What counts as a tropical rainforest
A tropical rainforest is a forest that grows in the warm, wet belt near the equator, where there is no real cold season and no long dry season. Temperatures stay fairly steady through the year, often somewhere in the warm 20s Celsius (roughly the 70s and 80s Fahrenheit), and rain falls in most months. Annual rainfall is typically well over 2,000 millimeters (about 80 inches), though the exact figures vary by region.
The largest areas are in the Amazon basin of South America, the Congo basin of central Africa, and parts of Southeast Asia and the islands around it. Smaller patches occur in Central America, West Africa, Madagascar, India, and northeastern Australia. Each region has its own mix of plants and animals, but they share a common set of conditions that shape how the forest works.
Not every forest in the tropics is a rainforest. Places with a pronounced dry season grow seasonally dry forests or open woodlands instead, and the line between them is gradual rather than sharp. Mountain, swamp and coastal forests in the wet tropics form their own special types.
Constant warmth and light, constant growth
In cooler regions, winter stops growth for part of the year. In a rainforest, the growing season is essentially all year. Sunlight is strong, water is usually available, and plants can photosynthesize in every month. This explains why the vegetation is so dense and why competition for light is intense. Seedlings on the ground receive only a small fraction of the sunlight that strikes the treetops, so many species wait in the shade for years until a gap opens above them.
Many rainforest trees are evergreen, although individual leaves are still replaced over time. Some species drop leaves at irregular moments, and some flower or fruit in loose, irregular cycles rather than by season.
A forest built in layers
Rainforests are often described as having vertical layers, each with its own conditions: a dim, humid forest floor, a shrubby understory, a closed canopy of interlocking crowns, and a scattering of taller emergent trees that rise above it. The real forest is messier than a diagram, but the idea is useful. Light, temperature, humidity and wind change sharply from the ground to the treetops, and many animals live mostly in one level. The details are covered in the article on rainforest layers from the floor to the emergent trees.
Climbing vines, called lianas, and plants that grow on branches without taking nutrients from the host, called epiphytes, add structure to the canopy and provide habitat in their own right.
Why so many species
Tropical rainforests contain a large share of the world’s plant and animal species relative to the area they cover, and a single hectare can hold far more tree species than a comparable patch of temperate forest. Researchers have proposed several reasons, including long-lasting stable climates, high energy input, and specialized pests and diseases that keep any one species from dominating. None of these ideas fully explains the pattern, and they probably work together. Many of the richest areas are recognized as biodiversity hotspots because they combine high species numbers with heavy pressure on remaining habitat.
Much of this diversity is small and hard to see: insects, fungi, and microbes make up most of the species. Many are unnamed, and scientists generally agree that a large number remain undescribed.
Fertile-looking forest, poor soil
Lush growth can give the impression of rich soil, but the reverse is often true. Heavy rain and constant warmth leach nutrients out of the ground and break down organic matter very quickly. Most of the nutrients are held in living plants and in a thin layer of leaf litter, and they are recycled rapidly by fungi, microbes and tiny roots near the surface. Soil quality varies from place to place: young volcanic soils and river floodplains can be quite fertile, while old, weathered upland soils are usually poor.
This tight recycling is why clearing the forest for crops can disappoint. Once the trees are gone, nutrients wash away or are lost, and bare ground is vulnerable to soil erosion during heavy storms.
Water and climate
Rainforests do more than receive rain. Trees draw up water through their roots and release it as vapor from their leaves, a process called transpiration. In large forest regions, a meaningful share of the rainfall may come from moisture that was recently cycled through the vegetation, which is why forests are sometimes said to help make their own rain. The extent of this effect depends on the region and the season.
Rainforests also hold large amounts of carbon in trees, roots and soil, and they take part in the global carbon cycle and the global water cycle. Large-scale loss of forest can affect local rainfall and temperatures, and it releases stored carbon, although the size of the effects is still being studied and differs between regions.
Pressures on the forest
Rainforests face clearing for farms and pasture, logging, roads, mining, fire, and the expansion of settlements. The drivers are covered in the article on tropical deforestation. Where cleared land is abandoned, secondary forests can regrow, though they usually differ from the original forest for a long time. Roads and clearings also lead to forest fragmentation, which splits habitat into smaller, more isolated pieces and makes remaining forest drier and more exposed at its edges.
Climate change adds further stress, including drought in some regions and a higher risk of fire in forests that evolved with little of it.
People and protection
Millions of people live in or near tropical forests and rely on them for food, medicine, building materials and income. Many Indigenous and local communities have managed forests for generations, and indigenous forest livelihoods are closely tied to keeping the forest standing. Conservation approaches include protected areas, community management, sustainable harvesting and restoration of damaged land. What works depends on local ownership, enforcement, economics and the state of the forest, and there is rarely a single solution.
Tracking change in these forests, through methods such as satellite imagery and ground surveys, helps show where forest is being lost or recovering. These records are imperfect, but they give a general picture of the trends in forest cover change over time.