The most accurate way foresters determine the age of a tree is by counting the growth rings of a severed tree stump or by taking a core sample using an increment borer. However, it is not always appropriate or practical to use these invasive methods to age a tree. There is a noninvasive way to estimate tree age in common trees where they are grown in a forest environment.

Growth Depends on Species

Trees have different growth rates, depending on their species. A red maple with a 10-inch diameter and competing with other forest-grown trees can easily be 45 years old while a neighboring red oak with the same diameter would only be approximately 40 years old. Trees, by species, are genetically coded to grow at about the same rate under similar conditions.

A formula was previously developed and used by the International Society of Arboriculture (ISA) to predict and determine a forestland tree's age. Running the calculations and comparing them to a species growth factor is regionally and species-specific, so these should be considered very rough calculations and can vary by region and site index.

The ISA says that "tree growth rates are affected tremendously by conditions such as water availability, climate, soil conditions, root stress, competition for light, and overall plant vigor. Further, the growth rates of species within genera can vary significantly." So, only use this data as a very rough estimate of a tree's age.

Estimating a Tree's Age by Species

Begin by determining the tree species and taking a diameter measurement (or convert circumference to a diameter measurement) using a tape measure at diameter breast height or 4.5 feet above stump level. If you are using circumference, you will need to make a calculation to determine the tree diameter: Diameter = Circumference divided by 3.14 (pi).

Then calculate the age of a tree by multiplying the tree's diameter by its growth factor as determined by species (see list below). Here is the formula: Diameter X Growth Factor = Approximate Tree Age. Let's use a red maple to calculate age. A red maple's growth factor has been determined to be 4.5 and you have determined that its diameter is 10 inches: 10 inch diameter X 4.5 growth factor = 45 years. Remember that the growth factors provided are more accurate when taken from forest grown trees with competition.

Growth Factors by Tree Species

Red Maple Species - 4.5 Growth Factor X diameter

Silver Maple Species - 3.0 Growth Factor X diameter

Sugar Maple Species - 5.0 Growth Factor X diameter

River Birch Species - 3.5 Growth Factor X diameter

White Birch Species - 5.0 Growth Factor X diameter

Shagbark Hickory Species - 7.5 Growth Factor X diameter

Green Ash Species - 4.0 Growth Factor X diameter

Black Walnut Species - 4.5 Growth Factor X diameter

Black Cherry Species - 5.0 Growth Factor X diameter

Red Oak Species - 4.0 Growth Factor X diameter

White Oak Species - 5.0 Growth Factor X diameter

Pin Oak Species - 3.0 Growth Factor X diameter

Basswood Species - 3.0 Growth Factor X diameter

American Elm Species - 4.0 Growth Factor X diameter

Ironwood Species - 7.0 Growth Factor X diameter

Cottonwood Species - 2.0 Growth Factor X diameter

Redbud Species - 7.0 Growth Factor

Dogwood Species - 7.0 Growth Factor X diameter

Aspen Species - 2.0 Growth Factor X diameter

Considerations for Aging Street and Landscape Trees

Because trees in a landscape or park are often pampered, protected, and sometimes older than forest-grown trees, it is more of an art to aging these trees without significant error. There are foresters and arborists with enough tree core and stump evaluations under their belts who can age a tree with a degree of accuracy.

It's important to keep in mind that it is still impossible to do anything but estimate a tree age under these conditions. For younger street and landscape trees, pick a genus or species from above and reduce the Growth Rate Factor by half. For old to ancient trees, significantly increase the Growth Rate Factor.