Tree (botany)

A tree is a type of plant composed of a woody stem, branches, twigs, and leaves or needles. Trees are related to shrubs and vines. They are perennial, which means they can live for many years. Trees are among the largest and oldest living organisms on Earth.

Numerous types of trees exist; some look the same and are related, but others have no resemblance and are not closely related. Pine, palm, oak, maple, and fir are some examples of the different types of trees. Subtypes exist within these types, such as ponderosa pine and limber pine. Each type of tree belongs to one of many different plant families. Some trees have leaves, and others have needles. Some produce fruit or nuts.

Botanists, or scientists who study plants, classify trees into two categories: coniferous and deciduous. Coniferous trees, also called evergreens or conifers, have needles and they usually grow upward. These types of trees look slender on top and full on the bottom. They do not lose their needles in the fall and winter. Instead they shed their needles gradually throughout the year. Cones containing seeds grow on this type of tree. Some examples of conifers include fir, juniper, pine, and spruce trees.

Deciduous trees have leaves and grow outward, giving them a rounded appearance on top. The leaves on deciduous trees change from green to hues of red, yellow, and orange in autumn before they fall to the ground. The trees grow new leaves in the spring. However, a few types of deciduous trees keep their leaves all year. Nuts or fruits containing seeds grow on deciduous trees. Apple, ash, elm, maple, and oak trees are examples of deciduous trees.

Trees are essential to the survival of humans, animals, and other living organisms. They provide food, oxygen, and shelter. Many products are made from trees. Without trees, Earth would suffer from serious problems, such as poor air quality, hot climate, drought, erosion (washing away of soil), and water scarcity. Humans would not be able to survive without trees.

Parts of a Tree

While the different types of trees may vary in size, appearance, and other characteristics, all trees have the same basic structure and parts, regardless of type.

The crown of a tree is the top part that contains the leaves (or needles), branches, fruits, and flowers. The crown is a very important part of a tree. It helps to filter particles from the air and cool the air. It also provides shade and protects the earth underneath from excessive rainfall.

Leaves and needles are the food-making parts of a tree. They contain chlorophyll, a green substance that helps aid in photosynthesis. Photosynthesis is the process in which a plant—in this case a tree—uses light from the Sun to convert carbon dioxide in the air and water into food. During this process, the tree releases oxygen, which goes back into the atmosphere.

Leaves on deciduous trees change colors in the fall months. This is because chlorophyll production deceases as the days become shorter and this causes other colors in a leaf to become dominant. Weather influences the colors: Warm, dry weather reveals less vibrant colors, while cool, moderately wet weather results in more intense colors.

After leaves change color, they typically fall off the tree as it prepares for winter. The winter months are too cold for a tree to complete photosynthesis because water freezes in the ground and cannot be transported through the tree. Instead a tree goes dormant for the winter. Coniferous trees retain their green color and needles in the winter months.

Roots are at the very bottom of a tree, mostly buried under the ground. They hold a tree upright. The roots soak up water and other nutrients and minerals from the soil. In addition, they store sugar. Roots can grow deep into the ground, but most of them grow within the upper foot or so of the soil where oxygen is located. Some roots, such as the taproot, can grow as far as fifteen feet downward. The roots branch into smaller roots that extend horizontally. Thousands of hairs cover each root, and help it absorb water and minerals.

The trunk is the large woody stem that shapes the tree and offers the crown support. The trunk usually comprises five layers of tissue from the center outward. They are heartwood, xylem, cambium, phloem, and outer bark. The inside layers and bark are made of dead tissue, while living tissue makes up several outside layers of the trunk. A network of tubes exists within these living layers. These tubes are similar to a plumbing system in a house. Their main purpose is to transport water and minerals from the roots to the leaves and carry sugar from the leaves to the branches, trunk, and roots.

Xylem, also called sapwood, houses layers of thick-walled cells and the tubes. Over time, xylem dies and forms heartwood. Heartwood composes the oldest layers of the tree, and its main purpose is to support a tree. Heartwood is the wood used by humans for many purposes. It is made up of sugar, dyes, oils, and other substances from the transportation process. These elements make heartwood a darker shade than the sapwood layer of a tree's trunk.

The trunk contains a very thin layer of growing tissue known as cambium. Cambium produces new layers of xylem and phloem (inner bark layer). Cambium widens a tree's trunk, branches, and roots.

The new layers of xylem produce a visible ring in the trunk that can be seen when the tree trunk is cut horizontally. Botanists use these rings (known as growth rings or annual rings) to determine the age of a tree. Inner rings are the oldest, while the outer rings are the newest. Some trees grow more than one ring a year; a wide and thin-walled ring known as springwood forms in the spring months, while a thick-walled layer known as summerwood develops in the summer months.

The phloem layer is between the cambium and outer bark layers. The living phloem transports sap from the leaves to the branches, trunk, and roots.

