The largest and oldest living things on Earth are trees. Trees are tall, woody plants, usually with one rigid stem called a trunk. Some trees may live for hundreds or even thousands of years. Well-known trees include aspens, birches, cypresses, eucalypti, firs, magnolias, maples, oaks, palms, and spruces.

Groups of trees

Scientifically, trees are not a separate family of plants. Trees are just one form that many plants can take. Dogwoods and junipers, for example, may be tall trees or short shrubs.

Scientists divide trees into groups based on how they reproduce. One group of trees, tree ferns, uses spores—particles that grow into new plants—to reproduce. Most trees, however, reproduce with seeds. Some seed-bearing trees grow their seeds in cones. Those trees are called conifers. The majority of trees grow their seeds in fruits or pods. Those trees are often called broad-leaved or flowering trees.

Another way to group trees is based on whether they lose their leaves. Trees that keep their leaves year-round are called evergreens. Evergreens are often conifers, but some flowering trees also keep their leaves all year—especially in tropical regions. Trees that lose their leaves during the winter are called deciduous trees.

Where trees grow

Trees grow nearly everywhere in the world. The climate affects which kinds of trees grow in a region. For example, pine trees thrive in high altitudes and cold climates. Palm trees, however, can survive only in warm climates. A huge number of trees grow in tropical rain forests because of the high level of moisture. The lack of rain in deserts means that few trees grow there.

Physical features

Trees are generally more than 10 feet (3 meters) tall. They have roots, a trunk, branches, and leaves. Most trees grow either cones or flowers and fruit.

Trees use their roots to pull water from underground springs and damp soil. Some trees have a shallow root system that lies just below the ground. Others have thick roots that travel deep into the soil.

The trunk and branches are made of fibers, called wood, that are protected by an outer covering called bark. Special tissues in the wood pull water and food through the trunk and branches. As the tree ages, the trunk and branches thicken. Each year, in many trees, new cells created in the center of the trunk push old cells outward in the shape of a ring. The number of rings in a trunk shows the approximate age of the tree.

As in all green plants, the leaves of a tree make food through a process called photosynthesis. Tree leaves can be broad, slender, or needlelike. Some have smooth edges, while others have sharp teeth. Running through the leaves are veins that carry water and food to and from the branches. Leaves are usually green. The leaves of deciduous trees often change to yellow, orange, or red in autumn and then fall off before winter. New leaves then grow in spring. The leaves of evergreens do not fall off every year.

Conifers grow oval-shaped cones on their branches. Male cones contain pollen. Female cones contain eggs that become seeds after pollination. Flowering trees grow flowers that may be large and showy or small and barely noticeable. Flowers contain both pollen and eggs. After the flower is pollinated, it produces a fruit or a pod that contains seeds.

Uses

Native peoples have long used tree wood, bark, and leaves for shelters, thatched roofs, canoes, baskets, and medicines. People today use wood from trees to build homes and furniture. They process wood fibers to make paper. Many people burn wood to cook and to heat their homes. The fluids inside maples and rubber trees provide maple syrup and rubber. Farmers plant orchards of trees that grow fruit for humans to eat. Popular foods that grow on trees include avocados, apples, cocoa beans, nuts, olives, and oranges.

People have also included trees in myths and religious rituals. Many centuries ago the Vikings and other peoples, for example, believed that a sacred tree connected the earth to the heavens. Today many people celebrate Christmas by bringing an evergreen tree into their homes.

Environmental issues

Trees sustain life on Earth. Along with other plants, trees help to keep the air clean. They release oxygen for animals, including humans, to breathe, and they take in the carbon dioxide that animals breathe out. Dead twigs and leaves release nutrients into the soil. Tree roots help to prevent soil erosion. Animals eat tree leaves and fruits. Both living and dead trees provide homes for animals, birds, insects, and other plants. Because of the importance of trees, many governments around the world have restricted the cutting down of trees in the wilderness.