What Are the Ages of Trees in Your Neighborhood?

by John Dwyer
Research Associate
The Morton Arboretum

Trees are often a distinctive feature of the urban landscape. Large trees are especially prominent, have a particularly significant influence on the urban landscape and symbolize permanence amid change.

We are curious about the age of these venerable giants as an indication of the history they have witnessed. Their ages also offer a clue to their significance by indicating how long we might have to wait for a replacement tree to reach their size.

To get some indication of the age of urban trees, Kerstin Von der Heide, Forester, Village of Downers Grove, Illinois measured tree diameters (4.5 feet above the ground) and counted the number of annual rings on the stumps of 328 trees representing 24 species removed from along village streets. The oldest was a 167-year-old white oak that measured 26.2 inches in diameter. All trees older than 135 years were white oak or burr oak. The largest tree measured was a 52.2-inch American elm 110 years old. Other trees over 40 inches in diameter were willow, green ash or honeylocust.

Following a study of forest-grown tree ages by Marlin Bowles and Michael Jones from The Morton Arboretum, I used a similar statistical approach to estimate the ages of urban trees at specified diameters for each of 12 species for which sufficient data were available. Limited observations restricted the ages for which results could be reported. The resulting estimates presented in the Table (below) are only approximations because there is a wide range of growth rates for individual urban street trees (even within a particular species), type of location (along streets), and village.

Other studies of the growth of street trees have shown considerable variation in tree size at a given age based on the condition of the tree and the environment in which it is growing. In addition, results may not be directly applicable to trees growing in other types of urban locations. For example, a tree growing unrestricted above and below ground in the middle of a large yard with good soil conditions may grow at a faster rate than trees along street corridors, consequently the yard tree may reach a given diameter in fewer years than a typical street tree.

A striking contrast between the urban trees studied here and forest grown trees studied by Bowles and Jones is that forest trees may be up to 80% older than urban trees at a particular size. These slower growth rates are probably caused by reduced available light and in some instances greater competition for soil resources under a forest tree canopy.

If you know the tree species and can find it in the Table (below). All you need is the tree diameter in inches measured at 4.5 feet above the ground. Then look up the estimated age. It is usually easier to

measure tree circumference than diameter, and diameter in inches can be calculated from circumference by dividing circumference in inches by 3.1415. If you do not have a measuring tape, you can put a string around the tree and then measure the string with a ruler. Because of the different growth rates of forest and urban trees, it is important to use the report by Bowles and Jones for estimating the ages of trees growing in forest conditions.

Many of the larger trees are most likely older than the residential developments in which they are found. Others may have been planted when the streets were established, the homes built or at other times.

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