How to Tell a Tree’s Age
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by Marlin Bowles, M.S.
Plant Conservation Biologist
Michael Jones, Research Associate
Tracing a tree back to its roots has never been simpler. With a newly developed aging chart, the life histories of the region’s old-growth trees are just a measurement away.
The tool in question resulted from a 1996 study of Chicago Wilderness’ old-growth forests. Morton Arboretum researchers Marlin Bowles and Michael Jones calculated the age of trees in Chicago region forests by collecting core samples from about 600 area specimens. The cores provided rings for Bowles and Jones to count without harming the tree. The extractions were only 3/16″ wide so the trees’ living tissue was minimally affected.
Bowles, a plant conservation biologist, says that old forest trees can help nature enthusiasts learn more about the history of their local environment. For example, an area with mostly young trees must have experienced a major logging or fire event that would have eliminated all the older trees. On the other hand, a stand of very old trees means that there has been no disturbance for a long time to cause disturbance to the tree stand.
The study found that the oldest trees in our area, mostly white oaks, date back to the early- to mid-1800s, when settlers began to thin some of the wooded groves in northern Illinois. The data also confirm the conventional wisdom that human fire suppression has caused the number of sugar maples to increase, while oak and shrub populations have declined.
Here’s how to calculate a tree’s age. Warning: It helps to have a little botany background and some math skills!
1. ID the tree species (a tree field identification guide will be helpful).
2. Measure the tree’s circumference with a tape measure. Wrap the tape around the tree at chest height (about 4-5 ft up) to produce an accurate measurement.
3. Divide the circumference measurement by pi (3.1416) to yield the tree’s diameter.
4. Check the chart (below) to determine the specimen’s age.
Was your tree starting out life as DuSable was building the first cabin in Chicago in 1779? Or when Illinois was just becoming a state in 1818? Or was it sprouting from an acorn when engineers reversed the Chicago River in 1900?
Some trees, because of poor growing conditions, may grow more slowly and, therefore, may be older than their size would suggest. On the other hand, trees grown in the open, like those in a suburban backyard, will be much younger for their size because the added sunlight available to them speeds their growth. Also, as the chart shows, different species grow at different rates.
Want to know more? Try this link. You’ll also find an article authored by Gerry Donnelly, PhD, President and CEO of The Morton Arboretum, on “Plant Science Gardens in Conservation,” in which he explains that botanical gardens and arboreta play a distinctive and integral role in protecting and preserving biodiversity.