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Introduction to Trees

Virginia Department of Forestry

A tree is quite wonderfully made, with each of its parts busily at work from the tiniest rootlet buried deep underground to the topmost twig of the highest branch. A tree lives and breathes steadily and constantly. It uses the light from the sun to convert minerals and water from the soil, and the carbon dioxide it receives from the air to sustain that life. A tree has three principal parts: root, trunk and crown. The root is the part that grows in the ground; the trunk grows above ground and the crown grows out of the trunk and is made up of limbs, twigs, leaves, buds, flowers and fruit. Like other living things, a tree may get diseased or have its limbs cut or broken. While the tree can often heal itself over time, it may sometimes be prudent to consult a professional on how to save the life of the tree. A healthy tree can be very long-lived, certainly much more so than its human counterpart. Indeed it will remain on a property, becoming a source of enjoyment for many generations to come.

Bark
Just as human beings have a protective outer layer all over their bodies known as skin, so do trees have a protective outer layer called bark. Damage to the bark can prove fatal to the tree. If someone cut the bark, around a tree to the wood beneath the flow of food would be disrupted and the tree will starve to death.

Many kinds of trees can be immediately recognized by their distinctive bark. Variations in markings, color and texture denote not only the type of tree, but even the age of the tree within that particular species.

The bark of a young tree and that on young parts of a mature tree are quite thin, but the bark of an older tree is thick and rough. For instance the bark of the giant and ancient west-coast Douglas-fir tree, may be more than a foot thick.

Every tree has two layers of bark, an inner layer and an outer layer. The inner bark, through which food passes up and down he trunk and along the branches, is soft and moist. The outer bark is hard and firm. The hardness and thickness of the bark protects the tree from injury and from the elements.

The older the tree, the thicker the bark grows. This is because each year a layer of inner bark hardens and becomes part of the outer bark. In this way the outer bark builds off, even though some of it will eventually fall off the tree in the form of scales.

Branch
On broad-leaved trees such as maples, oaks and elms, a bud forms during the summer at the point where each leaf joins the twig or branch. This bud remains when the leaf falls off the tree in the autumn, and the following spring when the warmth of the sun touches the bud, it bursts and sends out a new shoot at an angle from the branch. This shoot grows into a new branch, and as it grows during that summer, again new buds form along which in turn become other branches. This continues year after year until finally the tree is a fully matured maze of twigs and branches.

On the evergreen or coniferous trees, the formation of buds is different. On pine trees, the buds form only at the tips of the twigs. In spruce, buds form not only at the tips, but also back on the new shoot. On cedar trees, you cannot see any buds at all.

Leaves and branches perform necessary and symbiotic functions. The branches bring water and minerals to the leaves, where food is manufactured, and then return that nourishment back to the different parts of the tree.

Trunk This strong mainstay of the tree begins as a tender stem from which leaves begin to sprout. The trunk is the body of the tree, which not only supports the crown, but in addition internally channels sap and tree food from one part of the tree to another. A tree grows taller by adding new growth at the tip. In the spring, a new shoot starts to grow at the very tip of the tree. This is called the leader. Its length indicates how much a tree has grow over the course of a year. New shoots grow out sideways from the base of the leader. Each end of each branch has a similar growth of shoots. By summers' end, buds form on the new shoots, and from these buds will develop next year's shoots. The girth of a tree develops in quite a different way. Between the bark and the wood is a thin soft layer called the cambium. Each year this cambium produces a new layer of wood. You cannot see these layers as they are hidden by the bark. However, the age of cut trees may be determined by counting these layers- called annual growth rings - on the stump.

Leaf
Although there is an incredibly very wide variety of size, shape, texture, they all have one important thing in common. The leaf is where a tree's food is manufactured. Water and minerals from the soil combine with carbon dioxide from the air to ensure growth and development. A tree will suffer if insects eat all the leaves, as the nourishment needed cannot be made without them.

The leaf is divided into three parts - blade, vein system and stem. The blade of each leaf is made up of hundreds of tiny cells. The cells contain a green substance called chlorophyll, which is an important part of the food-making machinery. The veins of the leaf are the conduits, bringing in the sap that has been drawn up from the roots. Leaves collect carbon dioxide from the air and the sun provides the power to run the machinery. A product is formed that is much like starch. This food is sent to every part of the tree where it is used in building food, bark and other tissues.

We might imagine leaves having an arrangement with breathing with other animals and human beings. Animals use oxygen from the air giving back carbon dioxide to the air. The leaves take in the carbon dioxide, keep the carbon to build up the wood, and release oxygen into the air for all animals to use.

