, sandy soils that are extremely nutrient-poor. These forests are characterized by tree species that tolerate acidic, nutrient-deficient conditions, resulting in stunted growth compared to typical rainforests. More light reaches the forest floor, leading to dense undergrowth. Heath forests, also known as blackwater or caatinga forests, are drained by blackwater rivers and are primarily found in the Amazon Basin (especially in the Rio Negro drainage) and parts of Asia.

PEAT FOREST [news and information on peatlands]
Peat forests occur in small areas of Africa, northeastern South America, and large parts of Southeast Asia, particularly Borneo and Sumatra. These swamp forests form where dead vegetation accumulates as peat in waterlogged conditions. Peat retains moisture during dry periods and absorbs monsoon rains, acting as a natural sponge. However, when drained for agriculture, peatlands become highly flammable. During the El Niño event of 1997–98, thousands of fires raged through Indonesia's peat swamps, proving exceptionally difficult to extinguish because the fire continued burning underground in the peat layers.

TERRA FIRME FOREST
Terra firme, meaning "firm earth," refers to rainforests that are not seasonally flooded by river systems. These forests are noticeably taller and more diverse, with some areas supporting over 400 tree species per hectare. Terra firme forests grow on well-drained soils and are home to commercially significant species such as Brazil nut trees, rubber trees, and many tropical hardwoods.

MANGROVE FOREST [news and information on mangroves]
Mangrove forests grow in silt-rich, saline (brackish water) environments, primarily along river deltas, estuaries, and coastal regions. These forests have low tree diversity, consisting mainly of mangroves, and feature a broken canopy. Mangrove trees and shrubs are adapted to saltwater conditions with specialized breathing roots (pneumatophores) that emerge from oxygen-poor mud to absorb air.

Mangrove swamps support unique amphibious fish species, such as mudskippers (found from eastern Africa to Australia) and Anableps, or "four-eyed fish," from Central and South America. Mudskippers spend more time on land than in water, moving across tree roots and floating debris. Anableps, meanwhile, have split-lobed eyes that allow them to see both above and below the waterline simultaneously.

Mangrove forests are among the most threatened ecosystems on the planet due to their coastal location, which makes them prime targets for resort and real estate development. Additionally, their ecological value is often underestimated by local communities and governments. A study by the Food and Agriculture Organization of the United Nations found that 20 percent of the world's mangrove forests have disappeared since 1980, primarily due to farming, logging for timber and charcoal, freshwater diversion, urban expansion, and tourism-related development.

According to the Environmental Justice Foundation, approximately 38 percent of global mangrove deforestation is linked to shrimp farming. The large-scale clearing of mangroves for commercial shrimp and prawn hatcheries is especially widespread in Southeast Asia. Ironically, this form of aquaculture has replaced natural fish and shrimp nurseries, disrupting local marine ecosystems and fisheries.

The destruction of mangrove forests has severe consequences for the fishing industry, as these forests serve as critical spawning grounds and nurseries for many commercially valuable species. Additionally, mangrove forests play a vital role in coastal protection by acting as natural barriers against storm surges and erosion. Research conducted after the 2004 tsunami in Asia found that areas with intact mangrove forests suffered significantly less damage compared to regions without tree cover.

Mangrove forests are slow to recover from deforestation and degradation. For instance, in Nigeria, seismic lines cut through mangrove forests—just a few meters wide—were still visible from the air a decade after they were cleared.

REVIEW QUESTIONS

Review questions - Part I

• Where are rainforests located?
• How much land area rainforests do cover?
• What percentage of Earth is covered by rainforests?
• How many rainforest biogeographical realms are there?
• What biogeographical realm has the most rainforest?
• True or false - less than 5% of Earth's land is covered with rainforests.

Review questions - Part II

• Rainforests are generally broken into how many biogeographical realms?
• The largest expanse of rainforest is located on what continent?
• Most of the rainforest in Africa is found in what basin?
• How is African rainforest generally different from rainforests of Asia and South America?
• What is the world's second largest island?
• Does Australia naturally have monkeys?
• What is the Wallace Line?
• How did the Ice Ages affect islands and forests in southeast Asia?
• True or False—The Amazon River Basin is roughly the size of the forty-eight contiguous United States.
• Is the Amazon River the largest river, in terms of volume, in the world?
• Is the Amazon River the longest river in the world?
• What continent loses the most area of forest each year?

