How Is a Tree's Age Determined?

Written by darcy logan
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How Is a Tree's Age Determined?
Counting the rings is not the only method used to determine a tree's age. (Brand X Pictures/Brand X Pictures/Getty Images)

If you thought you needed to cut down a tree to determine its age, you would be wrong. That's only one method that is used. Scientists and historians have developed other methods to determine a tree's age. These methods might not be as accurate, but they can provide a good estimate without harming the tree.

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Counting the Rings

The most accurate measure of a tree's age involves counting the rings, known as dendrochronology. Every year, a tree grows new rings around its perimeter to transport water and nutrients. These rings are then counted to determine how many times the growth expanded. Counting rings is more difficult on tropical trees because they continue to grow year-round, so there is less definition between the rings. Trunks on some older trees may also become hollow, leaving no rings to count.

Diameter at Breast Height

The diameter at breast height (DBH) is determined by measuring around the tree's trunk 4 feet 3 inches from the ground, or breast height. If the tree has any growths, bulging or branches at this location, measurement is usually taken just above or below the area. The idea is to determine the average width of the trunk, which gives the circumference of the tree. The diameter is determined by dividing the circumference by pi (3.14). Once the DBH is determined, it can be compared to a chart that lists average diameter of the tree species at various ages.

Tree Aging Formula

The tree ageing formula can estimate a tree's age by multiplying the diameter of the tree by its growth factor. It is more accurate with forest and rural trees rather than urban trees, which are often grown in less ideal conditions that can stunt their growth. The tree ageing formula takes the diameter at breast height (DBH) and multiplies it by the tree's growth factor. The growth factor is determined by the tree's species. A tree that grows quickly, like cottonwood and aspen, have a low growth factor of 2. Slower-growing trees like dogwood and redbud have higher growth factors of 7.


If the tree is growing in an urban or residential area, old photographs might help determine its age. Photographs of an area can tell when the tree was planted or provide a minimum or maximum age. Historical records and firsthand accounts might also provide information about a tree's age. Comparing a tree to a similar tree nearby that has been cut down can also provide age clues.

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