Wood is a biological material that provides the structural integrity for trees, allowing them to grow tall and wide. It forms the vast majority of a tree’s trunk, offering the necessary support to withstand wind and gravity. Wood formation is a continuous biological function known as secondary growth, which enables the tree to expand its girth. This highly organized cellular process builds layers of specialized tissue year after year.
The Biological Engine of Growth
The increase in a tree’s width, or secondary growth, is driven by a single, thin layer of generative cells called the vascular cambium. This layer acts as the primary growth engine for the trunk and branches, located just beneath the bark. The cambium actively divides, generating new cells both towards the inside of the tree and towards the outside. Cells produced to the interior become wood, while cells produced to the exterior form the inner bark. This continuous division adds a new, concentric cylinder of growth each year, increasing the tree’s diameter.
Defining the Structure of Wood
The cells produced by the vascular cambium differentiate into two main transport tissues: xylem and phloem. Cells generated inward develop into secondary xylem (wood tissue), which transports water and dissolved minerals from the roots. Secondary xylem is also responsible for the tree’s mechanical strength because its cells have thick, rigid walls reinforced with lignin. Cells generated outward form the secondary phloem, which constitutes the inner layer of the bark and transports sugars produced during photosynthesis.
The wood itself is further divided into two distinct regions: sapwood and heartwood. Sapwood is the newer, outermost layer of xylem that remains biologically active and conducts water. As the tree ages, the innermost xylem cells cease to transport water and become filled with resins and other deposits. This inactive, central region is called heartwood, and it primarily serves a structural support function.
The Rhythm of Growth
The growth process is not uniform throughout the year, which leads to the visible pattern of annual rings. In temperate climates, the vascular cambium’s activity fluctuates with the seasons, creating distinct wood layers.
Earlywood and Latewood
When the growing season begins in spring, the cambium produces large, thin-walled xylem cells with wide internal cavities. This fast-growing tissue is known as earlywood (or springwood), and it appears lighter in color and less dense. As the season transitions into summer and fall, growth slows down, and the cambium produces latewood (or summerwood). Latewood cells are smaller, have thicker walls, and are packed more tightly, making this wood darker and denser.
The sharp contrast between the dense latewood formed at the end of one season and the lighter earlywood of the next creates a visible ring. Counting these alternating light and dark bands on a tree’s cross-section determines its age, as each complete band represents one year of growth. The width of these annual rings also provides a historical record, revealing the environmental conditions the tree experienced.