Establishing Dendrochronology (1929)

Sunday, May 23, 2010


The growth rings of an unknown tree species, at Bristol Zoo, England.
Andrew Ellicott Douglass (1867–1962) had a long and eminent career in astronomy, helping to establish and operate three major astronomical observatories—the Harvard College Observatory at Arequipa, Peru; the Lowell Observatory in Flagstaff, Arizona; and the Steward Observatory at the University of Arizona in Tucson—before he became involved in archaeology. And who would have thought that such an esoteric subject as astronomy could provide the dirt discipline of archaeology, at the other end of the scientific spectrum, with a scientific dating system that would change the writing of prehistory and history itself?

In the 1920s Douglass’s interest in the effect of sunspots on the earth’s weather led him to investigate the annual growth layers of Arizona pine trees to ascertain if there were any variations in tree-ring width. He discovered a relationship between rainfall and tree growth, and between cyclical variations in tree growth and sunspot cycles. Looking for extensive tree-ring records to help to substantiate his theories, Douglass asked archaeologists in Tucson for pieces of wood from the ruins of a Southwestern pueblo. Within a decade Douglass was able to date some of these wooden remains back to AD 100 and others to AD 700.

For the first time in the development of archaeology, here was a scientific way of determining the date of wooden material from sites, and therefore of the sites themselves. In achieving this, Douglass created a chronology that was independent of other chronologies devised from ceramics, stratigraphy, and of course, the written record.

Douglass went on to develop the study of tree rings into the science of dendrochronology or tree-ring dating. This type of dating made substantial contributions to archaeology in the Arctic, Britain, central Europe, and the Mediterranean Basin. Douglass also provided dendroclimatic and dendroenvironmental reconstructions for archaeology. He retired from astronomy to found and direct the Laboratory of Tree-Ring Research at the University of Arizona, which he helped to establish as the preeminent center for dendrochronological research.

Further Reading Douglass, A. E. 1946. Precision of ring dating in tree-ring chronologies. Tucson: University of Arizona. McGraw, D. J. 2001. Andrew Ellicott Douglass and the role of the giant sequoia in the development of dendrochronology. Lewiston, NY: Edwin Mellen Press. Nash, S. E. 1999. Time, trees, and prehistory: Tree-ring dating and the development of North American archaeology, 1914–1950. Salt Lake City: University of Utah Press.

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