How Do Rainforests Work? Layers, Rain and the Richest Life on Land
By the BrainSnail editorial team. How these articles are written and checked, and how to tell us when one is wrong.
A single hectare of Amazon forest can contain more species of tree than the whole of Europe, and a single tree in it can carry more species of ant than the British Isles. Tropical rainforests grow in a band around the equator where it is warm all year and rains most days, and they have turned those conditions into the most crowded, layered and competitive living system on land. They make much of their own rain, feed on almost nothing in the soil, and are being cut down at a rate that has made their workings a matter of urgent interest to people who will never see one.
Where and why
Rainforests form where two things hold all year: temperature above about 18 degrees and rainfall above about 2,000 millimetres, spread through every month. That describes the equatorial belt, where the sun is overhead, the air rises, cools and sheds its moisture as afternoon thunderstorms, and there are no seasons of cold or drought to stop growth. The three great blocks are the Amazon basin, which holds more than half of what remains, the Congo basin, and the forests of Southeast Asia and New Guinea, with smaller areas in Central America, West Africa, Madagascar and northern Australia. Together they cover about six percent of the land surface and are thought to hold at least half of all species on Earth, most of them still undescribed.
The layers
The defining structure of a rainforest is vertical. Light is the scarce resource, and the forest is organised as a competition for it:
- •The emergents: a few giant trees, 50 to 70 metres, standing above everything, exposed to sun, wind and eagles
- •The canopy: a continuous roof of crowns 30 to 45 metres up, where most of the light is caught, most of the flowers and fruit are produced, and most of the animals live, many never touching the ground
- •The understorey: shrubs, young trees and palms in deep shade, waiting for a gap to open above
- •The forest floor: dim, humid, receiving perhaps two percent of the light, where leaves fall and vanish within weeks
A forest that makes its own weather
Much of the rain falling on the Amazon has fallen there before. Each tree draws hundreds of litres of water a day from the soil and breathes it out through its leaves, so that the forest transpires a river into the air, and that vapour forms the clouds that rain back onto the forest downwind. Between a quarter and a half of the Amazon's rainfall is recycled this way, and the moisture the forest exports feeds the farms of southern Brazil and Argentina. The system has a threshold: clear enough of the forest and the recycling weakens, the dry season lengthens, and the remaining forest becomes vulnerable to fire and to conversion into savanna, a tipping point that some models put at twenty to twenty-five percent deforestation, against about seventeen percent already cleared.
Why the soil is poor
A visitor expects such a forest to grow on rich soil, and the opposite is true. Heavy rain over millions of years has leached the nutrients out of the ground, leaving red clays of iron and aluminium that would barely grow a crop. The forest survives by keeping its nutrients out of the soil altogether. Leaves, wood and animals that fall to the floor are decomposed within weeks by fungi and termites in the constant warmth and wet, and the roots, which are shallow and spread in a mat near the surface and are woven through with fungal threads, take the nutrients up again before the rain can carry them away. Almost all of the system's phosphorus and nitrogen is in living tissue at any moment. That is why land cleared for cattle grows grass for a few years and then fails: the fertility was in the trees that were burned, and it went up with the smoke.
What they do for everyone else
Rainforests hold about a quarter of the carbon stored in the world's vegetation, and cutting and burning them releases it; deforestation is responsible for roughly a tenth of humanity's greenhouse gas emissions, and the Amazon, once a large absorber of carbon, is now thought to be close to neutral or a net source in its worst-hit southeast. They regulate rainfall across continents, hold the wild relatives of crops, and have supplied a quarter of modern medicines, from quinine to the drugs that treat childhood leukaemia, out of a pharmacopoeia that has barely been catalogued. About 300 million people live in them. The rate of loss has fallen from its peak in the early 2000s and rises again with each change of government in the countries that hold them; roughly an area the size of Switzerland is still lost each year, and what grows back on abandoned land takes centuries to resemble what was there.
The takeaway
Tropical rainforests grow where it is warm and wet all year, organise themselves into vertical layers in a competition for light, recycle a large share of their own rain through their leaves, and keep almost all their nutrients in living tissue because the leached soil beneath holds nothing. They cover six percent of the land, hold at least half its species and a quarter of its plant carbon, and have a tipping point beyond which clearance turns the rest to savanna.