Trees and Forests

A tree is a plant that has solved the problem of height. Sunlight is the prize every plant competes for, and the plant that grows taller than its neighbours wins. Trees win by building wood, a material strong enough to lift a canopy a hundred metres into the air and keep it there for centuries. In doing so they have become the largest, heaviest and longest-lived organisms on the planet, and the forests they form are its richest habitats and its largest living store of carbon.

How a tree is built

Beneath the bark of a trunk is a thin living layer, no thicker than a sheet of paper, where all the growth happens. Each year it lays down new wood on the inside and new bark on the outside. The wood is made of long hollow cells, dead once mature, stacked end to end into pipes that carry water from the roots to the leaves. In a tall tree that water rises more than a hundred metres, pulled from above by evaporation from the leaves, in unbroken threads so thin that the pull does not break them. Just under the bark, a separate set of living pipes carries sugar down from the leaves to feed the roots and the growing trunk.

In temperate climates growth stops each winter and resumes each spring, and the alternation of fast pale spring wood and slow dark summer wood produces one ring a year. Count them and you have the tree's age; measure them and you have a record of every good and bad year it lived through. Rings from old timbers and preserved logs have been matched into continuous records stretching back more than 12,000 years, which tell us about droughts, volcanic eruptions and climate long before anyone was writing.

Below ground, the roots spread as far as the branches above, anchoring the tree and drawing up water and minerals. Nearly all trees share their roots with fungi, which extend far finer threads into the soil, collect nutrients the roots cannot, and are paid in sugar. Through these fungal networks, trees in a forest are physically connected, and there is evidence that they pass carbon and chemical signals to one another along them.

Giants and ancients

The coast redwoods of California are the tallest living things, the record holder standing 116 metres, higher than a 35-storey building. Their cousins the giant sequoias are the heaviest, with the largest, General Sherman, holding about 1,500 cubic metres of wood and weighing well over a thousand tonnes. Bristlecone pines on the dry mountains of Nevada and California grow so slowly, in such harsh conditions, that their dense wood resists rot and insects almost indefinitely; the oldest known individual germinated nearly 5,000 years ago.

Trees have no fixed lifespan in the way animals do. A tree dies when something kills it: fire, disease, wind, drought, a chainsaw. Protected from those, some can go on almost indefinitely, because the growing layer under the bark is renewed every year and a tree's oldest wood is simply dead scaffolding.

Forests

Trees rarely grow alone. Forests cover about 31 percent of the land and come in great belts: the dark conifer forests of the north, where few species survive the cold but cover immense areas; the broadleaf forests of the temperate zone that drop their leaves each autumn; and the tropical rainforests of the equator, which never stop growing and hold more than half of all species on land. A single hectare of Amazon forest can contain 300 species of tree, more than in the whole of Europe, and the animals, insects and fungi living in and on them number in the millions.

A forest is more than its trees. Its canopy shades and cools the ground, its roots hold soil on hillsides and soak up rain that would otherwise flood, and its leaves pump so much water into the air that large forests make their own rain; much of the Amazon's rainfall is water the forest itself released the day before.

Carbon

Wood is about half carbon by dry weight, and all of it came from carbon dioxide in the air. The world's forests hold more carbon in their trunks, roots and soils than the entire atmosphere does, and each year they absorb roughly a quarter of what humans emit. When a forest is cleared and burned that carbon goes straight back into the air. Deforestation, mostly to make farmland, destroys an area of forest about the size of Portugal every year and accounts for around a tenth of human carbon emissions. The planet has about three trillion trees, and about half as many as it did when farming began.

Why it matters

Trees made the land habitable. They built the soils, steadied the water cycle, sheltered most land species and filled the air with oxygen. They give us timber, paper, fruit, nuts, rubber, medicines and shade. And they are the simplest, oldest and still among the most effective tools we have for taking carbon back out of the atmosphere.