This was an Ellen find. In preparation for our trip to Arizona, she read Under Desert Skies, How Tucson Mapped the Way to the Moon and Planets, and it mentioned the Laboratory of Tree-Ring Research based at the University of Arizona in Tucson. A few minutes of internet browsing, and an e-mail later, and we were registered for the once weekly docent led tour of the facility.
To be honest we did not know exactly what to expect, and we more or less thought of it as a rest day between the days that we would be hiking in the Sonoran Desert.
But once we stepped into the lobby of the dedicated research lab building, we knew that we were about to experience something special. The lobby was a mix of history, art, and science, all polished with a sense of professionalism.
For me, the star of the lobby was this cross section specimen of a Sequoia that fell in Sequoia National Park during a storm in 1915. It is 10 feet in diameter and lived from 212 CE to 1915 CE, a span of 1703 years.
A series of historical dates of interest were projected onto the surface at the corresponding tree rings.
We were 12 minutes early and already the docent was familiarizing himself with his charges, ascertaining what brought them to this tour.
Our engaged guide, Larry Allen, was a retired chemical engineer. When I asked what led him to become a docent for this organization, he answered that he wanted to do something different than his career, and to learn something entirely new. But at the same time I think a relatively young science was the true lure.
I had a superficial understanding of tree-ring studies, primarily in regards to assessing past climatic patterns. But Larry immediately helped us understand that tree-ring analysis is used for much more than that. In fact, its first use was to assist archeologists in more precisely dating the age of ruins by comparing growth ring patterns on wooden artifacts with known specimens. Tree-ring study can more precisely date artifacts to specific years, whereas Carbon-14 dating generally dates things to decades.
Andrew Ellicott Douglass was the first to do this. By training he was an astronomer and his initial interest in tree-rings was the relationship between tree-rings’ characteristics and sunspot activity on the sun,. That research did not pan out.
Soon however, while working at the University of Arizona, he recognized the potential to use tree-ring studies for archeological dating. That is the uniqueness of scientists, they will take an apparent failure, reflect on it, ask additional questions, and turn it into a success. By 1929 he had developed two tree-ring chronologies, or timelines. The first was constructed using living tree specimens that extended back to 1260 AD. He also had a second chronology from artifacts collected at five Pueblo Indian sites across the Southwest. Because the Pueblo artifacts’ tree-ring patterns overlapped, these sites were all thought to be contemporary with each other, but could not be conclusively dated . Then in 1929 archeologists working for Douglass uncovered a buried burned roof timber from indigenous peoples at the Show Low site in Arizona, whose tree rings bridged his two chronologies , those of the living specimens and the Southwestern archeological artifacts, giving a specific timeline to the chronology of the Pueblo ruins. This allowed the archeologists to say precisely what years the different Pueblo sites were constructed:
Cliff dwelling at Tsegi Canyon (Arizona) – 1250 AD
Mesa Verde Group (Colorado) – Cliff Palace1073 AD, Oak Tree House 1112 AD, Spring House 1115 AD, Balcony House 1190 AD, and Square Tower House 1204 AD
Canyon de Chelly (Arizona) – 1150 AD
Aztec Ruin (Arizona)- various beams from 1110-1121 AD
Pueblo Bonito (New Mexico)- various sites, 919 AD, 1017 AD, 1033 AD, 1102 AD, and 1130 AD
That research was reported in National Geographic in December 1929, which was displayed in the lobby. I included a link to the original National Geographic article at the end of this post. It is a pdf and takes a while to load.
The laboratory itself was established by Douglass in 1937 and has become the world’s foremost authority on dendrochronology, a term developed by Douglass, – the science of dating events, environmental change, and archeological artifacts by using the characteristic patterns of annual growth rings in timber. The lab currently has over seven hundred and fifty thousand wood specimens in their possession. Think of it as a lending library of old wood where other researchers can obtain specimens to compare to the tree-ring patterns in their archeological relics, or utilize the expertise of the center’s staff to age artifacts.
By overlaying the tree-rings of numerous specimens from a specific geographic region, the laboratory can identify patterns of tree -growth going back up to12,000 years. The specimens are obtained from living trees, stumps from trees that were thousands of years old when harvested, fallen logs in old growth forests, and archeology artifacts. Specimens are obtained from living trees using an increment borer which drills into a tree for a core specimen, from which the tree recovers.
Thinly cut specimens are mounted on wooden stock for handling.
