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How do growth rings form in conifer wood?

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Gymnosperm Pine cone and exposed seeds of the pinyon pine (*Pinus edulis*). Pinyon pines are gymnosperms and bear their edible seeds, known as pine nuts, in protective cones instead of fruit.

In conifers, growth rings form through the cyclic activity of the vascular cambium, a layer of dividing cells located between the bark and the wood. During the growing season, this cambium produces secondary xylem, or wood, which accumulates in concentric layers.123

The visibility of these rings results from structural differences in the tracheids—the primary cells responsible for water conduction and support in conifers—produced throughout the season:13

Earlywood: Cells produced at the start of the growing season are larger with thinner walls, appearing lighter in color.3
Latewood: As the season progresses, the cambium produces cells that are smaller and have thicker walls, resulting in a darker, denser appearance.43

In temperate regions, this process typically corresponds to an annual cycle, as cambial activity ceases during winter dormancy. The sharp contrast between the dense, dark latewood of one season and the large, light-colored earlywood of the next creates the distinct boundary of the growth ring.15