Everything else being equal, in a wet year trees will produce a larger growth ring.In a dry year, trees will produce a narrow growth ring.
Tree-ring dating provides scientists with three types of information: temporal, environmental, and behavioral.
The temporal aspect of tree-ring dating has the longest history and is the most commonly known—tree rings can be used to date archaeological sites, such as the Cliff Dwellings found at Mesa Verde National Park (MVNP) or historic cabins.
“Tree-ring data are vitally important because they provide a temporal scale to studies that need to establish what environmental conditions were like in the past,” says Henri Grissino-Mayer, a dendrochronologist at Valdosta State University in Georgia (University of Tennessee as of 1 July 2000).
“These reference conditions are becoming increasingly important as we try to restore ecosystems disturbed by human activities.” Tree rings can provide information on such things as the structure and composition of past forests and the size and frequency of wildfires and other disturbances before human intervention.
In the American Southwest, the unbroken sequence extends back to 322 B. So, when an archaeologist finds a well-preserved piece of wood—say, a roof beam from an ancient pithouse—dendrochronologists prepare a cross section and then match the annual growth rings of the specimen to those in the already-established chronology to determine the year the tree was cut down. (Article available on the Indiana State University website.) The Laboratory of Tree-Ring Research in Tucson is the world's oldest dendrochronology lab; their website includes information for researchers and the general public.