A section of a tree stump showing the rings.

What Tree Rings Can Reveal

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What’s Up with Weeds?

The Hill is home to one of the world’s leading labs devoted to using tree growth patterns to study ancient history—and much more

By Beth Saulnier

For decades, it has been a curiosity to residents and visitors of York, a coastal tourist town south of Portland, Maine: the skeletal remains of a wooden ship buried under several feet of sand on a local beach. Every once in a while—going back at least as far as 1958—a storm briefly reveals what’s left of the vessel, and maritime history buffs have long speculated about its name and provenance. Now, the ship’s likely identity has been established—thanks, in large part, to a windowless lab in the basement of Goldwin Smith Hall.

Known as the Cornell Tree-Ring Laboratory, the facility is devoted to a field—at the intersection of science and the humanities—that uses the annual rings formed as part of a tree’s natural growth process to date archaeological sites and historic buildings, understand past climate patterns, and much more. In the case of the shipwreck, a maritime researcher sent samples of wood recovered from it—one piece each of oak, beech, and yellow birch—to the lab, where they landed under the microscope of senior research associate Carol Bliss Griggs ’77, PhD ’06.

Sturt Manning (at left in blue shirt) and staff in the dendrochronology lab in Goldwin Smith Hall.
Professor Sturt Manning (at left in blue shirt) and staff examine a tree ring sample in the dendrochronology laboratory in the basement of Goldwin Smith Hall. (Photo by Lindsay France/Cornell University)

The aim: to establish when the trees had been felled, and to see whether that fit with the construction date of the likeliest suspect, a sloop called the Defiance that went down in a 1769 storm carrying a load of flour, pork, and other goods to Portland from Salem, Massachusetts. “I was really doubtful that I could do anything with them, because of the different species and the uncertainty of where the wood came from,” recalls Griggs. “You have to take trees from the same climate region to be able to match them.”

But, she says, she was “astonished” to find that two of the samples (beech and birch) had growth patterns that matched each other almost perfectly—and that she was able to match those to a previously established timeline of growth patterns of trees in the Northeast. She fixed the year that the trees were cut down at around 1753; the Defiance was, in fact, built the very next year. “So it could be,” says Griggs. “I’m not saying this is definitely the Defiance, but it was a ship that was made at the same time.”

In the philosophical sense, you can find quotations going back to ancient Greece which imply that people understood that trees had rings and they mean time passed.

Professor Sturt Manning

One of the leading facilities of its kind in the world, the Cornell Tree-Ring Lab was established in 1976 by Peter Kuniholm, an expert in Mediterranean and Near East archaeology and a pioneer in the field of dendrochronology (the technical term, drawn from the Ancient Greek words for “tree” and “time,” for using growth rings to date historical objects and structures). Since Kuniholm retired with emeritus status in 2006, the lab has been led by Sturt Manning, a renowned dendrochronologist and archaeologist whose extensive fieldwork includes several ongoing projects in Cyprus.

“To write any form of history or investigate any aspect of the human story beyond the very recent period, one of the key things is to have an actual timeline,” says Manning, who’s the Goldwin Smith Professor of Classical Archaeology as well as the lab’s director. “It’s all very well to say that you’d like to study how complex societies developed or how any form of progress occurred, but you need to understand the time scale involved—whether this happened in the period of a few people’s lifetimes or over hundreds or thousands of years.”

A broader mandate

While the lab’s work in the Mediterranean, Aegean, and Near East regions has continued since Manning’s arrival and archaeological studies remain its focus, it has expanded to other parts of the globe including North America. It has also become home to work on related topics, such as dendroclimatology—the use of tree rings to study changes in climate—and dendrochemistry, in which the rings can offer a glimpse into the environmental conditions during a tree’s lifetime, such as the presence of pollution or volcanic eruptions.

