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Aging Trees Made Easier

Take a stroll through a local forest and you may be wondering, “How old is that big oak or enormous basswood?” Aging trees can be a fascinating perspective about the history of your local area, and a couple local scientists have made it a little easier.

Researchers Marlin Bowles and Michael Jones sought to figure out how old many trees were by collecting core samples from hundreds of area specimens. The cores provided rings for the scientists to count without damaging the tree; the extractions were only 3/16" wide and don’t impact the trees’ living tissue.

The study found that the oldest trees in the area, mostly white oaks, date back to the early- to mid-1800s, when settlers began to harvest some of the wooded areas in Chicago region. 

The process of calculating a tree’s age requires some tree identification and math. First, identify the tree species using a field guide or on a tree ID field trip with a naturalist. Next, measure the tree’s circumference with a tape measure or a length of string and a ruler. Wrap the tape around the tree at chest height to get an accurate measurement. Divide the circumference measurement by pi (3.1416) to get the tree’s diameter. Finally, check the chart below to determine the tree’s approximate age. 

This technique is 90 percent accurate for forest-grown trees listed on the chart. Trees that have spent part of their lives at a small size (perhaps due to poor growing conditions due to drought or lack of sun) may be older than their size would suggest. Trees grown in the open, like those in your backyard or an open savanna, will be much younger for their size, because the ample sunlight and available water speeds their growth. 

As the chart shows, different species grow at different rates.Tree core history 

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