Then, with the use of dissection microscopes and computers, the tree-rings are sequenced and corresponding patterns identified with known standards, allowing dates to be assigned to the artifacts.
Which trees are best for tree-ring analysis – specimens that are growing in stress conditions, such as poor soil or exposure to inclement weather. These work best as they are more susceptible to growth limiting factors like drought, which results in yearly tree-rings of variable width. If a tree grows under ideal conditions, like an area with consistent moisture, rings from each year will be similar in width and therefore not useful to determine a pattern of years of more stress and less growth.
After leaving the lobby we were escorted to a conference room where we we had an introduction to the tools of the dendrochronologist, including the increment borer that is used to obtain specimens. We also had a simplified exercise in trying to match a series of tree rings samples. Finding the pattern is like putting a puzzle together.
Next we were led to the curating suite where we met the Assistant Curator who outlined the process by which specimens are catalogued, as well as loaned out. The room was bordered by shelves, some holding labeled boxes of specimens, and others naked wood waiting to be processed.
Displayed on an antique lab table were were beautiful specimens of Ponderosa and Bristlecone Pine, two species that are particularly well suited for tree-ring study since they are long lived and grow in challenging environments. Bristlecone Pines are the longest living trees on Earth and can live for several thousand years. The title photo is a closeup of the Ponderosa Pine tree-rings.
An interesting interaction occurred when one of our fellow tour attendees, who was a professor of geology at Ohio State University, mentioned that he had collected some specimens for the laboratory in years past at Burnt Mill Canyon in Nevada. And to everyone’s surprise the curator retrieved a red box of his specimens from the shelf behind her.
He was able to see the results of his efforts – quite fun for everyone.
We were then taken upstairs and toured the actual research area of the facility. This was true science as demonstrated by the many poster abstracts of some of their completed research projects. Poster abstracts are one way scientific research is presented at conferences to allow for peer review.
In the labs you could see where they were trying to match up tree ring patterns. Unfortunately I did not ask how they utilize the bar code system noted on the specimens below.
Here we had some exposure to a project that they are working on. The dark wood artifacts are from Byzantine shipwrecks that were found in a now-landfilled harbor in Yenikapi, Turkey in 2004, when excavation was taking place for a subway tunnel. The wrecks range from the fourth century BC to 11th century AD, and the dendrochronologists will be able to determine the exact date of the timber artifacts over that broad span by comparing them to known specimens from that region.
As we toured the facility we found bonus exhibits mounted on the walls throughout the building. I literally could spend all day there.
These two photos are from the study of a tree from Sequoia National Park in California that had lived for over 2000 years. Tree-ring analysis showed evidence of significant fire exposure on 125 occasions over the course of its life.
Another exhibit, of the many we saw, was from a 2970 year old Sequoia, mounted beneath a cross cut saw that had fallen many of its brethren. The wood was collected in 1874 from a stump of a tree that had been cut in 1848.
In summary, our tour of the Laboratory of Tree-Ring Research was 2 hours well spent. To be honest, we did not anticipate a Footpaths article coming out of this visit and therefore only had phone cameras. But once we entered the lobby we knew that we were in for an unique experience. Who would I recommend this tour to – tree lovers, history and archeology buffs, folks who appreciate the scientific process, and woodworkers – for their love of the grain.
Note – we had two dilemmas as we wrote this article. First, is tree-ring truly hyphenated? You see it both ways in the literature, but since it was hyphenated in the title of the lab we carried hyphenated throughout the article. Also, the scientific literature fluctuated between the use of AD (Anno Domini) and CE (Current Era). We went with AD since most people of our generation are more familiar with it.
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Photo credits to Peggy Juengling Burns, Ellen Burns, and Patrick Burns
Overview:
Location – 1215 E Lowell St, Tucson, AZ 85721, in downtown Tucson.
Parking – in a pay garage just down the street from the the Research Center.
Kids – for the tour that we had perhaps 12 and older if they are scientifically inclined. They were hosting a grade school group when we were there but the nature of their tour was clearly different.
Links:
Fascinating article. if we ever get out there, my husband would love this place.
It is a real gem – for science types, woodworkers, tree folks, etc.
Patrick,
This is an outstanding overview of the Tree Ring Laboratory!
Thanks,
Larry Allen
Thanks Larry. We really enjoyed our visit. We were recently hiking at Sequoia, Kings Canyon, and Yosemite National Parks amongst the beautiful Sequoia trees and recalled the lessons we learned at the Tree-ring Laboratory. Several of our readers from across the country have expressed interest in visiting the Lab.