“It has this great interdisciplinary nature to it,” says Brita Lorentzen ’06, PhD ’14, a longtime researcher in the lab who majored in archaeology and Jewish studies as an undergrad and went on to earn a doctorate in geological sciences on the Hill. “I was always interested in combining the sciences and humanities, so it was a great way to manage those two interests.”

As Manning observes, dendrochronology—“dendro” for short—is both ancient and modern, simple and complex. “In the philosophical sense, you can find quotations going back to ancient Greece which imply that people understood that trees had rings and they mean time passed; Leonardo da Vinci comments on the fact that the rings change with the seasons, and you get differences if it’s very dry or wet,” says Manning, who was born in Australia and holds a PhD in classics from the University of Cambridge in the U.K. “But in practical terms, it’s a very new science.”

Roots in Arizona

The field traces its origin to the American Southwest around the turn of the previous century, when an astronomer named Andrew Ellicott Douglass was overseeing construction of a new observatory for the University of Arizona, Tucson—and noticed matching growth patterns in the local timber that he at first thought might be helpful in understanding how the Earth is affected by sunspots. Studying both living trees and prehistoric ruins in the region, he eventually was able to establish a continuous timeline of ring patterns stretching back to around 700 A.D.

Tree core samples in plastic tubes.
Tree core samples in the dendro lab. (Photo provided)

“He was, in a sense, lucky—and it also explains why the field took a long time to happen—that where he was in Arizona, you have some long-lived trees and it’s also extremely dry, so you have a very clear climate growth signal,” says Manning. “A lot of the time the trees are struggling with drought and every now and again you have some moisture, so you have dramatic peaks and troughs. Whereas in a lot of parts of the world, it’s much more even.”

For example, he says: consider Cornell’s Arts Quad, whose trees enjoy good soil and plenty of sunshine and moisture. “The rings are pretty much the same every year, therefore they’re not perfect candidates for dendrochronology,” says Manning. “It’s not that it can’t be done, but you don’t see clear patterns; to do it requires statistics and a lot of data, whereas Douglass was initially able to do this by eye, and then he came up with ways of doing it in a more quantitative and robust fashion.”

For decades, Manning says, it was assumed that dendro was unique to the Southwest; applying it elsewhere wasn’t considered practical. Except for one researcher in Germany in the 1940s—whose work on Iron Age sites, accomplished though it may have been, was overshadowed by the fact that it was done in the service of Nazi-era nationalism—dendro didn’t see wide application outside the Southwest until after the mid-1900s. “In terms of being all around the world,” he says, “it’s really a field that’s only half a century old.”

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‘Really low-tech’

While Manning admits that dendrochronology is “inherently a slightly destructive activity,” it’s practiced in a way that minimizes its impact on its study subjects. For example, taking a sample from a live tree entails extracting a slender, tube-shaped length of wood that’s only about five millimeters in diameter, which doesn’t harm it.

Two women taking a core sample from a tree.
PhD students Kathleen Garland (left) and Annapaola Passerini sampling a pine tree in southern Cyprus. (Photo provided)

“Contrary to what people might think, dendrochronology is really low-tech,” notes Annapaola Passerini, MA ’20, an Italian-born doctoral student in archaeological anthropology, who says that the lab was a major factor in her choice to study at Cornell. “It’s really sustainable in that sense. The core samples are tiny; we want to damage the tree as little as possible. We’re not here to destroy these important resources.”

In the case of historic buildings, a centimeter-wide sample can be taken from a beam in the cellar or attic—ideally, one with the bark still attached, to confirm the year it was cut down—or a small piece can be sliced off the end.

“Obviously you aren’t going to cut something where it’s structurally important,” says Manning. “If you’ve got an enormous, great beam and you drill a little hole, it’s the equivalent of a hypodermic needle in your arm. We can also fill in the hole, so afterward you basically can’t even see where we’ve been.”

