Many assume trees grow from the inside out, like inflating a balloon. However, trees do not grow by pushing their innermost wood layers outward. Instead, their width increases from the outside, just beneath the bark, while their height grows at the tips of branches and roots.
The Outward Growth of Trees
A tree’s outward expansion is controlled by a specialized layer of cells called the cambium. This thin, active layer is situated between the tree’s existing wood and its inner bark. The cambium continuously divides, creating new cells that contribute to the tree’s diameter. As cambium cells divide, they produce new wood (xylem) towards the inside and new inner bark (phloem) towards the outside. This constant addition of new layers pushes the older bark outward, making the tree wider year after year.
The Inner Workings of a Tree Trunk
Beneath the cambium and inner bark lies the bulk of the tree trunk, composed primarily of wood, which is essentially secondary xylem. This woody tissue is divided into two main parts: sapwood and heartwood.
Sapwood is the lighter-colored, outer portion of the wood, and it contains living cells that are actively involved in transporting water and dissolved minerals from the roots to the leaves. As a tree ages and its trunk expands, the older sapwood layers closer to the center of the tree gradually become inactive. These older cells die and cease water transport, transforming into heartwood. Heartwood, darker in color, primarily provides structural support for the tree and can contain compounds that offer resistance to decay.
Decoding Tree Rings
The annual growth of a tree forms distinct layers, visible as tree rings. Each ring represents one year of growth and is composed of two parts: earlywood and latewood. Earlywood forms in the spring when conditions are favorable, resulting in larger, lighter-colored cells. As the growing season progresses, the cambium produces latewood, which consists of smaller, denser, and darker-colored cells. The difference in cell size and density between earlywood and latewood creates the visible ring pattern. By counting these annual rings, one can determine a tree’s age and infer past environmental conditions based on each ring’s width and characteristics.