The Truth in the Tree Rings | American Archaeology

The University of Arizona’s Laboratory of Tree Ring Research was founded in 1937 by astronomer A.E. Douglass and is the premiere laboratory devoted to dendrochronology in the U.S. 

By Jasmine Demers

This is an article excerpt from the Summer 2020 edition of American Archaeology Magazine. Become a member of The Archaeological Conservancy for your complimentary subscription. 

The noted businessman, mathematician, and astronomer Percival Lawrence Lowell was obsessed with Mars. To study the distant planet, he founded an eponymous observatory in Flagstaff, in what was then the territory of Arizona, in 1894. Lowell hired another astronomer, A. E. Douglass, to design the telescope and observatory. Douglass, however, was preoccupied with solar variability rather than Mars, and he began studying trees to see if there was a correlation between tree-ring widths and sunspot cycles, which he believed affected the Earth’s climate. And so it was, curiously enough, that an astronomer founded the University of Arizona’s Laboratory of Tree Ring Research (LTRR) in 1937.

Archaeologist Tom Windes (left) and two colleagues record a wooden digging stick cache in Chaco Canyon in 1972. | Credit: Roger Huckins

Archaeologists Winston Hurst and Donna Glowacki map the interior of a small kiva in a cliff dwelling in southeastern Utah for the Cedar Mesa Building Murals Project, which was led by Ben Bellorado of the University of Arizona’s LTRR. The archaeologists collected thirty-five tree-ring samples from the cribbed roof of the kiva and determined that the structure was built just after A.D. 1215, and then remodeled just after A.D. 1229. | Credit: Ben Bellorado

Friable samples, which are typically charcoal, are wrapped in pure cotton batting for protection before being archived at the LTRR. The vast majority of the LTRR’s archaeological samples are charcoal rather than preserved wood. | Credit: Josh Bradford

Archaeologist Aaron O'Brien takes a core sample of a wooden beam from the interior wall of a cliff dwelling in southeastern Utah for the Cedar Mesa Building Murals Project. Analysis of the samples from this wall revealed that it was built just before A.D. 1250, which led to the dating of a mural that embellishes the exterior wall to between A.D. 1250 and 1275. | Credit: Ben Bellorado

Dendroarchaeologists work in the LTRR in 1974. The LTRR moved to the new Bryant Bannister Tree-Ring Building in 2012. | Credit: LTRR

The LTRR is the world’s premier laboratory devoted to dendrochronology—the study of tree ring dating. By the time the LTRR opened, Douglass had already been analyzing tree rings for over thirty years, fathering the science of dendrochronology in the process. Dendrochronology operates on the principle that in temperate climates like the Southwestern United States, trees grow one ring every year. In the best of circumstances, researchers can count each of these rings to find out how old a tree was when it was cut down.

 But dendrochronology, particularly as it is applied to archaeological samples, is considerably more complicated than simply counting rings. “The essence of tree-ring dating is matching the sequence of narrow and wide rings from one tree with those of other trees to establish a pattern of ring-width variabilityommon to all trees in the group,” said Jeff Dean, professor emeritus at the LTRR. “The matching process is called crossdating  and is the foundation of dendrochronology.”

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