Rainforests: How Earth's Wettest Forests Work and Why They Matter

What Makes a Forest a Rainforest?

A rainforest is not simply a forest where it rains a lot. Ecologists recognise rainforests by a set of features that appear together. The trees form a closed, continuous canopy, a roof of leaves so dense that little light reaches the ground. The plants depend on constant moisture and would not survive long dry seasons. The branches are crowded with epiphytes, plants that grow on other plants, and the trunks are wrapped in lianas, woody climbing vines. Finally, rainforests are normally too wet to burn: natural wildfires are practically absent.

The term itself is relatively young. The German botanist Andreas Schimper used the word Regenwald, meaning "rain forest", in a major work on plant geography published in 1898, and the name spread into English and other languages.

Rainforests are usually divided into two large groups: tropical rainforests, which grow near the equator, and temperate rainforests, which grow in cooler, wet coastal regions further from it. Scientists also describe other kinds, such as subtropical rainforests and tropical montane "cloud forests".

Tropical Rainforests: Life Near the Equator

Tropical rainforests lie in a belt around the equator, mostly between the Tropic of Cancer and the Tropic of Capricorn. Here the climate stays warm all year, with average temperatures usually around 25 to 28 °C, and there is no real winter. In the widely used Köppen climate classification, a tropical rainforest climate is one in which every single month receives on average at least 60 millimetres of rain. Many tropical rainforests receive more than 2,000 millimetres of rain a year, and some get far more.

The daily weather often follows a pattern. Mornings are calm and humid. As the sun heats the ground and the trees release water vapour, warm, moist air rises, forms towering clouds and turns into heavy afternoon thunderstorms.

The great rainforest regions

• The Amazon in South America is the largest tropical rainforest on Earth. It spreads across nine countries, with the biggest part in Brazil, and is drained by the Amazon River, the largest river in the world by the volume of water it carries.
• The Congo Basin in Central Africa holds the second-largest tropical rainforest. It is home to gorillas, forest elephants and the okapi, a shy relative of the giraffe.
• Southeast Asia and New Guinea contain rich rainforests; those of Borneo and Sumatra are the only home of wild orangutans.
• Smaller rainforests grow in Central America, West Africa, Madagascar, India, Sri Lanka and northeastern Australia, where the Wet Tropics of Queensland are among the oldest surviving rainforests in the world.

Altogether, tropical rainforests cover only around 6 percent of Earth's land surface. Yet scientists estimate that they are home to roughly half of all the species of land plants and animals on the planet.

A Forest Built in Layers

One of the most striking features of a tropical rainforest is its vertical structure. Life is arranged in layers, each with its own light, temperature, humidity and inhabitants.

1. The emergent layer. A few giant trees push above the rest of the forest, often reaching 45 to 55 metres and sometimes more. They face strong sun, wind and drying air. Eagles, such as the powerful harpy eagle of Central and South America, nest and hunt up here.
2. The canopy. Below the giants lies a nearly unbroken roof of tree crowns. This is the busiest part of the forest: most of the flowers, fruits and leaves are here, and so are many monkeys, birds, frogs, snakes and countless insects. Some animals spend almost their entire lives in the canopy and rarely or never touch the ground.
3. The understory. In the dim, still air beneath the canopy grow young trees, shrubs and plants with very large leaves, which help them catch the little light that filters down. Jaguars and leopards often rest and hunt at this level.
4. The forest floor. Only around 2 percent of the sunlight that strikes the top of the forest reaches the ground. The floor is dark, warm and humid, covered with fallen leaves, fungi and roots. Contrary to the image of an impassable jungle, the floor of an old, undisturbed rainforest is often fairly open, because so little light reaches it that few plants can grow there.

Epiphytes, Lianas and Clever Plant Strategies

In a rainforest the biggest challenge for a plant is often not water but light. Many plant strategies are ways of reaching the sun without growing a huge trunk.

Epiphytes such as orchids, ferns, mosses and bromeliads live high on the branches of trees. They are not parasites: they take no food from their host, only a place to sit. They collect water and nutrients from rain, mist and dust, and from bits of plant matter that gather around their roots. Many bromeliads have leaves arranged in a tight rosette that forms a small pool of water. These tiny ponds in the treetops become homes for insect larvae, and some poison dart frogs carry their tadpoles into them to grow.

