Counting rings is the tried and true method for determining the age of a tree. Dendrochronology is the science of counting tree rings to determine tree age and to identify extreme and unique environmental events. Scientists use certain trees, such as bristlecone pine, that have slow growth rates and live for a very long time, to map out long sequences of weather patterns. These sequences are use to compare to known events, and to help develop base lines for carbon dating. In some regions dendrochronologists have more than 10,000 years sequenced. There are some problems with counting rings. Some of them surmountable and some not.
The one big insurmountable obstacle for counting rings is that many large old trees have decayed to the point that the center of the tree is hollowed out. If there are no rings you can’t count them! In our region, live oaks are the prominent tree and are very prone to extreme hollowing. Although, this isn’t necessarily detrimental to the tree’s health, it does limit the ring counting technique.
One of the issues that we have worked around is accessing the rings (if they are still there). So, if you want to count the rings, you would have to cut down the tree, right? Wrong.
There are two ways to get around this. If you are lucky you might have a large broken limb somewhere up in the tree (or fallen on the ground) that you can use. Sure, you might think that you need to count the rings at the base of the tree to get a full count, but what we do is look at the rings on the limb and then count the number of rings-per-inch. Then, you measure the diameter of the tree’s trunk at the base and do the math to extrapolate the total number of rings. This isn’t a “direct” count, but it does give us a very close approximation. The alternative to this extrapolation method is to do core sampling. Core sampling is literally drilling out a tube of wood from the base of the tree to count the rings. This is probably a more accurate method if you are lucky enough to have no internal decay or hollowing, but it is damaging to the tree.
What are tree rings? Every year trees grow new rings around the perimeter of the trunk and limbs. These rings are the new vascular tissue that transport water and nutrients up the tree to the leaves. During the growing season the new growth that develops produces large open tubes to allow the large flows of water. During the dormant season the new growth is smaller, tight tubes reflecting the small amount of tree activity. The contrast between large and small tubes is what produces the “ring” effect.
The pictures in this article came from a limb that broke out of a 500 year old bur oak 50′ up from the ground. The guys from Arborilogical tree service up in Dallas saved a piece of the broken limb and Kevin Basset turned the wood to make it easy to see the rings.
Arborist have another good use for counting rings. We can use it to gain insight to tree health during the diagnosis process. Trees that are stressed out produce smaller tree rings. Often I’ll look for a limb that has recently died, cut a cross section and look at the rings to determine if the problem happened all the sudden or slowly over time. The rings will look normal if the problem happened all of the sudden. If the tree has had long term stress you will be able to see a history of small rings. Understanding the length of time a tree has been stressing can be instrumental for developing a confident diagnosis sometimes.
WARNING – Please don’t read this article and go out and start cutting limbs off your tree to count rings. This is fun, I do admit. But, it’s not worth ruining your tree. It would be wise to do this under the supervision of an arborist.