Review questions: - Part III

• What is the difference between primary and secondary forest?
• True or false—Cloud forest is found in mountainous areas.
• True or false—flooding is common in the Amazon rainforest.
• Why are mangrove forests important?
• Why are mangrove forests being destroyed?

CITATIONS

Citations - Part I

• D.W. Orr Earth in Mind: On Education, Environment, and the Human Prospect, Washington, D.C.: Island Press, 1994.
• David Quammen Wild Thoughts from Wild Places (New York: Scribner, 1998).
• M. McKloskey in "Note on the Fragmentation of Primary Rainforest," Ambio 22 (4), June: 250-51 1993 using analysis of satellite images.
• Deforestation rates and tropical forest cover are taken from the latest State of the World's Forests 2011 (SOFO) published by the United Nations Food and Agriculture Organization (FAO).

Citations - Part II

• Australia's rainforest coverage during the most recent ice ages is discussed in M. Hopkins and P. Reddell (Australia's CSIRO 1998) and van Osterzee (Where Worlds Collide, New York: Cornell University Press. 1997). T.F. Flannery (The Future Eaters, New York: Braziller 1995) also discusses vegetation shifts wrought by climate change and human influences.
• Van Osterzee (Where Worlds Collide, New York: Cornell University Press, 1997), Quammen (The Song of the Dodo, New York: Scribner 1996.), and Browne (The Secular Ark: Studies in the History of Biogeography, New Haven: Yale University Press 1983) provide an easily understandable review of the Wallace line biogeography including the current distribution of flora and fauna in the region and the impact of changing sea levels. Rubeli (Tropical Rainforest in South-East Asia, Kuala Lumpur: Tropical Press Sdn. Bhd., 1986.) discusses the link between flora of New Zealand, the Himalayas, and Borneo.
• The history of the Amazon River Basin is covered engagingly in Goulding (Amazon-The Flooded Forest, New York: Sterling Publishing Co., Inc. 1990).

Citations - Part III

• The mechanism responsible for the worldwide decline in amphibian populations is debated by Lips ("Decline of a montane amphibian fauna," Conservation Biology Vol. 12 No. 1 (106-117), Feb. 1998.), Sessions et. al. (Sessions, S.K. Franssen, R.A., Horner, V.L., "Morphological Clues from Multilegged Frogs: Are Retinoids to Blame?" Science 284 (5415) 1999), Tangley ("The Silence of the Frogs," U.S. World and News Report 8/3/98), and Tuxill ("The Latest News on the Missing Frogs," World Watch May/June 1998). For alternative commentary from an unlikely source see M. Fumento ("With Frog Scare Debunked, It Isn't Easy Being Green," The Wall Street Journal 5/12/99).
• The "Primary Cover versus Total Forest Cover" table is taken from Myers, N., "Tropical forests: present status and future outlook," Climactic Change 19 (3-32), 1991.
• Pearce correlates forest clearing in West Africa to falling precipitation in the African interior in "Lost Forests Leave West Africa Dry," The New Scientist 1-18-97.
• The Amazonian igapò is the subject of Goulding's Amazon-The Flooded Forest, New York: Sterling Publishing Co.,Inc. 1990.
• Brookfield, H., Potter, L., and Byron, Y. provide a short description of Indonesian peat forests in In Place of the Forest: Environmental and Socio-economic Transformation in Borneo and the Eastern Malay Peninsula (New York: United Nations University Press, 1995), while T. Nishizawa and J. I. Uitto, eds. (The Fragile Tropics of Latin America: Sustainable Management of Changing Environments, New York: United Nations University Press, 1995) review Latin American forest types.
• Threats to mangrove forest from shrimp aquaculture and oil activities are examined in Moffat, D. and Lindén, O., "Perception and Reality: Assessing Priorities for Sustainable Development in the Niger River Delta," Ambio Vol. 24 No. 7-8 (527-538), Dec. 1995; and Boyd, C.E. and Clay, J.W., "Shrimp Aquaculture and the Environment" Scientific American. Vol. 278, No. 6 June 1998, respectively.