A tree grows at different rates. In the spring and summer it grows more, and bands are light and wide. Growth slows in autumn, and during winter, it goes dormant and leaves a dark narrow ring. These tree rings provide a record of the past. The light and dark rings are a timeline to history.
Tree rings are a rainfall record during a year. During a wet year, a tree grows more, and makes the rings wider and light-colored. Years that have cool summer temperatures produce denser wood. This makes the light-colored bands of wood thinner. A researcher can tell what the area's climate was like hundreds of years ago by the thickness and density of the tree rings. Trees that grow in the same climate develop matching thick and thin ring patterns.
Other Climate Factors
Other factors influence tree growth and tree rings. The soil properties, wind, sun and location of the tree on a slope will affect growth rings. By noting the location and the climate influences on the tree, it is used to identify trees grown in similar conditions.
A tree grows one dark ring each year it's alive. Counting the dark tree rings gives the age of the tree when it died. By comparing tree rings of two contemporary trees grown in the same geographical and climate area, ring thicknesses may overlap, and sections of ring patterns match if they grew at the same time. This crossdating principle allows using tree rings to extend reliable dating techniques back over thousands of years.
Dendrochonology is the study of tree rings. A. E. Douglass developed the process of determining dates using tree-rings in the early 1920's. He founded the Laboratory of Tree-Ring Research at the U of A, University of Arizona in Tucson. Samples of tree-rings are stored at the university, and it has one of the largest tree-ring collections in the world. One source tree ring data is obtained is from trees that have been cut down. A section is cut from the tree and the tree rings counted. Live trees also yield data by core samples. A drill bit with a hollow shaft called an increment borer is used to remove a plug from a tree which shows the tree rings. These cores and sections continuously date tree-rings hundreds of years in the past. By matching site samples found with these known samples educators and scientists establish dates for the old wood in the same area.
Tree ring samples and dating have wide uses to determine climate and weather conditions in the past. It indicates the amount of carbon dioxide, forest population and other environmental conditions in the past. This allows archaeologists and others to establish dates for cities, settlements and past events.