How to age a tree?

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Have you ever looked at a huge, stately tree and found yourself making up stories about its being a little sprout when buffalo and Indians roamed our area, thinking about all the events it has witnessed during its lifetime, and wondering how just how old it really is? Dallas County Master Gardener, Steve Houser (1998), certified arborist and owner of Arborilogical Services, Inc., has found a method to determine a tree’s approximate age without inflicting any damage to the tree by increment boring (boring a hole around a quarter of an inch in diameter to the approximate center of the tree). He says that though this method is not an exact science, it can be used in place of a complete guess and he has verified its accuracy. It also is a very “doable” method for anyone to try.

To use the “calculated estimate method,” find the largest piece of dead wood in the tree and cut a thin”tree cookie,” a thin slice of a tree limb, from the base of the dead limb. In some cases the “tree cookie” must be stained in order to read the rings. On lighter colored woods, such as Ash, Maple or some Oaks, a darker colored stain (walnut or oak) works best. On darker colored wood, such as Bois d’Arc, Walnut or Eastern Red Cedar, use a clear or light color stain (clear or amber shellac, linseed oil or even motor oil). Once the rings can be read, or seen, use a ruler to determine an average growth rate per inch in diameter. You will notice that the growth rings are not all even. If some are close together and some are not, measure both (this is not an exact science) to find out how many years per inch the tree has grown and add them together. Then divide by two to get an average growth rate per inch. Remember that each year, the tree produces “spring and summer wood.” This means that two “rings” (one is usually darker than the other) equals ONE year of growth.

Once an average (current) growth rate is established, it is multiplied times the radius of the tree (diameter divided by two) to get an average age. To accurately measure for only this purpose, measure the circumference of the tree just above the root flare or root crown. This is the area just above the “flare” or expansion of roots at the soil level. Wrap a cloth measuring tape (or a string, then measure the string) in this location to get the circumference in inches. Divide the circumference by 3.1415 to get the trunk diameter, then divide by two to get the radius of the runk. This assumes the tree is perfectly round. If the tree is oblong or not round, a radius measurement may not accurately represent the true center of the tree. In this case, use a diameter measurement from the smallest or most narrow portion. This can be “guesstimated” by holding a steel measuring tape up to the narrowest portion of the trunk and”eyeballing” the measurement for the diameter. Then divide this number by two to obtain the radius. Though it can be argued that the tree grew faster when it was young than currently, one can adjust for this if it is a small or medium sized tree. If it is large and old, though it may have grown faster when young, it has been old for a long time and growing slower for many years. This more than offsets any difference.

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Steve recently used this method to help determine the age of two Post Oaks at a historic park in Farmers Branch. The results were surprising. The larger tree (around 28″ DBH (diameter at breast height, usually measured at 4.5 foot from the ground) was calculated to be around 300-400 years old. Since both trees were in good soil and healthy (given their age), he felt their ages were well over 300 years old, though, if asked to give their age to an official source, he said he would knock off some years to be safe. Steve says his method takes all existing site conditions into account but cannot reflect all droughts, insect problems, etc. from the past. For example, if a tree is over 100 years old, it survived the “dust bowl” and did not likely grow much during that time.

by Carolyn Bush, Originally appeared in The Newsletter of the International Society of Arboriculture Texas Chapter.

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