The Secret Life of Trees

Giants That Stand Still

Trees are so familiar that it is easy to walk past them without a second thought. They line our streets, fill our parks and cover whole mountainsides. Yet trees are some of the most remarkable living things on the planet. They are among the largest and oldest organisms ever to exist, they shape the air we breathe, and — as scientists have discovered in recent years — they live far more active and surprising lives than anyone once imagined.

A tree may look as if it is doing nothing at all. But behind that still, silent appearance, a tree is constantly at work: pumping water dozens of metres into the air, building its own food out of sunlight, defending itself against attackers, and even communicating with the trees around it. This book lifts the lid on the secret life of trees — how they grow, how they survive, and why life on Earth, including human life, depends on them.

Chapter 1: What Is a Tree?

A tree is a tall, woody plant with a single main stem, called a trunk, that supports a crown of branches and leaves. What makes a tree different from a smaller plant is wood — a tough, strong material that lets a tree grow far taller than a flower or a bush without falling over.

Trees belong to the huge family of green plants, and like all plants they make their own food. But their size sets them apart. Some trees are true giants. The tallest living tree, a coast redwood in California, stands higher than a thirty-storey building. The heaviest known trees, the giant sequoias, weigh as much as several jumbo jets. And some trees are astonishingly old: certain bristlecone pines have been alive for more than four thousand years, meaning they were already ancient when the pyramids of Egypt were new.

Trees grow in almost every part of the world where there is enough warmth and water, from steamy tropical rainforests to cold northern forests where they survive months of snow. Wherever they grow, they share the same basic body plan, made up of three main parts that we will meet next.

Chapter 2: The Three Working Parts

Every tree is built from three main sections, and each one has a vital job to do.

The roots spread out underground, often reaching as wide as the branches above. They do two important things. First, they anchor the tree firmly in the soil so that it does not topple in the wind. Second, they soak up water and minerals from the ground — the raw materials the tree needs to live and grow.

The trunk and branches form the tree's skeleton and its delivery system. Inside the trunk are tiny tubes that work rather like the pipes in a building. One set of tubes, called xylem, carries water and minerals up from the roots to the leaves. Another set, called phloem, carries food made in the leaves back down to feed the rest of the tree. Wrapped around the outside is the bark, a protective skin that shields the tree from heat, cold, insects and disease.

The leaves are the tree's food factories. Spread out to catch as much sunlight as possible, they are where the tree performs its most important trick of all — turning sunshine into food. To understand how a tree feeds itself, we need to look closely at what happens inside a leaf.

Chapter 3: Catching Sunlight

Unlike animals, a tree cannot go looking for a meal. Instead, it makes its own food out of thin air and sunlight, using a process called photosynthesis. The word means "putting together with light," and it is one of the most important processes in all of nature.

Here is how it works. A leaf takes in a gas called carbon dioxide from the air through tiny holes on its surface. The roots send up water from the soil. Inside the leaf is a green substance called chlorophyll, which gives leaves their colour and captures the energy in sunlight. Using that captured energy, the leaf combines the carbon dioxide and water to make a kind of sugar, which is the tree's food. As a by-product, the leaf releases oxygen back into the air.

That oxygen matters enormously. It is the very gas that people and animals must breathe to stay alive. Every breath you take contains oxygen that was, at some point, released by a plant or a tree. At the same time, by taking in carbon dioxide, trees help keep that gas from building up too much in the atmosphere. In this quiet, endless way, trees feed themselves and keep the planet breathable at the same time.

Chapter 4: How a Tree Grows

A mighty oak begins as a single acorn no bigger than a thumb. Inside that seed lies a tiny embryo and a store of food to get it started. When the seed lands in moist soil and the weather is right, it germinates: a root pushes downward and a shoot reaches upward toward the light. From this fragile seedling, given enough time, a forest giant can grow.

Trees grow in two directions at once. They grow taller at the tips of their branches and roots, stretching toward the light and water. And they grow wider, adding a fresh layer of wood around the trunk every year, just beneath the bark. This is why trunks thicken as trees age.

Those yearly layers leave a hidden record. Each year of growth forms a ring inside the trunk, and by counting the tree rings in a cut log, scientists can tell exactly how old the tree was. The rings reveal even more: a wide ring shows a warm, wet year with plenty of growth, while a thin ring shows a hard year of drought or cold. In this way, an ancient tree holds a diary of the climate stretching back centuries.

Chapter 5: Surviving the Seasons

Trees cannot move to escape bad weather, so they must endure whatever each season brings. Over millions of years, they have evolved clever ways to survive.

Many trees in cooler parts of the world are deciduous, which means they drop all their leaves before winter. This may seem strange, but it makes sense. In winter the ground can freeze, making water hard to draw up, and short, dim days provide little sunlight for photosynthesis. Rather than waste energy keeping leaves alive through a season when they cannot do much work, the tree pulls back the useful materials from its leaves, lets them fall, and rests through the cold months. Before they drop, the green chlorophyll fades, revealing the yellows, oranges and reds that paint autumn forests.

