How Do Rainforests Work? Layers, Rain and the Richest Life on Land

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 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

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.

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.

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.

For most of the twentieth century the canopy, where most rainforest life happens, was almost out of reach. Botanists identified tall trees from fallen leaves or by shooting branches down, and much of what lived thirty metres up was simply unknown. Since the 1970s researchers have found ways to get there: climbing ropes borrowed from caving, walkways slung between trunks, inflatable rafts lowered onto the treetops by airship, and construction cranes whose booms can carry a scientist in a gondola to almost any point above a patch of forest.

Some of the most startling results came from fogging, in which a single tree is misted with a quick-acting insecticide and everything that falls onto sheets below is collected and sorted. The American entomologist Terry Erwin did this in Panama and found so many beetle species unknown to science, many apparently tied to particular kinds of tree, that he argued the world's insect species might number in the tens of millions rather than the one or two million then assumed. Later estimates have come down, but his work showed how little of the canopy's life had been counted.

A forest that recycles its nutrients so tightly still leaks a little with every heavy rain, so something must top it up. Part of the answer blows in from across the Atlantic. Winds lift huge quantities of mineral dust from the Sahara, especially from the Bodele Depression in Chad, an old lake bed, and carry it westward. Some of it falls over the Amazon, bringing phosphorus, a nutrient the leached soils badly lack. Satellite studies have estimated that the amount arriving each year is roughly comparable to what the forest loses, linking the world's largest desert to its largest rainforest.

People have changed the soil too. Scattered across the Amazon are patches of dark, fertile earth that locals call terra preta, black earth, quite unlike the pale clays around them. Archaeologists have found them full of charcoal, pottery fragments, bone and other refuse, built up over centuries by settlements long before Europeans arrived. The charcoal helps the soil hold nutrients that would otherwise wash away, and these patches remain fertile today. They are part of the evidence that parts of the Amazon supported larger, settled populations than early European accounts suggested, and they interest scientists looking for ways to improve poor tropical soils.

When fires swept the Amazon in 2019, the claim spread widely that the forest produces twenty percent of the world's oxygen. Its plants do release enormous amounts of oxygen by photosynthesis, but that is only half the account. The same forest uses almost all of it again: the trees respire day and night, and the fungi, termites and microbes that break down fallen leaves and wood consume oxygen as they work. Measured over a year, a mature rainforest's net contribution to the oxygen in the air is close to zero, because what it makes and what it uses roughly balance.

The oxygen we breathe built up over hundreds of millions of years for a different reason. When living things grow they lock carbon into their tissues and release oxygen, and that oxygen stays in the air only if the carbon is never broken down again, which happens when a small fraction of dead material is buried in sediments, peat or ancient swamps that later became coal. The atmosphere now holds so much oxygen that burning every forest on Earth would lower it by only a fraction of a percent. About half of all photosynthesis happens in the ocean, in microscopic plankton, and the same rule applies there.

The sheer number of tree species raises a puzzle. If one species were even slightly better at competing for light, you might expect it to crowd out the rest, as a handful of species dominate many temperate forests. One influential answer was proposed independently by the ecologists Daniel Janzen and Joseph Connell around 1970. Seeds fall most thickly beneath their parent, and that is also where the insects, fungi and other enemies that specialise on that species are most concentrated. Seedlings growing close to their own kind therefore die at high rates, while those carried further away by animals or wind are more likely to survive.

The result is that no species can pack densely into one place, and the space beside each tree tends to be taken by a different species, which keeps diversity high. Field studies have repeatedly found seedlings dying faster near adults of their own species, and an experiment in Belize that sprayed seedling plots with fungicide found that the variety of surviving seedlings fell, just as the idea predicts. Ecologists do not think this is the whole story. The long, stable history of tropical climates, the abundant energy and the many ways species divide up light and soil probably all contribute, and how much each matters is still argued over.

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.