Interpreting Tree Rings: A Beginner's Guide

We’ve all seen tree rings before, but have you ever wondered what a tree ring actually is? And what can we learn from them? The information tree rings can tell us is so useful that there is a whole field of science, Dendrochronology, dedicated to their study.

Photo Credit: UCAR Center for Science Education

Tree rings are the new layer of woody growth a tree puts on during every growing season. Visually, they are the light and dark rings visible on the inside of a tree trunk. One year of growth is represented by the combination of one light ring and one dark ring. The lighter rings show the amount of growth during the spring and early summer when conditions are best for the rapid growth of less dense wood. The darker, denser rings represent growth during the late summer and fall when growth is slowing down. These rings can tell us a significant amount about both the individual tree and the conditions it grew in.

Photo Credit: NASA

The number of tree rings can tell us about the tree's age. In Minnesota and Wisconsin, there is only one growing season per year. Therefore, each light and dark tree ring combo signifies one year of growth, regardless of thickness. By counting up all of the rings, we can get an accurate estimate of the relative age of the tree.

The thickness of tree rings can tell us about the climate conditions of each year of the tree's life. For example, in a cold or dry year, a tree may put more effort into conserving energy than growing. This energy allocation would result in minimal growth and a thin tree ring. Comparatively, in a warm or rainy growing season, a tree will be less stressed and can put more of its energy into growth, resulting in thicker rings. Depending on the age of the tree, we can sometimes get hundreds of years of climate data through this method.

Irregularities in the rings can also provide information such as unusual growing patterns, disturbance history, or forest stressors. For example, trees growing on a slope will have off-center rings, forest fires may cause black blemishes called fire scars, and insect infestations may cause unusually narrow rings or blemishes such as holes and scars.

Photo Credit: NOAA

If you would like to learn more about the trees on your land, keep an eye out for recently downed trees or use an increment borer to take core samples of your living trees. With tree rings, there is much more than meets the eye!