Canopy to soil

Above the roof

A few trees stand clear of all the others. This one is a kapok, and its crown is wider than a tennis court is long.

Its leaves get all of the sunlight, and pay for it. By day this is the hottest, driest, windiest place in the forest: °C and m/s here, against °C and almost no wind at all on the ground below.

Harpy eagle. A metre tall, with hind claws as long as a grizzly bear's. It nests in the forks of trees like this one and hunts under the roof, taking sloths and monkeys off the branches.

Seeds on the wind

Wind is useful up here. The kapok packs its seeds in fluff and lets the air carry them: the white tufts drifting past.

That only works at the top. Under the roof the air hardly moves, and most trees have to pay an animal, in fruit, to carry their seeds.

Scarlet macaws cross above the crowns in pairs. They eat seeds, often unripe ones that are poisonous to most animals.

The roof

About 35 m up, the crowns close into a roof some ten metres thick. Most of the forest's leaves are in it. So this is where most of the sunlight is caught, most of the food is made and most of the animals live. Many of them never touch the ground.

Watch the light gauge on the way through. Above the roof it reads %. Below it, at 20 m, %.

White-throated toucan. The bill reaches fruit on twigs too thin to stand on. It is mostly air inside, and weighs little.

Where the light goes

Take every leaf above one square metre of ground and lay them flat: they would cover it nearly six times over ( times, measured near Manaus).

Each layer of leaves takes about half of the light that reaches it. Half, of a half, of a half, six times, leaves about 2 %. That is the whole reason the forest has layers: everything below here lives on what the roof lets through.

Red howler monkeys eat the roof itself, leaves and all. Their roar at dawn carries for kilometres over the crowns.

Gardens in the air

A big branch carries its own plants: orchids, ferns, mosses, and this bromeliad. They are epiphytes, plants that grow on other plants. They are not parasites. They take nothing from the tree except a place in the light.

With no soil, they live on rain, mist and whatever falls on them. About one in ten of all the world's plant species lives this way, most of them in forests like this.

A pond in a plant

The bromeliad's leaves overlap into a cup that holds rainwater: a pond in the treetops, with mosquito larvae, tadpoles, and things that eat both.

Dyeing poison frog, about 4.5 cm. The father carries his tadpoles up the trunks on his back, one or two at a time, to small pools held by plants. Usually that is a rain-filled hole in a tree, sometimes more than 20 m up. The colours are a warning: its skin is poisonous.

Ropes to the light

A liana is a woody vine. It roots in the ground like a tree, but it does not spend anything on a trunk strong enough to stand up. It climbs a tree instead and spreads its leaves on top of the tree's own.

In many tropical forests about one woody stem in four is a liana. For animals they are the roads from crown to crown.

Three-toed sloth. Leaves are poor food, so it spends very little: on most days it moves less than 40 m. The green tint is algae growing in its fur. About once a week it climbs all the way to the ground to relieve itself, and nobody is sure why.

The dim room

Below the roof the forest opens out. % of the daylight is left, and it has changed colour: the leaves above kept the red and blue for themselves and let the green through.

The air is still ( m/s) and close to saturated ( %). Leaves here are big, thin and dark, built to catch what little arrives. Many end in a long point, a drip tip, that sheds rain quickly.

Sunflecks

The roof has holes. As the Sun moves, a patch of full sunlight crosses a leaf for a few seconds or minutes and is gone.

For a plant down here those flecks can add up to half of all the light it gets in a day. Its leaves are quick to switch on when one arrives.

Blue morpho. Up to 15 cm across. The blue is not a pigment: tiny ridges on the wing scales reflect blue light and cancel the rest. The underside is brown, so in flight it seems to flash on and off.

The race for a hole in the roof

When a big tree falls it tears a hole in the roof, and for a few years sunlight reaches the ground.

Seedlings that have waited years in the shade, hardly growing, take off. Fastest are pioneers like this cecropia: hollow stems, huge leaves, two metres or more of height in a year, and a short life. Slower trees come up beneath them and close the roof again.

A forest is a patchwork of these gaps, all of different ages.

Red is for birds

Heliconias grow where light reaches the ground. The red parts are not petals but tough leaves that hold the flowers and a store of nectar.

Red is a colour birds see well and most insects see poorly, and the flower's tube is curved to fit one kind of bill.

Fork-tailed woodnymph, a hummingbird 10 cm long that weighs about as much as a sheet of paper. It carries pollen from plant to plant on its forehead.

The floor

% of the daylight gets here. With so little, not much can grow, and away from gaps and riverbanks you could walk through this forest easily. The tangled "jungle" of stories is what grows at a forest's edge, where light reaches the ground.

The tall fins at the foot of the kapok are buttresses. Roots stay shallow here, for a reason that is a little further down, and a 50 m tree on shallow roots needs props.

Jaguar. The largest cat in the Americas. It hunts mostly on the ground and swims well.

The nut and the agouti

Brazil nut trees are emergents. They drop their seeds from 40 m in a woody ball the size of a grapefruit, too hard for almost anything to open.

Red-rumped agouti. Its teeth can gnaw the ball open. It eats some of the seeds and buries the rest for later, and forgets some of them. Those are next century's Brazil nut trees.

A thin carpet

Leaves fall here all year, yet the litter is only a few centimetres deep. It is warm and wet every day, and things rot fast: most of a fallen leaf is gone within a year. In a cold northern forest the same leaf could last ten.

Fungi do most of that work. What you see are only their fruit: scarlet cup fungi, a veiled stinkhorn, and small pale mushrooms that glow green in the dark.

Leafcutter ants do not eat the leaves. They carry the pieces underground, feed them to a fungus, and eat the fungus.

Rich forest, poor soil

This soil is ancient, and rain has been washing through it for millions of years. Most of what a plant needs is long gone. What is left is clay, sand and the oxides of iron and aluminium, which is where the rust colour comes from.

So the forest keeps its nutrients in itself. Roots crowd into the top 20 to 30 cm, and even grow up into the litter, to take the minerals back out of each dead leaf before the rain can carry them away. Very little leaks.

Dust from the Sahara

What does leak has to be replaced. Some of it arrives by air. Each year winds carry dust from the Sahara across the Atlantic, and with it about 22,000 tonnes of phosphorus, which is roughly what rain and rivers wash out of the Amazon basin in the same year.

Cut the trees down and the store of nutrients goes with them. That is why cleared rainforest so often makes poor farmland.

The trade underground

Nearly every tree here has a fungus living in its roots. The fungus's threads are about a hundredth of a millimetre thick, far finer than any root, and they reach soil the roots cannot.

The fungus passes the tree phosphorus . The tree pays in sugar made in the leaves fifty metres above.

The threads are drawn glowing and far too thick. Real ones cannot be seen without a microscope.

Not settled: you may have read that trees feed and warn each other through these threads. Threads do link neighbouring plants. Whether forest trees pass each other useful amounts that way is disputed, and a 2023 review found the evidence much thinner than the idea's fame.

The ants' farm

This is where the leaf pieces went. Each chamber holds a pale, spongy garden of fungus growing on chewed leaf. A mature leafcutter nest can have hundreds of chambers, several metres down, and millions of ants.

Most roots stay near the surface, but not all. Where the dry season is long, roots have been found more than 8 m down, drawing up water that keeps the forest green through months without rain.

Fifty-eight metres

From the top of the kapok to here, the light fell fifty-fold, the wind dropped to nothing, the air cooled by five degrees and filled with water. Each layer lives in the weather the layers above make for it.

The numbers behind the picture are below.