Acting as an enormous "carbon sink", trees soak up carbon dioxide from the air, producing life-giving oxygen in return. In fact, a medium-sized tree generates the same amount of oxygen as each one of us needs to breathe.

In a tree, 'breathing' takes place in the leaf. Chlorophyll (the substance causing the green color) absorbs the CO2 and uses it along with water to dissolve minerals taken up through the roots. After the chemical reaction is completed, the leaf releases oxygen and water vapor through its pores.

Photosynthesis is a process by which CO2 and water are combined with sunlight and a pigment called chlorophyll. The chemical reactions result in the production of sugars which provide energy to the tree. The leaves use some of this energy, but the majority is transported, in the form of sugar solutions, to other parts of the tree that require it.

Transpiration, or water loss, also takes place in the leaves. As this occurs, water is drawn up from the roots through the vascular system to replace lost moisture.

In autumn broad-leafed trees display a brilliant coloring. This occurs when the removal of the green pigments (chlorophyl) takes place, leaving the yellow pigments. These along with other materials are stored in the branches during the winter are used by the tree to start further growth in the spring. Even the fallen leaf performs a necessary function. They benefit the soil by keeping it from being washed away by heavy rains and they prevent the ground from becoming too hard so that melting rain and snow can sink into the ground rather than flooding the surface of the earth.

The evergreen tree is a conifer. The leaves are in the shape of needles. These trees actually do get a new set of needles every year, but as the needles stay on the tree for more than a year they remain green. Amongst conifers there are exceptions. The larch in autumn turns yellow and the leaves fall off. The arbutus, native to the Pacific Northwest is a broad-leafed tree that keeps its green leaves all year round.

Root
Roots are the underground branches of the tree. They reach down into the ground to get the minerals and water for the tree's growth. The roots also anchor the tree in the ground. A tree's roots are its foundation and will keep pace with the size and dimension of the tree it will support.

The more the branches grow and expand in the crown, the more they grow deep and wide under the ground. The tree's roots form an amazingly intricate and complex system. Root tips may be covered with fine hairs to make it easier for the tree to absorb water and minerals. Trees absorb small amounts of moisture from the air through their leaves and their bark. Most of their water, however, comes via the roots.

Water enters the roots through thin membranes at their tips. The tree's vascular system draws the water up through the trunk and distributes it to the leaves. The leaves use the water to dissolve minerals. Excess water goes back to the air through pores in the leaf - a process called transpiration.

Roots are made up of a number of specialized components. The root hairs, tiny structures extending from the main root stems, have very thin walls which absorb water and minerals. This mineral solution is passed into the vascular core of the root from where it is transported throughout the tree. At the tip of the root, there exists a protective structure called the root cap. These loose cells are shed as the root grows into the soil.

Different trees have slightly different root systems. Pine trees have a strong central root called the taproot. This is usually larger than any other roots and often extends deep into the ground. Because substantial damage to this root can be fatal to the tree, trees with taproots are generally difficult to transplant. Other trees, such as the elm or maple, do not have a dominant taproot. Their root systems are characterized by a large number of roots often closer to the surface.

Generally, root growth is influenced by moisture and gravity. In other words, unless there are substantial amounts of moisture near the surface, roots tend to grow downwards through the soil. Roots are always growing and, like a tree's trunk, they grow both longer and wider. At the tip of the roots, the growing region is called the meristem. This is where most of the lengthwise growth takes place. In addition to this, wood is added to the inside of the root and phloem is added towards the outside.

Seed

The seed is the part of the plant that contains the germ of life that has the capacity to grow into another plant like the one from which it came. Although differing in size and shape, all seeds contain small leaves and the beginning of a trunk and root. Another thing all seeds have in common is the need for heat and moisture to begin the 'sprouting' process.

For a time the young tree will live on nourishment enclosed in the seed coating, but soon the root will reach down into the ground and the leaves will expand into the air and from that time on the young 'seedling' makes its own food from the materials it finds.

In order to grow, seedlings must have water, heat, light and air. In nature seeds fall from the ripened trees laying on the forest floor until they begin to sprout. Commercially, the forester will collect these tree seeds and transport them to nurseries where they are planted and tended in seedbeds. Two years later these 'seedlings' are then taken from the seedbeds and planted into fields called transplant beds. They are allowed to stay there from one to three years before finally being set out in rows to form a plantation, where they will eventually grow to maturity.

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