Why do trees have growth rings?
Short answer
Tree growth changes with the seasons, so the tree adds wood with different properties in a repeating pattern. In regions with seasonal growth, the boundaries can be seen as rings in a cross-section.
How it works
A layer of cells called the vascular cambium adds new wood to the trunk. During a fast-growing season, the wood can look lighter. Later, slower growth can produce denser, darker wood.
The combination of these contrasting areas forms a growth ring. In places with clear seasonal changes, one ring often corresponds to about one year, but that is not true for every tree or region.
Key point
Rings are not all the same width. Water, temperature, sunlight, disease, and other conditions change how much a tree grows. Scientists can use ring patterns to study past environments and events such as fires.