Lianas take another route. They root in the ground and climb up tree trunks, using the trees as support so they can spend their energy on growing long and reaching the canopy. Some lianas grow remarkably long, spreading from one tree crown to another.

The strangler fig combines both ideas. Its life often begins when a bird or a monkey leaves a seed high on a branch. The young plant first lives as an epiphyte, then sends long roots down to the soil. Over many years these roots thicken and fuse, wrapping the host tree in a lattice. The host may eventually die and rot away, leaving a hollow fig "tree" standing in its place.

Many large rainforest trees have buttress roots, wide wall-like flanges that spread out from the base of the trunk. They help support tall trees standing in shallow soil. Leaves often end in a narrow "drip tip", which helps rainwater run off quickly so the leaf surface does not stay wet and become covered with algae or fungi.

The Secret of the Poor Soil

It seems logical that such lush forests would grow on rich soil, but the opposite is usually true. Many tropical rainforest soils are old, heavily weathered and poor in nutrients. Constant heavy rain washes, or leaches, nutrients out of the ground, and many of these soils are red or yellow because they are rich in iron and aluminium compounds.

The forest survives by keeping its nutrients in living things rather than in the soil. When a leaf, branch or animal falls to the ground, fungi, bacteria, termites and other decomposers break it down very quickly in the warm, moist conditions. The released nutrients are captured almost immediately by a dense mat of fine roots near the surface, often helped by fungi that live in partnership with the roots, known as mycorrhizae. This tight recycling system is why cleared rainforest land often loses its fertility within a few years of farming: once the trees are gone, the nutrients are gone too.

Remarkably, part of the Amazon's supply of phosphorus, a nutrient that plants need, is delivered from across the Atlantic Ocean. Winds carry dust from the Sahara Desert in Africa, and some of that dust settles over the Amazon, helping to replace nutrients washed away by the rain.

Temperate Rainforests: Cool, Green and Mossy

Not all rainforests are hot. Temperate rainforests grow in mild, very wet regions, usually along coasts where moist winds from the ocean meet mountains and drop their water as rain and fog. Summers are cool, winters are mild, and the air is damp for most of the year.

The most extensive temperate rainforests stretch along the Pacific coast of North America, from northern California through Oregon, Washington and British Columbia to southeastern Alaska. Here grow giant conifers such as Sitka spruce, western hemlock and western red cedar, and in northern California the coast redwood, the tallest tree species in the world. Other temperate rainforests are found in southern Chile, New Zealand, Tasmania, Japan, Norway and parts of the British Isles.

Temperate rainforests also exist around the Black Sea and the Caspian Sea. In western Georgia, the Colchic rainforests grow in the humid lowlands and mountain slopes of the old region of Colchis. These forests, together with the region's wetlands, were added to the UNESCO World Heritage List in 2021. They shelter many plant species that survived there from much older times, when similar forests covered larger parts of Europe and Asia.

Compared with tropical rainforests, temperate rainforests have fewer species, but they are extraordinary in another way: they can store more wood and other living matter per hectare than almost any other ecosystem on Earth. Their branches and trunks are covered in thick coats of mosses, lichens and ferns, and fallen trees decay so slowly that they become "nurse logs", on which new seedlings sprout in rows.

Other Kinds of Rainforest

Between and beyond these two main types there are several others:

• Subtropical rainforests grow somewhat further from the equator, where winters are a little cooler but rainfall stays high, for example in parts of eastern Australia and southern China.
• Montane rainforests grow on tropical mountains. At certain heights they are wrapped in clouds for much of the time and are called cloud forests. Their trees capture water directly from the fog, and they are rich in mosses, orchids and ferns.
• Monsoon forests grow where a long dry season alternates with a very wet one. Many of their trees drop their leaves in the dry months, so they are usually treated as a separate type of tropical forest rather than a true rainforest.

An Explosion of Life

Rainforests are the richest land ecosystems on Earth. A single hectare of tropical rainforest can contain hundreds of different tree species, while a hectare of forest in a cool climate usually holds only a handful. In one famous survey in the Peruvian Amazon, scientists counted more than 40 species of ants living on a single tree.

