How do annual rings form in the stems of woody plants?

In woody plants, annual rings form through the activity of the vascular cambium, a lateral meristem that produces new secondary xylem toward the inside of the stem each growing season.1

The visibility of these rings results from structural differences between earlywood and latewood. Earlywood, produced at the start of the growing season, typically consists of larger cells with thinner walls to facilitate rapid water conduction. As the season progresses, the cambium produces latewood, which is characterized by smaller cells with thicker walls, resulting in higher density and often a darker appearance.213

The sharp contrast between the dense latewood of one year and the large-celled earlywood of the following year creates the distinct boundary seen in a cross-section of the stem. While these increments are often annual in temperate regions, their formation is a response to seasonal environmental cycles; consequently, variations such as "false rings" can occur due to fluctuations like drought.21