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How do growth rings form in tree wood?

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temperate softwoods and hardwoods selected to show variations — wood Temperate softwoods (left column) and hardwoods (right column), selected to highlight natural variations in colour and figure: (A) Douglas fir, (B) sugar pine, (C) redwood, (D) white oak, (E) American sycamore, and (F) black cherry. Each image shows (from left to right) transverse, radial, and tangential surfaces. Click on an individual image for an enlarged view.

Growth rings in tree wood form due to seasonal variations in the activity of the vascular cambium, a lateral meristem that produces secondary xylem (wood) toward the inside of the stem.123

These rings are visible because of structural differences between earlywood and latewood produced during a single growing season. Earlywood, typically formed at the start of the season, consists of larger, thin-walled cells that facilitate rapid water conduction. As the season progresses, the cambium produces latewood, which is characterized by smaller, thicker-walled cells. The sharp contrast between the dense latewood of one season and the large-celled earlywood of the next creates the distinct boundary of a growth ring.142

While often annual in temperate regions, ring formation is a response to cyclic environmental conditions, such as water availability. Variations like drought can cause the formation of "false rings" or result in no ring being formed at all, while factors like soil quality and climate influence the width of each increment.12