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How does vascular cambium make a tree trunk thicker?

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Tall giant sequoia trees with reddish bark and thick trunks rise closely together in a dense forest at Sequoia National Park, California. — tree A stand of giant sequoia trees (*Sequoiadendron giganteum*) in Sequoia National Park, California.

The vascular cambium is a lateral meristem—a layer of actively dividing cells—responsible for the secondary growth of a tree, which is the process of increasing its diameter or thickness.123

This process occurs through the following mechanisms:

Cell Division: The vascular cambium consists of two types of initial cells: fusiform initials and ray initials. These cells divide tangentially to the stem's surface, producing new cells toward the inside and outside of the tree.451
Tissue Production: The cambium generates secondary xylem (wood) toward the central axis of the stem and secondary phloem toward the outside. Because the cambium produces more cells internally than externally, the tree accumulates significantly more secondary xylem, which forms the bulk of the trunk's thickness.145
Circumferential Expansion: As new layers of xylem are added to the inside, the vascular cambium itself must increase in circumference to accommodate the growing diameter of the stem. This is achieved through the radial division of the cambial initials, allowing the cylinder of dividing cells to expand outward as the tree thickens.5

This secondary growth allows the tree to develop the structural support and conducting tissues necessary for its size.43