Tree Dating Methodology
Growth factor table used to estimate tree ages: https://www.treehugger.com/estimating-forest-trees-age-1343321
DBH = circumference divided by Pi
DBH x Growth Factor = APPROXIMATE age
Understanding the only way to determine the most exact age of a tree is counting the rings either by boring into the tree and taking a sample or by counting the rings on the stump when it has been cut down. Size of tree does not necessarily correlate to age. Trees growing in a wide-open field in good conditions may grow much larger much faster than those growing in competition or in less-than-ideal circumstances, meaning trees of the same species and age can be much different in size.
The following information about the calculations and determining the age of a tree comes from: https://www.fs.usda.gov/nrs/pubs/jrnl/2018/nrs_2018_smith-k_002.pdf
The potential for a tree to reach a great size and to live a long life frequently captures the public’s imagination. Sometimes the desire to know the age of an impressively large tree is simple curiosity. For others, the date-of-tree establishment can make a big difference for management, particularly for trees at historic sites or those mentioned in property deeds, literature or historical documents (Figure 1).
If we know with certainty that a tree is 150 years old, we can be sure that the shade of that tree didn’t inspire a poet or revolutionary from 200 years ago. We know that trees tend to increase in size as they age.
Can age be determined for mature landscape trees by simply measuring tree circumference or diameter?
Some organizations produce tables of “growth factors” of various tree species to provide age estimates from size measurements. The process usually involves passing a tape measure around the trunk at 4.5 feet above groundline to determine the stem circumference.
The circumference can be divided by Pi (π, approximately 3.14) to yield the diameter at breast height (DBH).
Specialized diameter tape measures have the circumference-to-diameter conversion already made in the scale markings.
Depending on the particular table, either the diameter or circumference is multiplied by the listed growth factor to give tree age, usually accompanied by a caution that the age given is an approximation.
Based on local experience, a forester or an arborist may know that a 2-foot DBH red maple is usually about 75 years of age, while an eastern cottonwood of the same size at the same location might be less than half that age simply due to inherent differences between species. The amount of confidence to have in such estimates is hard to assess, as they are rarely checked against actual observations. Even when valid, this sort of experience is not readily transferred to other locations or growing conditions.