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The study and dating of tree growth rings to understand past climate is called dendrochronology. Every year, a tree will add a growth ring around its trunk. Scientists use these rings to determine the tree's age (by counting backwards from the bark inward), as well as which years were good for growth and which were difficult.
Pictured here is a cross section taken from a Douglas fir in the Santa Catalina Mountains near Tucson, Ariz. This specimen's annual tree rings date from 1600 to 1976, revealing a detailed history of drought (narrow rings) and wetness (wide rings). The darker bands--called "latewood," because they develop late in the year near the end of the growing season--form the boundary between each ring and the next.Image credit: ©Daniel Griffin, Laboratory of Tree-Ring Research
