How Tree Rings Record Growth
Why earlywood and latewood form visible rings inside tree trunks.
When you look at a cut tree stump, you see a series of concentric circles running from the center to the outer bark. These tree rings show the age of the tree, but they also record the weather conditions the tree experienced during each growing season.
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Tree rings form because of changes in how the tree grows throughout the year. In spring, when water is abundant and temperatures rise, the tree produces a layer of xylem tissue known as earlywood. The cells in earlywood are large with thin walls, designed to transport large volumes of water to the expanding canopy. This tissue appears light in color.
As summer turns to autumn, water becomes less available and growth slows down. The tree produces latewood, which consists of smaller cells with much thicker walls. This tissue provides structural support and appears darker.
The contrast between the dark latewood produced at the end of one year and the light earlywood produced at the start of the next creates a visible line. By measuring the width of these rings, scientists can determine past rainfall levels, as wet years produce wide rings and drought years produce narrow ones.
