{"id":470,"date":"2021-08-24T19:06:30","date_gmt":"2021-08-24T19:06:30","guid":{"rendered":"https:\/\/alumni.cornell.edu\/cornellians\/?p=470"},"modified":"2022-07-01T11:55:06","modified_gmt":"2022-07-01T15:55:06","slug":"what-tree-rings-can-reveal","status":"publish","type":"post","link":"https:\/\/alumni.cornell.edu\/cornellians\/what-tree-rings-can-reveal\/","title":{"rendered":"What Tree Rings Can Reveal"},"content":{"rendered":"\n

The Hill is home to one of the world\u2019s leading labs devoted to using tree growth patterns to study ancient history\u2014and much more<\/p>\n\n\n\n

By Beth Saulnier<\/strong><\/p>\n\n\n\n

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\u2014going back at least as far as 1958\u2014a storm briefly reveals what\u2019s left of the vessel, and maritime history buffs have long speculated about its name and provenance. Now, the ship\u2019s likely identity has been established\u2014thanks, in large part, to a windowless lab in the basement of Goldwin Smith Hall.<\/p>\n\n\n\n

Known as the Cornell Tree-Ring Laboratory<\/a>, the facility is devoted to a field\u2014at the intersection of science and the humanities\u2014that uses the annual rings formed as part of a tree\u2019s 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\u2014one piece each of oak, beech, and yellow birch\u2014to the lab, where they landed under the microscope of senior research associate Carol Bliss Griggs \u201977, PhD \u201906. <\/p>\n\n\n\n

\"Sturt
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)<\/em><\/figcaption><\/figure>\n\n\n\n

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<\/em> that went down in a 1769 storm carrying a load of flour, pork, and other goods to Portland from Salem, Massachusetts. \u201cI was really doubtful that I could do anything with them, because of the different species and the uncertainty of where the wood came from,\u201d recalls Griggs. \u201cYou have to take trees from the same climate region to be able to match them.\u201d<\/p>\n\n\n\n

But, she says, she was \u201castonished\u201d to find that two of the samples (beech and birch) had growth patterns that matched each other almost perfectly\u2014and 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<\/em> was, in fact, built the very next year. \u201cSo it could be,\u201d says Griggs. \u201cI\u2019m not saying this is definitely the Defiance<\/em>, but it was a ship that was made at the same time.\u201d<\/p>\n\n\n\n

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.<\/p>Professor Sturt Manning<\/strong><\/cite><\/blockquote>\n\n\n\n

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 \u201ctree\u201d and \u201ctime,\u201d 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>, a renowned dendrochronologist and archaeologist whose extensive fieldwork includes several ongoing projects in Cyprus. <\/p>\n\n\n\n

\u201cTo 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,\u201d says Manning, who\u2019s the Goldwin Smith Professor of Classical Archaeology as well as the lab\u2019s director. \u201cIt\u2019s all very well to say that you\u2019d like to study how complex societies developed or how any form of progress occurred, but you need to understand the time scale involved\u2014whether this happened in the period of a few people\u2019s lifetimes or over hundreds or thousands of years.\u201d<\/p>\n\n\n\n

A broader mandate<\/strong><\/h2>\n\n\n\n

While the lab\u2019s work in the Mediterranean, Aegean, and Near East regions has continued since Manning\u2019s 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\u2014the use of tree rings to study changes in climate\u2014and dendrochemistry, in which the rings can offer a glimpse into the environmental conditions during a tree\u2019s lifetime, such as the presence of pollution or volcanic eruptions.<\/p>\n\n\n\n

\u201cIt has this great interdisciplinary nature to it,\u201d says Brita Lorentzen \u201906, PhD \u201914, 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. \u201cI was always interested in combining the sciences and humanities, so it was a great way to manage those two interests.\u201d<\/p>\n\n\n\n

As Manning observes, dendrochronology\u2014\u201cdendro\u201d for short\u2014is both ancient and modern, simple and complex. \u201cIn 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\u2019s very dry or wet,\u201d says Manning, who was born in Australia and holds a PhD in classics from the University of Cambridge in the U.K. \u201cBut in practical terms, it\u2019s a very new science.\u201d<\/p>\n\n\n\n

Roots in Arizona<\/h2>\n\n\n\n

The field traces its origin to the American Southwest around the turn of the previous century, when an astronomer named Andrew Ellicott Douglass<\/a> was overseeing construction of a new observatory for the University of Arizona, Tucson\u2014and 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. <\/p>\n\n\n\n

\"Tree
Tree core samples in the dendro lab. (Photo provided)<\/em><\/figcaption><\/figure>
\n

\u201cHe was, in a sense, lucky\u2014and it also explains why the field took a long time to happen\u2014that where he was in Arizona, you have some long-lived trees and it\u2019s also extremely dry, so you have a very clear climate growth signal,\u201d says Manning. \u201cA 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\u2019s much more even.\u201d<\/p>\n<\/div><\/div>\n\n\n\n

For example, he says: consider Cornell\u2019s Arts Quad, whose trees enjoy good soil and plenty of sunshine and moisture. \u201cThe rings are pretty much the same every year, therefore they\u2019re not perfect candidates for dendrochronology,\u201d says Manning. \u201cIt\u2019s not that it can\u2019t be done, but you don\u2019t 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.\u201d <\/p>\n\n\n\n

