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Inside every tree trunk is a record of every year it lived. For nearly a century, the University of Arizona has been the world's leading institution for reading that record. 

Trees grow one ring per year, and the width of each ring reflects the conditions of that year. A wide ring signals a wet, favorable season. A narrow one points to drought or stress. Because those patterns repeat consistently across trees of same species and region, scientists can match ring sequences between samples to build continuous timelines stretching far beyond the lifespan of any single tree. That process, known as dendroecology, was pioneered in Tucson. 

The Laboratory of Tree-Ring Research was founded in 1937 by astronomer Andrew Ellicott Douglass, who recognized that growth patterns in trees could be cross-matched across regions and extended back through time. The method gave archaeologists, climate scientists and historians something no other tool could offer: a precise, year-by-year record of the natural world.

Oldest lab of its kind

The LTRR is the oldest dendrochronology laboratory on earth. It holds the world's largest collection of tree-ring specimens, cores, cross-sections and ancient beams drawn from sites around the world. Researchers tap that collection to date artifacts, reconstruct pass droughts and flood cycles, and track how ecosystems have responded to long-term climate variability. By overlapping specimens from living trees, ancient stumps and archeological timber, the lab has assembled continuous chronologies going back more than 12,000 years. 

That record has had reach beyond tree science. When Willard Libby, a chemist, was developing radiocarbon dating in the 1950s, he used precisely dated wood from the LTRR to calibrate his method – work that contributed to a Nobel Prize.

Still expanding the science

Today, the lab's researchers work across archaeology, paleoclimatology, fire ecology and isotope geochemistry. One team used tree rings to help pin down the date of the ancient Thera volcanic eruption, tying together timelines across the Mediterranean world. Another probed ancient wood for evidence of massive solar storms, detecting ancient solar storms through radiocarbon signals preserved in wood. Active work closer to home is focused on how fire history can directly shape land management.

"One of the things we can study with tree rings is the history of forest fires. We are working on a project to extract information from that fire history to inform prescribed burning and fire management in American forests, specifically by looking at the seasonality of fires and when is the best time to burn," said Valerie Trouet, a Distinguished Scholar and professor at LTTR.

New tools, new territory

New technology is also pushing the science into regions it could not previously reach. Working with colleagues at Ghent University in Belgium, LTRR researchers are applying an X-ray scanning technique that measures ring density in broadleaf trees for the first time. 

"Density is really important because it is the part of the tree ring that is most responsive to changes in temperature," Trouet said. 

The LTRR has helped build dendrochronology programs and institutions worldwide, training researchers and establishing methods that now reach well beyond Tucson. Nearly 90 years on, the lab that invented the discipline remains the standard-bearer for a science now practiced throughout the world.