Towering above us, branches reaching to the sky, trees are a familiar feature of the landscape, whether a forested hillside, a busy city park, or our own backyard. In the summer, trees form a green canopy shading and cooling the ground below. In winter, they stand silently braced against the cold and snow. With some 100,000 different species worldwide, trees come in a variety of shapes and sizes and thrive in a wide range of habitats. But what makes a tree a tree?
By definition, trees are large, woody, perennial plants. From underground roots to leafy branches, each part plays an important role in the survival of the tree. The roots of the tree anchor it in the earth and absorb the water and minerals the tree requires. Though roots may descend downward ten or more feet, they generally grow horizontally in the top few feet of soil. A tree’s root system can be quite vast and may extend underground far beyond the branch tips, often reaching outward in diameter more than twice the height of the tree.
Covered by protective bark, the trunk of the tree connects the roots to the branches and leaves. Just inside the bark is a thin layer of actively dividing cells known as the cambium, which produces xylem (wood) cells to the inside and phloem (inner bark) cells to the outside. Phloem delivers the sugars produced in the leaves throughout the tree. The active portion of the xylem is called sapwood, which carries water and dissolved minerals up to the leafy crown. As sapwood cells age and become inactive, they transform into heartwood, which is often darker in color due to the build-up of resins and minerals. Wood, composed of cellulose and lignin, provides important structural support for the tree.
Trees increase in size each year as the cambium produces new layers of cells that circle the stem, branches and roots. The oldest part of the tree’s trunk is at the center, and the newest wood is just under the bark. In temperate regions, trees grow fast in the spring, slow down as summer comes, and stop in the fall and winter, resulting in visible annual rings. If growing conditions are good, the growth ring may be many cells wide, and in poor years, it is relatively thin. In very harsh growing conditions – on a mountaintop or a cliff – a tree may not have any extra resources to lay down new wood. In an excellent site, a sugar maple can add an inch of girth to its trunk in three years’ time, while in a poorer site, it might take as much as ten years. Generally, tree girth is calculated by measuring the diameter at breast height or “DBH.”
A cross section of wood can reveal a good bit about the life history of the tree, providing evidence of drought, blight, stress, or injury. Some trees, such as ash, oak, and chestnut, are ring-porous. Their spring wood cells are much larger than the summer wood cells, making their annual rings quite distinct. Other trees, like maple, birch, and cherry, are diffuse-porous. The spring and summer wood cells in these are similar in size, and it can be quite hard to see the rings. Rays are fine lines that seem to radiate out from the center of the trunk. These cells carry nutrients and water horizontally within the sapwood and also transport waste products from the cambium to the heartwood, where they protect wood from bacteria, fungi, and insects.
Bark protects the tree from weather, disease, insects, fire, and other injuries. A special cork cambium layer in the bark produces new outer bark cells to accommodate the expanding trunk. As a result, the bark on young trees often looks very different from older trees of the same species. Bark characteristics such as color, pattern, thickness and texture vary greatly from species to species and so aid in identification. Beech bark is thin and gray; shagbark hickory bark peels and hangs in long vertical strips; black locust bark is deeply fissured. Another distinguishing characteristic on some types of tree bark are lenticels, which appear as horizontal or vertical slits and allow for the exchange of gases. The horizontal lines on white birch bark are a familiar example.
Rising from the trunk, a tree’s branches and twigs hold the crown of leaves up to the sunlight. The arrangement of branches and leaves on a tree helps to maximize exposure to the sun’s energy. Trees and other plants turn sunlight into energy through the process of photosynthesis. The process begins when light energy is captured by chlorophyll, a green pigment in the leaves. In the cells, light energy is transformed to chemical energy. This energy is used to split water molecules into hydrogen and oxygen. The hydrogen combines with carbon dioxide from the air to produce simple sugars. These sugars are transported via the inner bark (phloem) to the roots, stems and fruits of the plants. Amazingly, some ninety percent of a tree’s biomass comes from carbon dioxide absorbed from the air. And a byproduct of photosynthesis is oxygen, so critical to our own survival.
One of the challenges any plant faces is getting enough sunshine to produce the energy needed to grow and reproduce. To find a place in the sun, trees grow vertically, many reaching heights of up to 100 or more feet. They grow at different rates depending on the species and on where they are growing. In an average site, a sugar maple may reach fifty feet tall in fifty years, but in a really good site, it can reach 100 feet in the same time. One important factor is competition with other trees for light and nutrients. A tree growing in a forest would, in general, grow more slowly than one that is out in the open. The tallest tree on earth is a coast redwood in California that stands 379 feet tall!
In order to reproduce, trees make either flowers or cones of some sort. Some tree flowers, like those of apples and locusts, are showy and fragrant to attract insect pollinators. Others, like maples and oak, depend on the wind for pollination, and their delicate flowers are easily overlooked. Some trees have separate male and female flowers, and in some species there are even separate male and female trees. And from these diverse flowers come an assortment of seeds. Some have a fleshy covering – a fruit or berry – while others grow inside nuts or pods. Conifers make their seeds in cones. Other trees, like maple and birch, have seeds with wings that are dispersed by the wind.
The journey from tiny seed to mighty tree is full of risk. First the seed must find a fertile spot to take root, one where it can find the resources needed to germinate. The tiny seedling competes with other plants for a place in the sun and access to water. Its tender buds and leaves are a welcome food source for many animals. As it grows, the tree is vulnerable to ice and wind damage, disease, and injury from animals. Despite all these risks, many trees can be quite long-lived. Lifetime in trees ranges from decades for birches to hundreds of years for maples and hemlocks. Why, a bristlecone pine in California is nearly 5,000 years old!
Trees grow and change throughout their lives, just like people do. The shape, bark, and posture of each tree tells its history – a branch broken in an ice storm, a dark slash from a lightning strike, a woodpecker cavity high in the crown. In this way, a familiar tree helps us tell our own story about the place we call home.
Suggested Reading
Eastman, John. The Book of Forest and Thicket. Mechanicsburg, PA: Stackpole Books, 1992.
Heinrich, Berndt. The Trees in My Forest. New York: Harper Collins, 1997
Sibley, David. The Sibley Guide to Trees. New York: Alfred K. Knopf, 2009.
Symonds, George W.D. The Tree Identification Book. New York: Harper Collins, 1958.
Tudge, Colin. The Tree. New York: Crown Publishers, 2005.
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