Some rainforest inhabitants are especially remarkable:

• Sloths move so slowly that algae grow in their fur, giving them a greenish tint that helps them hide among the leaves.
• Leafcutter ants do not eat the leaves they carry. They chew them into a paste on which they grow a fungus, and the fungus is their food. They are, in a sense, farmers.
• Poison dart frogs display bright colours that warn predators they are toxic.
• Rafflesia arnoldii, found in Sumatra and Borneo, produces the largest single flower of any plant. It smells like rotting meat, which attracts the flies that pollinate it.
• Howler monkeys of Central and South America are among the loudest land animals; their calls can be heard across long distances through the forest.

New species are discovered in rainforests every year, and many scientists believe that most rainforest insects have still not been described.

Why Rainforests Matter to Everyone

Weather and water

Rainforests help to make their own rain. Trees draw water from the soil and release it through their leaves as vapour, a process called transpiration. Over the Amazon, a large share of the rain that falls is water that has been recycled by the forest itself. Moist air carried by the wind from the forest also brings rain to farmland and cities far away, which is why these streams of vapour are sometimes called "flying rivers".

Carbon and climate

Rainforests store huge amounts of carbon in their trunks, roots and soils. As long as the forest stands, that carbon stays out of the atmosphere. When forests are cut down and burned, much of it is released as carbon dioxide, a gas that contributes to global warming.

The Amazon is often called "the lungs of the planet", but this nickname is misleading. The forest does produce a great deal of oxygen through photosynthesis, but its plants, animals and microbes use almost the same amount when they breathe and decompose. Its real global importance lies in storing carbon, shaping rainfall and protecting biodiversity.

Food, materials and medicine

Many everyday products come from plants that originated in rainforests or forest regions near them. Cocoa, the source of chocolate, comes from a small tree native to the Amazon region. Natural rubber was first obtained from the Amazonian rubber tree. Vanilla is the fruit of a climbing orchid from Mexico and Central America. Brazil nuts are still gathered from wild trees in the Amazon.

Plants from tropical forests have also given us important medicines. Quinine, from the bark of the cinchona tree of the Andes, was for centuries the main treatment for malaria. Curare, a poison used by some Indigenous peoples of South America to tip hunting darts, led to drugs that relax muscles during surgery.

Threats and Protection

Rainforests once covered a much larger part of the planet than they do today. The main causes of their loss are clearing land for cattle pasture and crops such as soy and oil palm, logging, mining and the building of roads that open remote areas to settlement. In Borneo and Sumatra, the spread of oil palm plantations has destroyed much of the habitat of orangutans, which are now critically endangered.

Because rainforest soils are poor, cleared land often supports farming for only a short time, and the forest recovers very slowly, if at all. Scientists also warn that if too much of the Amazon is lost, parts of it could reach a tipping point, beyond which the remaining forest would no longer produce enough rain to sustain itself and could gradually turn into drier, savanna-like vegetation.

There are reasons for hope. Many countries have created national parks and reserves to protect rainforests. Studies have shown that territories managed by Indigenous peoples, who have lived in and cared for rainforests for thousands of years, often have much lower rates of deforestation than surrounding areas. Satellite monitoring now allows illegal clearing to be spotted quickly, and choosing products certified as coming from responsibly managed sources can reduce pressure on the forests.

Fascinating Rainforest Facts

• Because rainforests are so wet, natural wildfires are almost unknown in them. Most fires in rainforests today are started by people clearing land.
• In the canopy, some trees carry entire small gardens of epiphytes, and the weight of these plants and the water they hold can become so great that branches break.
• The coast redwoods of California's temperate rainforests include the tallest living trees known, more than 110 metres high.
• The Wet Tropics of Queensland in Australia preserve plant families whose ancestors date back tens of millions of years, offering a glimpse of what ancient forests may have looked like.

From the steaming Amazon to the misty slopes of Colchis, rainforests are places where water, light and life are woven together in extraordinary ways. Understanding how they work is the first step toward keeping them standing for the generations to come.