How Much of a Tree Is Alive? Understanding Living and Non-Living Tree Cells

Only a very small percentage of a dormant mature tree is biologically living. The rest of the tree is composed of non-living, structural wood cells. This means that very little of a tree's woody volume is composed of metabolizing tissue.

Here, we review the anatomy of a tree and why the ratio of living to non-living cells is so important to the tree's overall survival.

Anatomy of a Tree

There are many parts of a tree—both living and non-living—and they can be separated into three major categories:

• Crown: the upper parts of the tree that include the leaves, branches, and any flowers or fruit produced.
• Trunk: the base of the tree, which serves as a transport for nutrients to travel from the roots to the crown. The trunk contains major anatomical components: the bark, cambium, sapwood, and heartwood.
• Roots: the lower parts that anchor the tree to the soil and collect water and nutrients.

Most of a tree consists of its trunk, and most of the trunk is not living. The outer bark is comprised of non-living cells, whereas the inner bark is alive for a period of time. The bark protects the cambium, the thin layer of living cells within the trunk that keep the tree growing. Specifically, the cambium facilitates diameter growth, producing a new layer of bark (and protection) each year.

The Essential Role of Non-Living Cells

The non-living cells in the bark serve as a line of defense against insects and disease, which could affect the vulnerable living tissue of the cambium. If anything happens to the cambium, the tree could be damaged or die.

When new cells are formed, the living cells cease metabolization as they transform into transport vessels and protective bark. This is a cycle of creation—beginning with rapid growth and ending with cell death as the tree climbs into a healthy, full plant.

When Wood Is Considered Alive

Wood is considered to be the product of living cells in trees. It is only technically considered dead when it's separated from the tree itself. In other words, while wood is largely made of non-living cells, it is still considered "alive" if it is attached to the tree and participating in the vital cell life cycle processes.

However, if a branch falls off or a person cuts down a tree, the wood is considered "dead" because it no longer transports living matter through itself. Wood that has been separated will dry up as the once-living protoplasm hardens. The resulting protein is the wood one might use in a fireplace or for building a shelf.