The most common ways to calculate tree age, first, being when the tree was planted while the second way to estimate tree age is to count the
annual growth rings.
Calculating Tree Age Using Annual Rings
The annual growth rings can be counted using two different methods. First, you can extract an increment core from the tree using an increment borer.
Most people, however, do not have access to an increment borer, and in fact this instrument does result in an injury to the tree. For trees that are dead and have been cut down, you can count the rings on the stump. This provides an accurate estimate, but for live trees it just won’t work!
See Also: Tools for Measuring Tree Diameter
Calculating Tree Age Using DBH (Diameter at Breast Height)
Now, when we talk about the age of a live tree, you can estimate the age of a living tree, without knowing when the tree was planted, by the following method.
In the example below, we are measuring an oak tree.
Step – 1: Measure the circumference (c) of the tree trunk using a measuring tape that measures in feet and inches. This should be done at 4.5 feet above the ground which is the Diameter at Breast Height (dbh). Here, the oak tree measures 12 feet, 10 inches in circumference.
c = 12 feet, 10 inches = 154 inches
(multiply 12 feet x 12 inches per foot, then add 10 inches)
Step – 2: Calculate the diameter (d) and radius (r) of the trunk in inches.
Divide the circumference by “pi” 3.14.
c/3.14 = d; 154 inches/3.14 = 49 inches
Divide the diameter by 2 to get the radius (r).
d/2 = r; 49 inches/2 = 24.5 inches= r
NOTE: You should deduct at least .5 inch from the radius for the width of the tree’s bark, and more for trees with very thick bark (1.0 inch), less for trees with very thin bark (.25 inch). In our example, we will deduct .5 inch, for a radius of 24 inches.
Step – 3: Determine the average width (w) of an annual ring of wood for the species of tree you have measured. In ring-porous species of trees (distinct
annual rings), the annual ring will contain both early or spring wood (large vessels) and late or summer wood (small vessels). In diffuse porous woods, it is much more difficult to see where the rings begin and end. A hand lens or magnifying glass is useful.
- The width of an annual ring of wood for a particular species depends upon many factors, including:
- Genetic differences
- Tree age
- Height above ground
- Local site and climatic conditions
- Competition from other trees
- Water availability
- Nutrient availability
- Presence of insects or diseases
- General tree health
You can also calculate the average width of an annual ring using cross sections of wood from the same species. Measure the radius from just inside the bark to the center of the trunk, count the number of rings, and divide the radius by the number of rings. The result will be the average width per ring.
10 inches/50 rings = .2 inches/ring
Conversely, if we divide the number of rings by the number of inches, the result will be the average number of rings per inch.
50 rings/10 inches = 5 ring/inch
If the absence of actual data for white oak we will use in our example an assumed average of .2 inches per ring, or 5 rings per inch.
Step – 4: Divide the radius (r) by the average width (w) of one annual growth ring to get the approximate age of the tree.
r/w = approximate age
24 inches/.2 inches per year = 120 years!
In this way, the age of a living tree can be estimated.
Courtesy: For more information, contact the Athens-Clarke County Community Forester at (706)613-3561 voice, (706)613-3566 fax, or by e-mail at email@example.com.