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Determining the Age of a Tree

I’ve been wondering about the age of a large white oak tree guarding the edge of the forest for years. Not enough to actually do something about it, mind you, but wondering nonetheless. Then a hike with old growth trees last Sunday triggered a burning curiosity in me about the age of the trees we hiked amongst, and by extension, the age of the trees in my own backyard. I found myself having to know.

There’s an easy way to gauge the age of a tree: you count the rings. The problem with that method is you’re really measuring the age of death of the tree. I prefer to keep them around, especially when they’re my elders. But rest assured, there’s another method for estimating the age of a tree, and that’s doing some basic math and adding a bit of educated guessing. All of this is searchable online, of course, but I found the instructions from Purdue University to be particularly helpful.

Step one is to measure the DBH of the tree. What’s this? Another acronym in a world of acronyms? Sorry! But this one is easy to remember. DBH stands for Diameter at Breast Height. Take a flexible tape out to your tree of interest, measure 4 1/2 feet up from the base of the tree and there’s your DBH. Now anchor the end of the tape measure (or have someone hold it) and walk around the tree back to your starting point. This is the circumference of the tree. Convert this to total inches. In the case of my stately white oak, it measures 92 inches in circumference.

The next step is to determine the diameter of the tree, which simply means dividing the circumference by 3.14. For the white oak, this was 29.29. So far, so good. And now we rely on something called the growth factor to figure out the rest. This is where science meets estimation. For a tree on the edge of the forest in optimal growing conditions, the growth factor is pretty straightforward. For a tree on a sidewalk in downtown Boston or near the summit of Mount Jackson in New Hampshire, well, that tree’s growth factor is going to be pretty compromised by the stress of everyday living. You’ll need to factor that in to the equation at some point.

Back in my backyard, our white oak is happy as a tree can possibly be in this crazy world. The growth factor for a white oak in this happy situation is 5. You multiply the diameter by the growth factor and my favorite white oak tree turns out to be around 146 years old! And those old growth trees I saw hiking? They’re roughly 300 years old, which is about how long a healthy white oak typically lives. I hope they beat the odds with that good clean living.

So around 1875 when the fields were no longer being farmed or grazed in this patch of Southern New Hampshire land an acorn sprouted and grew in the sun. It witnessed the forest grow around it, protecting it from the worst of the winds and the whims of humans looking for firewood and lumber. And then I became its neighbor and guardian at age 124. And we became fast friends.

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