Where rainforests are
Tropical rainforests grow in warm regions near the equator where rain falls throughout most of the year, typically well over two meters annually, and temperatures stay warm and fairly steady. The largest areas are in the Amazon basin of South America, the Congo basin of central Africa, and parts of Southeast Asia and New Guinea. Smaller areas occur in Central America, West Africa, Madagascar and northeastern Australia. There are also temperate rainforests in cooler, very wet regions, such as parts of the Pacific coast of North America, but this article focuses on the tropical kind.
Rainforests cover only a modest share of Earth’s land surface, but they hold a disproportionate share of its plant and animal species. Scientists generally agree that tropical forests are among the most species-rich habitats on land, although exact totals are uncertain, because many species, especially insects, have not yet been described.
Layers of the forest
A mature rainforest has a vertical structure with several layers, each with its own conditions and residents.
- Emergent layer. A few very tall trees rise above the general roof. They are exposed to strong sun, wind and temperature swings.
- Canopy. The main roof of interlocking treetops, often tens of meters above ground. It catches most of the sunlight, and much of the forest’s food, flowers and fruit are produced here. Many birds, monkeys, insects and tree-dwelling frogs spend almost all their lives in the canopy.
- Understory. A dim, humid layer of shorter trees, shrubs and young trees waiting for a gap. Plants here often have large leaves to catch scarce light.
- Forest floor. Very little sunlight, often only a few percent, reaches the ground. It is covered in leaf litter, fungi, fallen wood and the animals that feed on them.
Epiphytes, such as orchids and bromeliads, grow on branches rather than in soil. Vines called lianas climb tree trunks to reach the light. Together these add many more niches, or ways to make a living, which is one reason so many species can coexist.
A fast-moving but fragile system
Rainforest soils are often surprisingly poor, especially in old, heavily weathered regions like parts of the Amazon basin. The reason is that most of the nutrients are held in living plants and the thin layer of decaying litter on top. Warm, wet conditions let decomposers such as fungi and bacteria work very fast, and roots, often with partner fungi, take up the nutrients almost immediately. Rain would otherwise wash many nutrients away.
This helps explain a common problem. When forest is cleared for farming, the soil can lose fertility within a few years, because the main store of nutrients was in the trees. The system works through rapid recycling, a pattern that follows the general rules of ecosystem balance and energy flow, with very little spare capacity.
Wildlife and interdependence
Rainforest species are tightly linked. Many trees depend on animals to carry their pollen and disperse their seeds. Fruit-eating birds, bats and monkeys spread seeds across the forest, and insects and other animals pollinate flowers. Some plants and animals are highly specialized, such as certain frogs that breed in the water held in bromeliad leaves, and specialized relationships make some species vulnerable if their partners disappear. Rainforests also support complex food webs, from leaf-cutter ants to jaguars and harpy eagles.
Many rainforest species have small ranges, which means a loss of forest in one area can eliminate species that exist nowhere else. This is a major reason that tropical deforestation is a leading concern in the study of species extinction.
Rainforests and climate
Rainforests influence climate in a few well-established ways.
Carbon storage. Trees take up carbon dioxide through plant photosynthesis and store carbon in wood, leaves, roots and soil. Large forests hold a great deal of carbon. When forests are cut or burned, much of that carbon returns to the atmosphere as carbon dioxide, adding to the greenhouse effect. Intact forests are therefore considered important for limiting climate change, although how much extra carbon a given forest is absorbing in any year can vary, and some regions have been observed to lose their capacity during droughts or fires.
Water and rainfall. Trees draw water from the soil and release it as vapor through their leaves, a process called transpiration. In large forests like the Amazon, this recycled moisture contributes to local and regional rainfall. Studies suggest that extensive clearing could reduce rainfall in some areas, but how far this might go, and where any tipping points may be, remains an active research question.
Oxygen. Rainforests are often called the lungs of the planet, but this is misleading. Forests do produce oxygen through photosynthesis, but they use nearly as much through respiration and decay, so the net contribution is small. Much of the world’s oxygen supply has come from the oceans’ tiny algae and from the long-term burial of carbon over geologic time.
People and rainforests
Many Indigenous and local peoples have lived in rainforests for thousands of years, and they continue to rely on them for food, medicines and materials. Rainforests also supply timber, crops such as cocoa and coffee in some regions, and compounds studied for medicine, though claims about medical potential are sometimes overstated.
The main threats are conversion to agriculture, including cattle pasture and plantations, logging, mining, road building and fire. Rates of forest loss vary widely between countries and years, and trends can change with policy and economics. Protection, restoration and sustainable land use are the main approaches discussed to slow the loss. Rainforests are one of the places where the connection between wildlife, climate and human livelihoods is easiest to see.