Bark covers the outer layer of the trunk, branches, and twigs. Dead phloem cells compose the bark. After they die, they are pushed to the outer part of the trunk, forming a hard protective cover for the tree. Bark protects trees from extreme weather elements, insects, disease, and more.

Dangers of Extinction and Deforestation

According to the World Wildlife Fund (WWF), forests cover about one-third, or 33 percent, of the planet. In 2024, Eight Billion Trees reported that 15 billion trees are cut down each year, which is about four trees for every person. However, many trees around the world are dying at rapid rates and many species are in danger of becoming extinct. This can be due to a variety of factors, but especially because of deforestation. Deforestation is the process of clearing all trees in an area. This can be done intentionally and unintentionally. When areas are cleared of trees, this creates a host of other problems affecting the environment, wildlife, air quality, and more.

Intentional deforestation is caused by humans, who clear trees from areas for numerous reasons. The illegal logging industry is a big contributor to the loss of forests around the world. In most countries, loggers must follow laws and rules when clearing land, for example, replanting cleared areas and respecting protected species. However, some loggers violate these laws in an effort to make money. Some unscrupulous loggers clear wood from protected areas, harvest more than is allowed, or take endangered species of trees. In addition, some people illegally clear timber from forests to use as fuel.

Another reason people clear land is for agricultural purposes. Farmers need open lands to raise livestock and plant crops. Commercial and residential development is another reason lands are being cleared. Much space is needed for new businesses and housing developments.

Unintentional deforestation includes uncontrollable events such as fires, invasive species, and more. While fire is sometimes used as a natural way to shape landscapes, wildfires destroy millions of acres of forest lands every year around the world, according to the WWF. Uncontrolled fires can ravage huge swaths of trees in very little time, especially in rural or arid areas that are far from water sources. Fire can also cause other problems such as altering water cycles and charring soil. When an area has no growing trees to soak up water, this excess water has no place to go. Excess water causes erosion of the land and greatly affects water cycles, causing rivers and other bodies of water to flood.

Because trees absorb carbon dioxide, they help alleviate emissions of carbon dioxide and other greenhouse gases. However, when fewer trees exist in an area, this process decreases. When fires burn trees, carbon dioxide is released into the atmosphere. This increase of greenhouse gases can cause the climate to change and temperatures to rise and may alter weather patterns. Climate change causes many problems. It can affect the habitats of living things and decrease food and water available in a particular area. This could cause living organisms to die or force them to move to other areas for survival.

Invasive species such as insects and plants also are responsible for killing trees. Some insects such as beetles congregate to certain types of trees and feed on them until they die. Invasive plants such as cheatgrass grow rapidly around trees, taking needed nutrients and space that trees need for survival.

Human Usage of Trees

Trees are vital to human survival. People use trees for many purposes, including food, shelter, and oxygen. They also make numerous products from trees. Wood, which is used as fuel and a building material, is a major product made from trees. Humans use wood to build many items, such as houses and some forms of transportation. They also burn it for fuel.

Trees produce paper items, such as notebooks, tissues, cardboard, paper towels, grocery bags, and packaging for products. Other items made from trees include chewing gum, rubber, fibers, dyes, oils, resins, and waxes. In addition, trees produce food items such as berries, fruits, nuts, and maple syrup.

Because trees absorb carbon dioxide and release oxygen, they are responsible for filtering the air and making it clean for humans to breathe. Without trees, the earth would suffer from other problems such as drought and climate changes that would make it unsuitable for living organisms.

Throughout time trees have been associated with religion and mythology. Some religions hold trees as sacred, and trees are the central theme in many myths.

Many examples of the importance of trees in religion exist. Siddhartha Gautama (Buddha) was said to have been enlightened under the Bodhi tree. In the Bible, trees represent longevity, strength, and pride, and stories about numerous trees, including the Tree of Life and the Tree of Knowledge of Good and Evil, appear throughout the religious work. The Celtic Druids were said to have worshipped in groups around sacred oak trees.

In mythology trees were regarded as mysterious creatures, sometimes possessing magical powers. The sycamore held special status in Egyptian mythology as the manifestation of the goddesses Nut, Isis, and Hathor. In Greek mythology, the god Zeus held the oak tree in high regard, while the olive tree was sacred to the goddess Athena. Also, men and women sometimes turned into trees, or trees were personified in the female form in Greek myths.

Some of the myths and religious beliefs about trees have carried over into popular culture with many books and films personifying trees. A few of these include the angry apple trees in The Wizard of Oz (1939); the Whomping Willow that beats things to pieces in Harry Potter and the Prisoner of Azkaban (2004); the beautiful truffula trees that teach a lesson about the importance of ecology in The Lorax (1971); and Charlie Brown's Christmas tree that reminds people of the real meaning of Christmas in A Charlie Brown Christmas (1965).

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