Trees are among the largest and longest-lived organisms on Earth. Their sustained growth is possible because of specialized regions of perpetually dividing cells, which enable them to expand both vertically and in girth. Understanding tree growth requires examining the biological machinery that generates energy and directs the formation of new tissues. This process converts atmospheric and soil components into the massive wooden structures that define these organisms.
The Engine of Growth: Photosynthesis
The energy and raw materials needed for a tree’s physical expansion are supplied by the process of photosynthesis. This biochemical reaction takes place within the chloroplasts, primarily located in the leaves, which contain the green pigment chlorophyll. Chlorophyll captures light energy from the sun, which is then used to convert water absorbed from the roots and carbon dioxide taken from the air into sugar and oxygen.
The chemical reaction uses carbon dioxide and water, plus light energy, to produce glucose (sugar) and oxygen. Glucose is the tree’s food source, providing energy for cellular respiration and the carbon-based building blocks for new tissues. This sugar is transported throughout the tree via the phloem cells to areas of active growth, where it is used to create complex polymers like cellulose and lignin that form wood.
The Mechanics of Height and Width
Tree growth occurs in two distinct ways, each controlled by specific groups of perpetually dividing cells called meristems. Height and length increase, known as primary growth, occur only at the tips of the shoots and roots. This vertical expansion is managed by the apical meristems located at the apex of the branches and roots.
As cells divide in the shoot apical meristem, they elongate, pushing the tip of the stem or root further outward or upward. Growth that occurred lower down the trunk will never increase in height again; a mark placed five feet up the trunk will always remain five feet from the ground. In contrast, the increase in girth, or secondary growth, happens all the way down the trunk, branches, and roots.
This radial expansion is the function of the vascular cambium, a thin cylinder of meristematic tissue located just beneath the bark. The cambium is a bifacial meristem, dividing its cells in two directions. It produces secondary xylem cells toward the inside of the tree and secondary phloem cells toward the outside. The xylem cells become the wood, while the phloem cells become part of the inner bark, which transports photosynthetic products. The continuous production of new xylem cells inward causes the trunk to expand in diameter, supporting the tree’s canopy.
Building the Structure: Wood, Roots, and Rings
The wood generated by the vascular cambium ultimately differentiates into two main types: sapwood and heartwood. Sapwood is the newer, outermost layer of xylem, which remains active in the transport of water and dissolved minerals from the roots up to the leaves. As the tree ages, the innermost layers of sapwood cease water transport, and their cells die, forming the heartwood.
Heartwood often darkens as it becomes infused with chemical compounds like resins and phenols, which offer structural support and resistance to decay and insects. While the trunk and branches expand above ground, the root system simultaneously grows to provide anchorage and absorb water and nutrients from the soil.
The cambium’s seasonal activity leaves a visible record of the tree’s life in the form of annual growth rings. In temperate climates, growth begins rapidly in the spring when moisture is abundant, leading the cambium to produce large, thin-walled cells known as earlywood, which appear light in color. As the season progresses, growth slows, and the cambium produces smaller cells with thicker walls called latewood, which form a darker, denser band. The shift between the dense latewood of one year and the light earlywood of the next creates a distinct ring boundary, allowing determination of the tree’s age and analysis of past environmental conditions.