The process by which a tree increases its girth, or diameter, is called secondary growth. This widening is accomplished by a thin, continuous layer of generative tissue that wraps around the entire stem and root system. This layer provides the structural foundation for the tree’s trunk size.
The Vascular Cambium
The engine of a tree’s lateral expansion is the vascular cambium, a cylindrical layer of cells that acts as a lateral meristem. This tissue is located just beneath the bark, separating the wood from the inner bark. It is present as a complete ring in woody plants, extending from the roots to the smallest branches.
The cambium is composed of cells capable of repeated cell division. Its function is to produce new tissue that expands the diameter of the tree, providing the raw material for the tree’s two main transport systems.
Building the Trunk Xylem and Phloem
The vascular cambium increases the trunk’s diameter by dividing and producing new tissues both inward and outward. Cells produced toward the center differentiate into secondary xylem, which is the wood we commonly recognize. The accumulation of this secondary xylem is the primary reason a tree trunk increases in size and structural stability.
The xylem cells transport water and dissolved nutrients from the roots up to the leaves. These cells develop thick, lignin-rich walls and become non-living, creating the rigid, load-bearing tissue of the trunk. Conversely, cells produced toward the outside mature into secondary phloem, which constitutes the inner layer of the bark. The phloem transports sugars produced during photosynthesis downward to the rest of the tree.
The cambium produces significantly more secondary xylem than secondary phloem each growing season. This imbalance means the wood layer continuously thickens, while the phloem layer remains relatively thin. The constant production of new wood pushes the older phloem layers outward, contributing to the formation of the outer bark.
Managing the Exterior The Cork Cambium
As the accumulation of secondary xylem forces the trunk to expand, the outermost protective layers must adapt. The original epidermis and cortex layers are not elastic enough to accommodate this continuous radial growth. The increasing girth causes these outer tissues to stretch, rupture, and eventually be shed.
To replace the lost protection, a second layer of meristematic tissue, the cork cambium, develops in the outer tissues. This cambium produces the periderm, which serves as the tree’s new protective outer layer, replacing the primary epidermis.
The cork cambium generates cork cells toward the outside, which are dead at maturity and contain water-resistant substances. This dense, protective layer forms the familiar outer bark, shielding the vascular cambium and inner phloem from damage and water loss.
Reading the Growth Annual Rings
The activity of the vascular cambium is not uniform throughout the year, creating a visible record of the tree’s life in the form of annual growth rings. When growth resumes in the spring, the cambium is highly active, producing earlywood (or springwood). This wood is characterized by large cells with thin walls, designed for efficient water transport during rapid growth.
As the growing season progresses into the late summer and fall, the cambium slows its production, creating latewood (or summerwood). These cells are smaller, have thicker walls, and are denser, providing greater structural support.
The sharp contrast between the dense latewood of one year and the porous earlywood of the following year creates the distinct boundary of the annual ring. Counting these rings allows for the determination of the tree’s age and provides a historical record of past climate conditions.