A tree’s growth rings are the circles or rings you see when you cut through a limb or the trunk of a tree. For each year that a tree grows, it produces two new sets of growth rings. One set is developed in the spring and one in the late summer or early fall (sometimes referred to as earlywood and latewood). Each set of growth rings is typically a different color, with one lighter in color (earlywood) and the other darker (latewood). Each year, as a tree grows it adds a new growth ring near the outer edge of the bark with the original or first year’s growth rings in the center of the limb or trunk.
Counting either the light or dark-colored growth rings on a stump or limb from the center of the rings to the outer edge will provide the tree’s approximate age. In research using growth rings to determine the age of a tree, there are potential deviations in the number of growth rings. It is possible to have extra or false growth rings due to an unseasonably warm spell during a dormant period, which stimulates new growth. It is also possible to have missing or indistinct growth rings due to damage to the tree or during a drought.
The distance between the growth rings is an indication of a tree’s health, as well as the cultural or environmental conditions during each year of its life. Years of higher rainfall may produce larger growth rings, whereas years of lower rainfall may produce smaller rings. The National Weather Service can corroborate a great deal of tree growth ring data, primarily because the size of the growth rings is directly related to past weather conditions and droughts for a geographic area.
The cultural conditions of a site relating to a tree such as past pruning, irrigation, lightning damage, or root/trunk damage, will also have a direct effect on a tree’s health and growth rates. If a tree is growing on shallow rock, in poor soil conditions, or suffers from a lack of sun or other environmental conditions, its growth rates can be greatly reduced. Trees growing in a lower-lying area or near a creek will typically enjoy better soil and grow at faster rates than trees growing on shallow rock.
Growth rates can also vary greatly by species. Those species known to be faster growing, such as Mulberry (Morus spp.) or Cottonwood (Populus spp.), can produce large sets of growth rings each year. Slower growing species, such as Post Oak (Quercus stellata), can produce very small growth rings, especially when growing on a rocky site with no irrigation.
For additional information on false or missing growth rings or determining a potential age range of a tree, please see the book on Comanche Marker Trees of Texas published by A&M University Press.
The lighter ring color is Earlywood and represents the tree’s growth during the spring.
The darker ring color is Latewood and represents the tree’s growth during the late summer or early fall.