Tree
What is a Tree?
by Michael Kuhns, Extension Forestry Specialist
What is a tree? How are trees different from grasses, sunflowers, dandelions, and other types of plants? And what do trees have in common with shrubs and woody vines? The answers to these questions seem obvious at first. But unless you are a botanist you may find that the answers don't come easily. Let's look a little closer to find some common characteristics of trees, shrubs, and woody vines.
Trees, shrubs, and vines belong to many different plant families. Some are closely related, like ponderosa pine and limber pine. Others are not closely related at all, like eastern redcedar and silver maple. However, trees, shrubs, and vines all have one thing that separates them from the rest of the plant world: a woody stem that is perennial or that lives for many years. Grasses and certain other plants may be perennial, but their tops are regrown year after year from rhizomes, bulbs, or other organs found at or just under the soil surface.
Let's look at each of the parts of a woody plant's stem and see how they are put together and how they grow larger year after year. If you could tunnel into a woody plant's stem, whether the trunk of a tree or a twig on a shrub, you would first encounter bark, then cambium, and finally wood or xylem. Bark is the outer covering on the trunk, twigs, and woody roots. The outer bark we are familiar with is a layer of dead corky cells protecting the rest of the stem. The inner bark, or phloem, is a live spongy layer just inside the outer bark that moves sugars and other substances from the leaves to the stem, roots, and other places where they are needed. Inner bark eventually grows out to form part of the outer bark.
New bark is constantly being made on the inside and pushed out. This is why older trunks usually have rough outer bark that peels or flakes away. Bark is highly variable, though. Young trees of most species have fairly smooth bark. To see what a tree's bark looked like when it was young, look at the young bark on upper branches and twigs.
Just inside the bark, but outside the wood, is a single layer of cells called the cambium. This layer repeatedly divides, first in then out, to form all of the new wood and bark. Wood, or xylem, makes up everything inside the cambium on tree trunks, branches, twigs, and woody roots. Wood is made up of fibers for strength and hollow tubes of different sizes. These tubes are like straws that conduct water from the roots to the leaves. These tubes and fibers, as well as other types of cells, are packed tightly together to make the wood inside a woody plant.
As woody plants grow in diameter a new layer of wood is produced each year by the cambium. This layer is called an annual ring or growth ring. The rings in the center of a trunk or twig are the oldest and those near the outside are younger. Wide rings usually indicate good growth conditions for that year and narrow rings indicate poor growing conditions.
As a stem ages and grows, eventually some of the wood in the center is not needed for water movement. This wood dies one ring at a time and becomes heartwood. Heartwood often is filled with dark colored substances that help it resist decay, as in redcedar's red to purple heartwood. The active living wood on the outside of the stem, one to many rings wide, is called the sapwood. It is usually lighter in color than heartwood. Sapwood is responsible for all water and mineral movement through the stem.