A tree is a large plant with a woody stem or trunk, covered with a protective layer of bark. There are two main groups of trees: the broadleaves and the conifers. Trees are a valuable resource. They give us fuel, timber, medicines, food, paper, rubber, fruits, oils and even soap. Even more importantly, they take in carbon dioxide and give off oxygen, so maintaining the balance of gases in the atmosphere.

How a tree lives

Like most plants, a tree has roots and a shoot. The shoot is made up of a trunk, its stem, and branches bearing leaves, buds, flowers and fruits (or needles and cones). The trunk holds up the tree while the branches and twigs spread out the leaves so that they receive as much sunlight as possible. The leaves themselves grow in a spiral pattern to avoid shading those below.

Water containing nutrients from the soil—xylem sap—is drawn up from the soil to the leaves through the sapwood. The leaves use the water and sunlight, as well as carbon dioxide in the air to make food by photosynthesis. This food passes from the leaves to all other parts of the tree as sap through the inner bark (phloem).

Rings

Each year of a tree’s life the growth layer of the trunk makes new bark on the outside and new sapwood on the inside. In this way, the trunk becomes thicker every year. If you look at the stump of a freshly cut-down tree, the sapwood that is added each year appears as a single ring. Count the number of rings and you will find the age of the tree.

Broadleaves

Broadleaved trees are flowering plants that produce fruits with seeds inside. Many broadleaved trees are deciduous: their leaves drop in autumn, or, in tropical or subtropical latitudes, during the dry season. Some broadleaves are evergreen: their leaves do fall, but not all at the same time.

Conifers

The word conifer means “cone-bearing”. All conifers are bushes or trees. Firs, pines, spruces and redwoods are examples of conifers.

Most conifers have long, narrow leaves called needles. The needles are tough and can survive wind, frost and long periods of drought. This means that they can grow in areas where the climate is cold or dry. Nearly all conifers are evergreen. Their needles are shed over the course of a year, but not all at the same time.

Palms

A third kind of tree is the palm tree. It is a monocot flowering plant which grows in hot countries. Palms usually have no branches and only a few large evergreen leaves in a crown at the top of the tree. The leaves are either fan-shaped or feather-shaped and grow in a spiral arrangement at the top of the stem. Coconut palms grow along tropical seashores. Their seeds ripen into coconuts. These may fall into the sea and float to other beaches where they grow into more coconut palm trees. A date palm can grow up to 30 metres (100 feet) high and live for 200 years.

Tallest and heaviest

The tallest trees alive today are coast redwoods growing on the slopes of north California’s Coast Ranges. The tallest of them all is believed to be a specimen growing in Redwood National Park, measured at about 115 metres (377 feet) tall.

The heaviest living tree, and the most massive living thing on Earth, is one of another kind of redwood from California. Called "General Sherman", this giant sequoia tree, perhaps as old as 2700 years, is, at an estimated weight of 2000 tonnes, thirteen times heavier than a blue whale, the largest animal. More than 90 metres (300 feet) tall, its trunk is 25 metres (82 feet) wide just above the ground. You can punch its thick, spongy bark without hurting your hand.

Baobab

Baobab trees, from Madagascar, mainland Africa, Arabia and Australia, have huge trunks shaped like bottles. These store water to help them survive the dry season. A large baobab tree can measure more than 50 metres (165 feet) around its trunk—the largest circumference of any species of tree. In long droughts, some baobabs may actually shrink in size.

Ginkgo

Probably the oldest kind of living tree is the maidenhair, or ginkgo, a native of a small area in central China. Its fan-shaped leaves look almost exactly like fossil leaves preserved in rocks known to be more than 160 million years old. It is probably related to trees known from fossils dating back 270 million years. Ginkgos thus existed in the Age of Dinosaurs, and very likely millions of years before.