How To Determine the Age of a Tree

So you may have heard that you can count a tree’s age by counting the interior rings, but what if you want to determine the age of a tree that is still standing, and you’re not sure of the year that it was planted?

The good news is that determining the age of a tree is a simple process that doesn’t require tree felling, core boring, or other methods that might harm the tree. In this article, we break it down step-by-step. While there is a little bit of math involved, it’s nothing more complicated than some basic geometry…

FIRST METHOD

To start, simply measure the circumference of the tree using a “soft” measuring tape (used for measuring fabric). Make sure to measure the breadth of the tree at chest height, or roughly 4.5 feet from the ground.

The next step will be to take your circumference, and to divide it by pi (3.14). From there, you can find the radius by dividing the diameter in half by 2. For example:

126 inch circumference

126 inches ÷ 3.14 (pi) = 40 inches (diameter)

40 inches (diameter) ÷ 2 = 20 inches (radius)

To make our measurement as precise as possible, we’ll now want to subtract the thickness of the bark from the radius. Depending on the the species of the tree, we’ll want to subtract anywhere between 0.25 in – 1.00 in. For quick reference, black oak trees will tend to have a thickness of around 1 inch, while birch trees will have a thinner bark of around 0.25 inches. If you’re just looking for a ballpark, your best bet is to use 0.5 inches as a nice happy medium!

20 inches (radius) – 0.5 inches = 19.5 inches (radius without the bark)

Going forward, it will be helpful to track down a similar tree of the same species that has been felled. For example, perhaps you’ve had a tree on your property that has fallen or has been cut down with the stump in tact. By measuring the radius and counting the number of rings, you’ll end up with the average ring width. For example:

Let’s say we come across a tree stump with a 30 inch radius and 120 rings…

30 inch radius ÷ 120 rings = 0.25 inches (average ring width)

NOTE: In this case, we are assuming that the average ring width will be the same for the tree that you are looking to measure. Since they are the same species of tree, and in the same geographical location, we can safely say that the two trees will have the same ring width.

Now that we have the radius (minus the thickness of the bark) and the average ring width, we have all the pertinent information that we need. Time to put it all together!

To estimate the age of the tree, divide the radius by the average ring width. Continuing along with our example:

19.5 inches (radius) ÷ 0.25 inches (average ring width)

= 78 YEARS OLD

ALTERNATIVE METHOD

Alternatively, if you don’t happen to have a stump on your property to compare with, you’ll be able to Google the average growth rate of the tree in question for most species. Keep in mind that growth rates may differ between forested versus urban areas so you may want to consider using two different growth rates, calculating the age of the tree, and then averaging them together.

In this example, you can divide the circumference of the tree by the average annual growth rate.

126 inches (circumference) ÷ 1.25 inches (annual growth rate) =

= 101 YEARS OLD

NOTE: The results of this method may differ first method, but not to worry – both methods of calculating a tree’s age are more like rough estimates than precise measurements.

And there you have it! In just a few basic steps using the methods above, you’ll be able to estimate the age of your trees using nothing more than some simple math and a measuring tape.

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