Trees yield secrets at famous campus lab

By Joseph Altman Jr.

Arizona Daily Wildcat

Next time you go to a home football game, you might be sitting on top of the world's leading dendrochronology lab.

Dendrochronology is the science of telling time by trees, and the scientists and researchers at the Laboratory of Tree-Ring Research, nestled under the west side of Arizona Stadium, are members of the largest group in the world to perform research related to tree rings. Ironically, though, the whole thing started by accident.

In 1902, Victor Douglas, a young astronomer from Harvard University, started working at Lowell Observatory in Flagstaff. He wanted to find out if signs of variations in the sun, such as sunspot cycles, could be found on Earth, said Malcolm Hughes, the current lab director. Through his research, Douglas found that tree stumps consist of concentric circles, the sizes of which correspond to specific periods of time.

While Douglas's work never did succeed in finding a connection between tree rings and the sun, he did stumble across a technique that people had described over a hundred years ago but never put to practical use using tree rings to date a tree or piece of wood.

After that, Douglas served as University of Arizona president from 1910-11, and was "the most significant person on this campus in terms of scholarship up to about 1940 or 1950," says Hughes. By 1937, Douglas convinced the Board of Regents to make the Laboratory of Tree-Ring Research a department on campus with the mission of promoting the teaching and research of dendrochronology.

"Since then, we've diversified the applications of the study of tree rings into many different areas," Hughes said.

Steven Leavitt, associate

study of tree rings into many different areas," Hughes said.

Steven Leavitt, associate professor of dendrochronology, has been using radiocarbon dating as another method of analysis to date the contents of pack rat middens.

Pack rats collect material from their surroundings and store it in their dens, creating piles of trash called middens. Analyzing the contents of the middens, using radiocarbon dating, allows Leavitt to discover what type of plant life was present up to 40,000 years ago.

When carbon dating was developed by Willard Libby, which won him the Nobel Prize for Chemistry in 1960, the method was based on the assumption that the content of radiocarbon in the atmosphere is constant through time.

But through tree-ring research, the UA's lab played an important part in correcting that misconception about radiocarbon dating, which is used to date organic substances.

"Tree rings became a really critical instrument by which we can now determine just how much radiocarbons change in the atmosphere," Leavitt said.

Since the exact age of a tree can be discovered from its rings, that information can be used to correct the age given by radiocarbon dating, Leavitt said.

With tree-ring data available for the past 9,000-10,000 years, tree rings can make radiocarbon dating more accurate than before.

"So, if we found some corn in a hearth of some archaeological site, we could date it and get the radiocarbon age and correct it on a calibration curve (from tree-ring data)," Leavitt said.

Leavitt's research of the rat middens will help scientists better understand climatic conditions and atmospheric composition thousands of years ago based on the types of vegetation that flourished thousands of years ago.

"Tree rings and the isotopic composition of tree rings represent atmospheric carbon dioxide as well as being representative of precipitation which fell at sites and provided water," he said. "It's very successful in the southwest United States in making inferences about weather in the past."

Other research being done in the tree-ring lab includes tracing climate patterns and their relation to forest fires.

Hughes said scars that form on a tree during a fire show when fires occurred and how damaging the fires were. By comparing that data with the size of tree rings, which indicate the amount of precipitation, Associate Professor of Dendrochronology Tom Swetnam has been able to study the "climatology of fire" for the southwestern United States

The U.S. Forest Service has even incorporated Swetnam's research into its management plans, Hughes said.

Tree-ring data from all over the world has also been used to build a picture of how climate has changed to gain a better understanding of the "greenhouse effect."

While actual temperatures have only been recorded for less than 100 years, Hughes said, tree-ring records for the past several hundred years can show longer cycles of temperature changes which show that the "greenhouse effect" is actually a natural part of the world's normal temperature cycle.

"You can't tell if someone has a fever if you don't know what his temperature is when he's healthy," says Hughes.

There is a wide variety of concentrations using tree-ring dating as a part of their research, including anthropologists, climatologists, forest ecologists and hydrologists, Hughes said.

Currently, eight professors of dendrochronology work in the Laboratory of Tree-Ring Research and there are 30 to 50 graduate and undergraduate students working in the lab, Hughes said.

Many international scientists have also been at the UA to learn from the leading center for dendrochronology research in the U.S.

"Anybody who's anybody passes through here," Hughes said. "We're a very major player."

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