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How do annual growth rings form in woody tree stems?

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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.

Growth rings in woody tree stems form through the activity of the vascular cambium, a lateral meristem that produces secondary xylem (wood) toward the inside of the stem.12

The visibility of these rings results from structural differences between earlywood and latewood produced during a single growing season. Earlywood, typically formed at the start of the season when water is abundant and hormone production is high, consists of larger, thin-walled cells. As the season progresses, the cambium produces latewood, which is characterized by smaller, thicker-walled cells that are denser. This transition creates a clear demarcation between the latewood of one ring and the earlywood of the next.341

While often annual in temperate regions, ring formation is primarily a response to cyclic environmental conditions, such as seasonal changes in water availability. Factors like drought can cause the formation of "false rings" or result in missing rings, as the rate of cell division and expansion is tied to current weather conditions.315