Arborists have a formula for estimating the age of forest-grown trees; we found it at the Morton Arboretum’s website. Here’s what to do:
1. Determine the species.
2. Measure the circumference of the tree, four to five feet up on the trunk.
3. Divide the circumference by pi (3.14). The answer is the diameter.
4. Compare your tree’s diameter with this chart from the Morton Arboretum to find the approximate age.
Keep in mind that this formula is based on average growth rates of trees in forests. Trees growing in the open, where they don’t have to compete for water, nutrients or sunlight, can grow faster than forest trees. Therefore, a tree in a suburban back yard may be slightly younger than its diameter suggests.
The Morton chart lists mostly oaks and ashes. For help in aging other species, check out this page on estimating tree age from About.com. It relies on the same formula, but it lists additional species and another mathematical step.
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