What happens to trees each year?

During this century, archaeologists have discovered that trees can be a valuable tool for measuring time. In scientific terms, this method is called dendrochronology. Dendrochronology is based on the scientific discovery that each year a tree grows a new layer of wood. When you cut down a tree, for example, you can see and count the layers of concentric circles. These circles will reveal a great deal of information. The layers vary in color and in thickness. These variations are a result of differing climate conditions from year to year that affect the growth of the annual tings. Cold weather, for example, creates thinner and lighter tings while hot weather produces thicker and darker rings. All trees that grow in the same region will have roughly the same pattern of annual rings, and this can be used as a source of comparison and measurement.
     Early in this century, archaeologists began using the rings to date ruined structures and figures built in the southwestern United States. They did this by comparing the rings of the wood found in the ruins with the rings of recently cut trees. Slowly but surely, the system was extended backwards in time to measure older ruins. The heavy forestry of the United States, coupled with the relative youth of its history, led to a fairly uncomplicated study. Eventually, dendrochronology produced an accurate time and weather record for the American southwest that extended over 2000 years.