NOVA Online | Tracking El Niño | How are tree rings made?

How are tree rings made?

Diagram of closeup of tree rings aligned with growth rates

A tree grows mostly during the springtime, when there is plenty of rain and sun to nourish it. It starts its spring growth by dividing a layer of cells known as the cambium that lay between the old wood and the tree's bark. This first growth, called earlywood, is light colored. As the growth rate slows and stops during fall the cells laid down are smaller and denser (and therefore appear darker). This layer, called latewood, marks the end of the growing season. The area from the beginning of the earlywood and end of the latewood periods represent one year of growth.

Not all trees form rings—they only occur in trees that have a down time when they do not grow, such as fall and winter. Trees in the tropics that grow year-round do not form annual rings.

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Photo: ©P.R. Sheppard

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