Tree rings form each year as a trunk adds a new layer of wood beneath its bark.

The longer version

Inside a trunk, a thin layer of cells called the cambium divides throughout the growing season. Cells produced early in the season tend to be large and light, while those formed later are smaller and denser, and that contrast marks the boundary of one annual ring.

Because the pattern depends on how good each season was, rings vary in width. A wet, mild year usually leaves a broad band, and a hard year leaves a narrow one. Counting them gives an approximate age for the tree, and matching patterns between living and dead wood helps date older timber.

Where this entry sits

Plants and Trees gathers ordinary and surprising details about how green things live, from the tallest forest giants down to the moss on a garden wall. Expect notes on flowers, seeds, roots, leaves, fungi, and the quiet tricks plants use to spread and survive. The full list sits on the Plants and Trees page, where every entry is listed in order.