Dendrochronology is the extraction of chronological and environmental information from the annual growth rings of trees. This technique uses well established tree ring sequences to date events. Reconstruction of environmental occurrences, droughts for example, which took place when the trees were growing, is also possible based on traits such as changes in tree ring thickness. Tree-ring dating allows dates to be assigned to archeological artifacts; reconstructed environmental events shed light on the ways that human societies have changed in response to environmental conditions.
Dendrochronology was developed in the early 1900s by the American astronomer Andrew Ellicott Douglas as part of his research on the effects of sunspots on Earth's climate. Douglas developed a continuous 450-year record of tree ring variability, which he succeeded in correlating with the winter rainfalls preceding the growth years.
The technique very quickly proved useful for dating wood and charcoal remains found in the American southwest. By 1929, dendrochronology had become the first independent dating technique to be used in archeology. Since then, approximately 50,000 tree ring dates from about 5,000 sites have yielded the finest prehistoric dating controls anywhere in the world. Tree ring dating later proved successful in other parts of North America, including Alaska and the Great Plains. Today, the technique is practiced in one form or another throughout the world.
The key to successful dendrochronolgical dating is cross-dating—comparing one tree's rings with other trees in the area. This may be done by looking for covariations in tree ring width, or comparing other tree ring attributes to identify overlapping sequences.
By incorporating overlapping sequences from multiple trees, it has been possible to produce chronologies that go back further than any of the individual tree ring specimens. In this way, it has been possible to extend the chronology for the southwest as far back as 322 B.C. The longest individual tree ring chronologies developed to date have been for an 8,700-year California bristlecone pine sequence, and a 10,000-year sequence in Europe.
Besides chronological information, archeological tree ring dating yields information about the way wood was used in an ancient culture, and about past climates.
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