There are two ways to find out how old a tree is. The first is to wait until the tree is cut down and count the rings. The second is to use an increment borer and count the rings on a trunk cross section. The last method is most preferred as the tree is not harmed.
Usually small diameter cores are taken using the increment borer where nearly all ring counts can be confirmed and data collected. If done correctly, these samples can even be used for chemical studies in commercial labs to determine suitability of the trees for different purposes.
In the urban areas, city or state foresters may use the core to determine not just the age, but the structural condition of the tree. A hollow tree or a tree with substantial rot may be hazardous and have to be removed.
