Trees, unlike most animals, exhibit indeterminate growth, meaning they can continue to grow throughout their entire lifespan, given the right conditions. Many people assume that older, massive specimens stop growing, but this is a common misunderstanding. The question of where growth occurs in an old tree reveals a fundamental split in how a tree develops its structure. The tree must balance reaching upward for sunlight and spreading roots for resources against the need for a strong, wide trunk to support its increasing mass. This process of adding height versus adding width is managed by two distinct types of growth.
Primary Growth Versus Secondary Growth
Plant biologists categorize tree development into two fundamental processes: primary growth and secondary growth. Primary growth is responsible for the lengthening of the tree, extending its shoots upward toward the sun and pushing its roots deeper into the soil for stability and water absorption. Secondary growth, in contrast, increases the girth or thickness of the stem, branches, and roots. This widening creates the wood and bark, providing the necessary structural support as the tree grows taller and its canopy expands. All trees undergo primary growth, but only woody plants experience significant secondary growth.
Where Elongation Continues
Primary growth continues in an old tree and is confined to specialized regions called apical meristems. These are small, active areas of dividing cells located exclusively at the very tips, or apices, of the plant’s shoots and roots. The shoot apical meristem is found within the terminal buds at the end of every branch, including the highest point of the tree. When these cells divide and subsequently elongate, the branch extends in length, pushing the new growth further outward and upward. This growth only occurs at the tips, meaning the trunk and older branches do not get taller from the bottom. Root apical meristems at the root tips are responsible for the constant exploration of new soil volume, allowing the tree to persistently seek new resources and expand its overall reach.
The Dominance of Secondary Growth in Mature Trees
While primary growth adds length at the extremities, secondary growth becomes the dominant factor in the mass and structure of an old tree. This increase in girth is governed by lateral meristems, which are cylinder-shaped layers of active tissue running parallel to the length of the trunk and branches. The most prominent of these is the vascular cambium, a thin layer located just beneath the bark. This tissue layer annually produces new wood (secondary xylem) on the inside and new inner bark (secondary phloem) on the outside. Another lateral meristem, the cork cambium, produces the tough, protective outer bark that insulates the living tissues from the environment.