How Old is My Tree?
'Trees, typically, are not as old as they look and look older than they are!' - (West Country Folklore)
Estimating the age of a tree is as much art as science. There are at least half a dozen different methods, each with their own strengths and weaknesses and because of this, none of them universally accepted. The following is a relatively simple method with, of course, exceptions.
With age, the height and spread of a tree reaches a maximum and then declines. Neither height nor spread can, therefore, be used to estimate the age of a tree. However, the trunk increases in circumference throughout its life and this measurement can be used to estimate the age of a tree.
Most trees reach a point when fully mature (and with a full crown) when the circumference is 1 inch (2.5cm) for each year of growth. Hence a tree that is 15 feet or 180 inches (4.50m) in circumference, measured at breast height i.e. at 5 feet (1.50m) above ground level, is approximately 180 years old.
When juvenile or semi-mature, trees will add some 1½ inches (3.75cm) in circumference but when mature and over mature will slow down their rates of growth annually to increments of ¾ inch (1.80cm) in circumference.
Exceptions to the Rule
Trees growing in a woodland environment typically have a restricted crown and therefore increase in circumference at approximately 50% of the rate of fully crowned trees. Hence, a tree that is 7½ feet or 90 inches (2.25m) in circumference, measured at breast height, is approximately 180 years old.
Fast growing trees, including most Eucalyptus, some Fir trees, Willows, Larch, Giant Sequoia and a handful of others, can add 2-3 inches (50-75 mm) in circumference.
Slow growing trees, especially Yew, may only add ½ inch (1.25cm) in circumference per year and in their maturity possibly as little as 1/5th inch (0.50cm) per year.
Scots Pine, Horse Chestnut and Common Lime grow at the average rate for perhaps the first 100 years and then slow down to perhaps half the average rate, and possibly even less.
The Trees of Britain and Northern Europe by Alan Mitchell