For decades, Manning says, it was assumed that dendro was unique to the Southwest; applying it elsewhere wasn\u2019t considered practical. Except for one researcher in Germany in the 1940s\u2014whose 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\u2014dendro didn\u2019t see wide application outside the Southwest until after the mid-1900s. \u201cIn terms of being all around the world,\u201d he says, \u201cit\u2019s really a field that\u2019s only half a century old.\u201d<\/p>\n\n\n\n

\u2018Really low-tech\u2019<\/strong><\/h2>\n\n\n\n

While Manning admits that dendrochronology is \u201cinherently a slightly destructive activity,\u201d it\u2019s 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\u2019s only about five millimeters in diameter, which doesn\u2019t harm it. <\/p>\n\n\n\n

\"Two
PhD students Kathleen Garland (left) and Annapaola Passerini sampling a pine tree in southern Cyprus. (Photo provided) <\/em><\/figcaption><\/figure>
\n

\u201cContrary to what people might think, dendrochronology is really low-tech,\u201d notes Annapaola Passerini, MA \u201920, an Italian-born doctoral student in archaeological anthropology, who says that the lab was a major factor in her choice to study at Cornell. \u201cIt\u2019s really sustainable in that sense. The core samples are tiny; we want to damage the tree as little as possible. We\u2019re not here to destroy these important resources.\u201d<\/p>\n\n\n\n

In the case of historic buildings, a centimeter-wide sample can be taken from a beam in the cellar or attic\u2014ideally, one with the bark still attached, to confirm the year it was cut down\u2014or a small piece can be sliced off the end. <\/p>\n<\/div><\/div>\n\n\n\n

\u201cObviously you aren\u2019t going to cut something where it\u2019s structurally important,\u201d says Manning. \u201cIf you\u2019ve got an enormous, great beam and you drill a little hole, it\u2019s the equivalent of a hypodermic needle in your arm. We can also fill in the hole, so afterward you basically can\u2019t even see where we\u2019ve been.\u201d<\/p>\n\n\n\n

In situations where physical samples can\u2019t be taken, such as works of art or musical instruments\u2014for instance, Griggs has helped establish the age of violins and basses\u2014the 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.<\/p>\n\n\n\n

\u201cWhereas 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\u2019t do that anymore\u2014you\u2019d get arrested,\u201d says Manning. \u201cNow we have to be more selective in where we work, and to build formal collaborations with partners there.\u201d For several recent projects, they\u2019ve made arrangements to polish, prepare, and scan samples on site. \u201cThe only thing that\u2019s being exported,\u201d he says, \u201cis the digital file that we then work on back at Cornell.\u201d<\/p>\n\n\n\n

On a global scale<\/strong><\/h2>\n\n\n\n

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\u2014many of which are icons painted on wood panels\u2014that they contain. As they explain, while the churches are UNESCO heritage sites and have been much studied, some questions remain. <\/p>\n\n\n\n

\"A
Brita Lorentzen taking a core sample from a historic church in Cyprus. (Photo provided)<\/em><\/figcaption><\/figure>
\n

Thus far, the Cornell dendro team has built what\u2019s known as a \u201cchronology\u201d\u2014a continuous timeline of annual growth patterns\u2014of 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. <\/p>\n\n\n\n

They\u2019ve also built a separate, older chronology of more than 250 years from the wood in the Byzantine churches, which they\u2019ve dated using a method called radiocarbon wiggle matching; while less precise than dendro, it can pinpoint the wood\u2019s age to within about half a decade.<\/p>\n<\/div><\/div>\n\n\n\n

One of the team\u2019s 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. \u201cThe 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,\u201d says Manning. <\/p>\n\n\n\n

\u201cAlmost all of the walls have survived, but everyone assumed that the Paphos Gate was pretty recent\u2014that it was either fixed up by the British when they took over Cyprus or restored in the 1930s. We\u2019ve done dendrochronology on it, and the ring sequence matches perfectly against our Byzantine church record. We\u2019ve done radiocarbon dating as well; this wood is fifteenth century. It\u2019s been hanging there for probably 550, if not 600 years. So it\u2019s a major national historical monument.\u201d<\/p>\n\n\n\n

Branching out into North America<\/h2>\n\n\n\n

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. <\/p>\n\n\n\n

\u201cWe 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,\u201d he says. \u201cThat 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\u2019ve been sort of writing the wrong history of indigenous cultures, because we haven\u2019t had the timeline right.\u201d<\/p>\n\n\n\n

Griggs\u2014the veteran lab staffer who worked on the Defiance<\/em> shipwreck\u2014has 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\u2019s 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\u2014whether the annual rings are larger or smaller\u2014which can illustrate the environmental and climate conditions in which they lived. <\/p>\n\n\n\n

\u201cThe trees are about a foot in diameter, and that\u2019s not normal for a really cold climate, so the climate was getting generally better,\u201d she observes. \u201cAnd 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\u2019s an indicator of how quickly the climate changed.\u201d<\/p>\n\n\n\n

Top: A cross-section of a tree trunk reveals the rings. (Photo provided)<\/em><\/p>\n\n\n\n

Published October 5, 2021<\/p>\n\n\n\n

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