How Can You Tell How Old a Tree Is?
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 study of tree rings to calculate a trees’ age is called dendrochronology.
When a tree is cut down, you can usually see rings in the tree stump. By counting these rings, you can tell how long the tree was growing. Each year a tree adds a layer of new wood around the old wood. When the trunk of the tree is sawed across, each layer of wood shows as a ring.
Tree trunk growth results primarily from the xylem layers and is counted to determine the age. The tree produces large xylem cells in the spring and small cells of xylem in the summer, making it easy to note the difference between years. The tree rings are larger in wet years than in dry years.
In some hardwood trees, the rings are so small that a hand lens and pins may be needed to count the rings. The size of the rings is now being used to determine past weather patterns, even with petrified trees.
Trying to establish weather history is a very difficult process and more than one or two samples of wood is needed.
The second way is to use an increment borer and count the rings on a trunk cross section. While a tree is still standing, scientists can tell its age by extracting long, thin cores of wood with a special wood borer. In this way, the tree’s annual growth rings can be counted without destroying the tree.
To obtain a trunk cross section sometimes called ‘beaver cookies’, ‘tree cookies’, or ‘hockey pucks’, the tree must be killed. To get an accurate age of the tree, the rings must be near the base (ground) of the tree.
The borer takes a small (0.200 inch diameter) straw-like sample from the bark to the pith of the tree. Though this hole is small, it can still introduce decay in the trunk.
The tree’s age can therefore be obtained by counting the annual rings of either a ‘core’ or trunk cross section. We can also guess at a tree’s age from its height, size, and species.