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Why do light and dark bands appear in conifer growth rings?

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The massive Grizzly Giant sequoia tree stands with a wide, reddish-brown trunk and a hollowed base, surrounded by other tall trees in Mariposa Grove, Yosemite National Park. — Conifer A giant sequoia (*Sequoiadendron giganteum*) estimated to be between 1,900 and 2,400 years old, Grizzly Giant is the oldest tree in the Mariposa Grove, Yosemite National Park, California.

In conifers, the appearance of light and dark bands within growth rings is caused by structural differences between wood produced at different times during the growing season.12

Light bands (Earlywood): At the beginning of the growing season, the tree produces cells with larger diameters and thinner walls to facilitate rapid water conduction. These cells appear lighter in color.312
Dark bands (Latewood): As the growing season progresses, the tree produces cells that are smaller in diameter and have thicker walls. These denser, thick-walled cells appear darker in color.3412

The juxtaposition of these small, dark latewood cells from the end of one season against the large, light earlywood cells that mark the start of the next season creates the distinct, visible boundary of the growth ring. In temperate regions, this cycle typically corresponds to one year of growth, as no wood is produced during winter dormancy.14