In situations where physical samples can’t be taken, such as works of art or musical instruments—for instance, Griggs has helped establish the age of violins and basses—the researchers use digital scanning. Scans are also employed when doing field work in countries, like Turkey and Egypt, that have imposed strict rules against the export of cultural heritage objects.

“Whereas thirty years ago you could drive around a lot of this part of the world in a van and collect things and bring them back, you can’t do that anymore—you’d get arrested,” says Manning. “Now we have to be more selective in where we work, and to build formal collaborations with partners there.” For several recent projects, they’ve made arrangements to polish, prepare, and scan samples on site. “The only thing that’s being exported,” he says, “is the digital file that we then work on back at Cornell.”

On a global scale

Lorentzen and Manning have done extensive work on Byzantine-era churches in Cyprus, exploring the age and provenance not only of the materials used to build them, but also the large collections of religious art—many of which are icons painted on wood panels—that they contain. As they explain, while the churches are UNESCO heritage sites and have been much studied, some questions remain.

A woman on a ladder taking a core sample from a historic church in Cyprus.
Brita Lorentzen taking a core sample from a historic church in Cyprus. (Photo provided)

Thus far, the Cornell dendro team has built what’s known as a “chronology”—a continuous timeline of annual growth patterns—of more than 500 years going back from the present day, a feat accomplished using both living trees and historic lumber; by identifying growth patterns that overlap from one set of specimens to another, they can piece together an unbroken timeline.

They’ve also built a separate, older chronology of more than 250 years from the wood in the Byzantine churches, which they’ve dated using a method called radiocarbon wiggle matching; while less precise than dendro, it can pinpoint the wood’s age to within about half a decade.

One of the team’s most striking discoveries in Cyprus has involved Paphos Gate, one of three entrances in the defensive walls built around the capital city of Nicosia after the island became part of the Republic of Venice in 1489. “The Walls of Nicosia are one of the most famous Venetian architectural structures in the world; they stood against the Turks, who shot their way through when they captured Cyprus in the sixteenth century,” says Manning.

“Almost all of the walls have survived, but everyone assumed that the Paphos Gate was pretty recent—that it was either fixed up by the British when they took over Cyprus or restored in the 1930s. We’ve done dendrochronology on it, and the ring sequence matches perfectly against our Byzantine church record. We’ve done radiocarbon dating as well; this wood is fifteenth century. It’s been hanging there for probably 550, if not 600 years. So it’s a major national historical monument.”

Branching out into North America

Closer to home, in recent years the lab has done groundbreaking work in North America, including studies of Iroquois sites in southern Ontario and Central New York. Historically, Manning says, such sites have been roughly dated through the presence or absence of certain European trade goods, like glass beads. But if sites have artifacts like charcoal or wooden posts, researchers can use dendro and radiocarbon dating to get a much more accurate measure.

“We went to the largest Iroquois site ever excavated in North America, just northeast of Toronto, and dated it really carefully, and the numbers are fifty to seventy-five years different from the so-called conventional wisdom,” he says. “That led us to look at a number of sites across the region, and in quite a few cases we found significant differences. It means that for the past hundred years, we’ve been sort of writing the wrong history of indigenous cultures, because we haven’t had the timeline right.”

Griggs—the veteran lab staffer who worked on the Defiance shipwreck—has conducted several projects in Upstate New York, including studying dozens of preserved logs that had been excavated along with mastodon bones at various sites dating from about 10,000 to 14,000 years ago, at the end of the region’s last glacial age. In addition to doing radiocarbon dating on the trees, she has studied differences in how much they grew from year to year—whether the annual rings are larger or smaller—which can illustrate the environmental and climate conditions in which they lived.

“The trees are about a foot in diameter, and that’s not normal for a really cold climate, so the climate was getting generally better,” she observes. “And there are periods of about 200 years where there was a lot of warming and then a little bit of cooling over 100 years. So it’s an indicator of how quickly the climate changed.”

Top: A cross-section of a tree trunk reveals the rings. (Photo provided)

Published October 5, 2021

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