Cottage Q&A: Can I tell the age of a tree without counting its rings?

Close up of a tree trunk By mothelifeStudio/Shutterstock

Is there some way, some formula, perhaps, to gauge the age of trees without cutting them down and counting the rings?—Jeff Moore, French River, Ont.

Yes, there is a formula, originally developed by the International Society of Arboriculture. It works because trees of the same species under the same conditions grow at roughly the same rate. Here’s how you can use it: measure the circumference of the tree’s trunk about 4.5 feet from the ground. Divide that by pi (3.14); this gives you the “diameter at breast height” (or the “DBH” in arborist lingo). You then multiply the DBH by the tree species’ typical growth factor (you can find charts online). This is a number assigned to each species that corresponds to how fast it typically grows. 

The problem with this formula? In Canada, what’s typical varies widely. Different regions have different growing seasons, and “there are many factors that can influence a tree’s size,” says Steve Smith, an arborist with Bartlett Tree Experts in Calgary: soil conditions, root stress, water availability, or even light availability. “Trees that are in the understory of the forest often don’t grow as much in diameter as trees in better growing conditions,” says Smith. “I once had to cut down a spruce tree that was roughly 60 years old but only had a trunk diameter of about 15 cm.” A healthy, 60-year-old spruce should be a thick tree. “Here in Alberta, I’d expect one to have a diameter of 60 to 80 cm.”

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So, the formula is just a rough estimate, and it can be pretty inaccurate. There’s got to be a better way! There is—getting a tree core sample using an increment borer, says Peter Quinby, the chief scientist at Ancient Forest Exploration and Research in Powassan, Ont. “This is a tool used to extract a long, narrow, tube-shaped piece of the tree trunk—the bole—at 4.5 feet above the ground and perpendicular to the length of the trunk,” he says. “The idea is to get this ‘tree core’ so that you can examine the growth rings extending from the outer-most ring to the inner-most ring.”

Not that you’d want to start taking chunks out of your cottage trees—the process is intrusive and creates a wound, says Smith. “But it’s much less intrusive than cutting the whole thing down.”

This article was originally published in the May 2023 issue of Cottage Life.

Got a question for Cottage Q&A? Send it to answers@cottagelife.com.

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