Trees

A tree is a tall plant with a single woody trunk. Trees are the largest living things on Earth, and they include some of the oldest as well. The tallest tree alive today (Hyperion, a coast redwood) stands 116 metres tall. The oldest single tree (Methuselah, a bristlecone pine in California) is over 4,800 years old. Trees provide us with oxygen, food, fuel, building materials, paper, medicine and shelter, and they hold soil together and slow climate change. The world would be a vastly different and worse place without them.

• Number of trees on Earthapprox. 3 trillionapprox. 422 per person
• TallestHyperion (redwood)116 m, California, USA
• Oldest single treeMethuselahA bristlecone pine, approx. 4,800 years old
• Most massiveGeneral ShermanA giant sequoia, approx. 1,400 tonnes
• Largest organismPando aspen colonyA single root system, 80,000 tonnes
• Tree species worldwideapprox. 73,000Many still being discovered

What makes a tree a tree?

There is no single botanical definition of a tree (it is more of a useful general description than a strict scientific category), but most trees share these features:

• A single woody trunk that supports the rest of the plant.
• Tall enough to stand above the ground, usually at least 4-5 metres.
• Long-lived: most trees live for decades or centuries.
• Make new wood every year, adding to their trunk and branches.

How a tree is built

The trunk of a mature tree is mostly dead wood, made of old layers of xylem that have stopped transporting water. The living parts of the tree are concentrated in just a thin layer near the bark:

• Bark: the protective outer skin.
• Phloem: a thin layer just inside the bark, carrying sugar down from the leaves.
• Cambium: a single-cell layer that produces new wood each year.
• Sapwood: the active xylem, carrying water up from the roots.
• Heartwood: the dead inner wood, providing strength to support the tree.

If you cut a tree down and look at the stump, you can see all these layers as concentric rings. Each ring represents one year of growth.

Tree rings and a tree's life story

Every year, a temperate tree adds a new layer of wood. In spring the wood grows fast and light (the early wood); in late summer it slows and the wood becomes darker (the late wood). The boundary between one year's late wood and next year's early wood shows up as a clear dark ring. By counting the rings on a tree stump you can find out how old the tree was when it was cut.

Even more impressively, scientists called dendrochronologists can read climate history from the rings. Wet years produce wide rings; dry years produce narrow ones. By matching tree-ring patterns from one tree to the next, going back generations, scientists have built continuous year-by-year climate records stretching back over 12,000 years.

Why trees matter

Trees provide an extraordinary range of benefits.

• Oxygen: trees produce roughly a third of all the oxygen on land.
• Carbon storage: a single mature tree absorbs around 20-25 kg of carbon dioxide per year, helping to slow climate change.
• Habitat: a single old oak can be home to over 2,000 different insects, mosses, lichens, birds and small mammals.
• Water cycle: trees release water vapour into the air through their leaves, helping form clouds and rain.
• Soil: tree roots stop soil washing away and pump nutrients to the surface.
• Food and materials: fruit, nuts, wood, paper, rubber, cork, maple syrup and much more.
• Mental health: studies show that spending time among trees lowers stress and improves mood.

Fact General Sherman, a giant sequoia in California, is the largest single tree by volume in the world. It is 84 m tall, 11 m wide at the base, and weighs around 1,400 tonnes, more than 200 elephants. It is estimated to be around 2,200 years old: alive since before Julius Caesar invaded Britain. Yet General Sherman is still adding wood to its bulk every year, growing slowly but unstoppably.

Forests and trees in trouble

Despite all the benefits, trees and forests are under enormous pressure. Around 10 million hectares of forest are cut down or burned every year, mostly for cattle ranching, palm oil plantations, soy farming or wood. The Amazon, Congo and South-East Asian rainforests are losing trees at alarming rates. Climate change is making fires worse, and many older trees in temperate forests are dying from drought, disease or pests.

The good news is that, given a chance, trees can come back fast. Many countries are now running large-scale reforestation projects: China alone has planted billions of trees in the last few decades. If you have a back garden, planting even a single tree makes a small but real difference.

Did you know? Some trees can communicate with each other through underground fungal networks that connect their roots. These networks (sometimes called the "wood wide web") allow trees to share sugars, send chemical warnings about insect attacks and even feed dying neighbours. A single forest is far more interconnected than it looks from above.

Deeper dive: the secret life of the wood wide web

For most of botanical history, trees were thought of as solitary individuals competing for light, water and nutrients. Then, starting in the 1990s, Canadian forest ecologist Suzanne Simard and her colleagues began publishing remarkable evidence that trees are actually deeply connected.

The connections are made by mycorrhizal fungi: thread-like fungal networks that wrap around (and grow into) tree roots. The fungus gets sugar from the tree; the tree gets extra water and minerals from the much wider reach of the fungal network. Crucially, the same fungal network often connects many trees together, sometimes linking dozens or hundreds of individual trees of many different species across a whole forest.

Simard's experiments showed that trees actually send sugar to each other through these fungal networks. Older "mother trees" feed sugar to younger saplings of their own species, especially when those saplings are growing in shade and cannot photosynthesise much. Trees can also send chemical warnings through the network: if one tree is being attacked by an insect, nearby trees receive the message and start producing defensive chemicals before the insects reach them.

This view of the forest as a connected, communicating community (sometimes called the "wood wide web") is one of the most exciting recent shifts in ecology. It changes how we think about cutting down forests: even removing a few key "mother trees" can disrupt networks that took centuries to build.

For other plants, see flowering plants and what is a plant. For trees in ecosystems see also the rainforest biome.