Tropical rainforests

Tropical rainforests grow in a band near the equator where it is warm all year and rain falls heavily, usually more than 2,000 millimetres a year. The largest are the Amazon in South America, the Congo Basin in Central Africa, and the forests of Southeast Asia and New Guinea.

Although they cover only around 6 percent of Earth's land, rainforests are estimated to hold more than half of the world's land species. Many of those species have never been formally described by scientists.

• Emergent layer: scattered giant trees, some over 60 metres tall, that rise above the rest.
• Canopy: a dense roof of branches and leaves that captures most of the sunlight and holds much of the forest's animal life.
• Understorey: young trees, shrubs and plants adapted to dim light.
• Forest floor: dark and humid, where fallen leaves and wood decompose quickly.

Ecologists have several explanations that probably work together. Constant warmth and moisture let plants grow all year, providing a steady supply of food. Tropical forests have also existed for a very long time without being scraped away by ice ages, giving species more time to evolve.

The forest's complex structure creates many niches, from the tops of trees to rotting logs. Specialised relationships add to the diversity: some plants rely on a single species of insect for pollination, and many trees are kept rare by pests that specialise on them, leaving room for many different tree species.

It surprises many people that rainforest soils are usually poor. Heavy rain washes nutrients out of the soil, and warm, damp conditions let fungi, bacteria and insects break down dead material within weeks. Most of the nutrients are held in living plants, not in the ground.

Tree roots and fungi capture nutrients almost as soon as they are released. This is why cleared rainforest land often loses fertility within a few years of farming, pushing farmers to clear more forest.

Rainforests store enormous amounts of carbon in trees and soil. When they are burned or cleared, that carbon enters the atmosphere as carbon dioxide. They also shape rainfall: the Amazon releases so much water vapour through its leaves that it helps create its own rain and feeds rainfall far beyond the forest.

Forests provide food, materials and medicines. Quinine, long used against malaria, comes from the bark of the cinchona tree, and many other drugs have origins in tropical plants. Hundreds of millions of people, including many Indigenous communities, depend directly on these forests.

The main drivers of tropical deforestation are clearing for cattle pasture, soy and palm oil plantations, logging, mining and roads that open remote areas to settlement. Fire is often used to clear land and can spread into standing forest.

Scientists warn that if enough of the Amazon is lost, parts of it could pass a tipping point and turn into drier savanna-like landscapes. Protection works best when it combines enforcement against illegal clearing, recognition of Indigenous land rights, which studies link to lower deforestation, and economic incentives to keep forests standing.