Other trees are evergreen and keep their leaves all year. Many evergreens, such as pines and firs, have narrow, needle-shaped leaves coated in a waxy layer. This shape loses very little water and can shrug off snow and frost, which is why evergreens thrive in cold northern forests and on high mountains where deciduous trees struggle.

Chapter 6: The Hidden Network

For a long time, people imagined that each tree stood alone, silently competing with its neighbours for light and water. Recent science has revealed something far more surprising: forests are connected by a vast, living network hidden beneath the soil.

The connectors are fungi — the same family of organisms that produces mushrooms. Threads of fungus, far thinner than a hair, wrap around and even grow into tree roots, spreading through the soil and linking the roots of many different trees together. Scientists have nicknamed this underground web the "wood wide web."

This partnership benefits everyone. The fungi help the tree absorb water and minerals from the soil far more efficiently, reaching places the roots alone cannot. In return, the tree shares some of the sugar it makes through photosynthesis to feed the fungi, which cannot make their own food. Even more remarkably, trees can use this network to pass nutrients to one another. A large, healthy tree may share food with a younger or struggling neighbour through the fungal threads that connect them.

Chapter 7: Trees That Defend Themselves

Trees may be rooted to the spot, but they are far from helpless. They have evolved an impressive range of defences against the insects and animals that try to eat them.

Many trees produce bitter or poisonous chemicals in their leaves. When a hungry caterpillar starts munching, the tree can flood that leaf with substances that taste foul or even make the attacker ill, encouraging it to move on. Some trees go further still. When attacked by certain insects, they release chemical signals into the air — a kind of invisible alarm. Neighbouring trees that detect these airborne signals can begin to strengthen their own defences before the pest even reaches them.

Trees also send warnings through the underground fungal network. A tree under attack from disease or insects can release signals into the soil that prompt nearby trees to ramp up their protective chemicals. In this slow, silent way, a forest behaves a little like a community that looks out for its members — though it is important to remember that trees do all this through chemistry and instinct, not through thoughts or feelings as we have them.

Chapter 8: Why Forests Matter

Trees do not only matter to themselves and to the creatures that live among them; they shape the health of the entire planet.

Forests are sometimes called the "lungs of the Earth" because of the oxygen they release and the carbon dioxide they absorb. By soaking up carbon dioxide, a greenhouse gas, forests help to slow the warming of the climate. Their roots hold the soil in place, preventing it from washing away in heavy rain, and they help control flooding by soaking up water like a sponge and releasing it slowly. Through their leaves, trees release water vapour into the air, which forms clouds and influences rainfall, sometimes far away.

Forests are also among the richest habitats on Earth. A single old tree can be home to hundreds of species of insects, birds, fungi and mammals. And humans depend on trees in countless ways: for the timber that builds our homes, the paper we write on, the fruits and nuts we eat, the medicines that come from forest plants, and the cool, shady, beautiful places that lift our spirits.

Chapter 9: Protecting the Trees

Despite everything trees do for us, forests around the world are being cut down at an alarming rate. This clearing of forests, called deforestation, happens when land is cleared for farming, for timber, or to make way for roads and cities. When a forest disappears, the animals that lived there lose their homes, the soil can erode away, and the carbon dioxide that the trees once absorbed is no longer captured.

The encouraging news is that people everywhere are working to turn the tide. Governments and organisations protect ancient forests so they cannot be cut down. Around the world, huge tree-planting projects aim to restore woodlands that have been lost. Foresters now practise sustainable methods, taking only what can be replaced so that woodlands keep renewing themselves. And each of us can help in small ways: by using less paper, recycling wood and paper, and supporting efforts to plant and protect trees.

What We Learned

The next time you pass a tree, you will know that it is far busier than it looks. We have discovered that a tree is a giant woody plant with three working parts — roots, trunk and leaves — each doing a vital job. We saw how trees feed themselves through photosynthesis, capturing sunlight to make sugar and releasing the oxygen we breathe. We followed a tree from a single seed into a towering giant, learned to read its life story in its rings, and discovered how it survives the seasons by dropping its leaves or wearing tough evergreen needles.

Most surprising of all, we found that trees are not the silent loners they appear to be. Through the hidden "wood wide web" of fungi, they share food and send warnings, and they defend themselves with clever chemistry. Forests shape our climate, guard our soil and shelter countless living things. Trees gave us the air in our last breath and will give us the air in our next — which is reason enough to understand them, value them, and protect them.

Want to keep exploring the living world? Discover the tiny creatures all around us in The World of Insects, or journey through Earth's habitats in Amazing Animals of the World.