How Trees Tell Time: Dendrochronology
Did you know that trees are some of the best ‘clocks’ on the planet? You may have heard that you can determine a tree’s age by counting its rings, but if you count the rings on lots of trees you can date archaeological sites going back tens of thousands of years!
Previously on StoneAgeMan, we covered relative and radiocarbon dating methods. While those are the most common dating techniques, there’s another method that can date archaeological remains to an exact calendar year, and make radiocarbon results more accurate. It’s called dendrochronology, or tree-ring dating.
Basic Premises of Dendrochronology
Trees produce rings each year that they grow. Trees grow more during wet years, producing wide rings, and less during dry years, leaving narrow rings. Since no two years have the same precipitation levels, this generates unique patterns of wide-to-narrow rings. However, because trees from the same region receive similar amounts of moisture, they develop ring patterns that are close enough to be synced up.
Dendrochronology as an Absolute Dating Method
It was an astronomer named Andrew Ellicott (A. E.) Douglass who first used tree rings to date archaeological sites. He began studying tree rings in the Southwestern United States in 1901 to see if they’d reflect sun spot activity, and he soon realized their usefulness for archaeology.
Douglass, an anthropologist named Clark Wissler, and several other researchers worked with the indigenous peoples of the Southwest to collect samples from as many trees as they could. As they obtained these samples, Douglass and his colleagues were able to arrange them in chronological order using a method called crossdating.
Crossdating starts with a tree of known age, so that dendrochronologists can match each of its rings to a specific year. Dendrochronologists then produce a “skeleton plot” by lining up a piece of graph paper with a cross-section of the tree, or a narrow core that they’ve drilled out of it, and recording all of the narrow rings.
Dendrochronologists can then use this skeleton plot to date an older tree whose lifespan overlapped with the younger one.
With the help of the skeleton plot, dendrochronologists find which pattern of narrow rings matches that of the younger, dated tree: allowing them to identify the years in which both trees were alive. By counting backwards from the last overlapping year, dendrochronologists can then determine the age of the older tree. They can also construct a skeleton plot of the tree, and then date even older specimens.
An example of a skeleton plot. Image from Page 280 of “Brigham Young University Science Bulletin” (1955) from Internet Archive Book Images. No known copyright restrictions.
Douglass and his teammates employed crossdating to establish a “master sequence” of tree-ring patterns in the U.S. Southwest going back to 700 AD, which dendrochronologists have now extended to 6700 BC.
What this means for archaeologists is that if they find a piece of timber in a Southwestern archaeological site, and if they can match its tree-ring pattern with the master sequence, then they can attach a precise calendar year to when that site was active (provided the tree wasn’t long dead before people used it). The same goes for any other region in which dendrochronologists have developed master sequences.
The main problem with dendrochronology – at least as an absolute dating method – is that it’s geographically limited.
Dendrochronology is only useful in climates that are arid enough to produce distinct ring patterns. In moist regions, like the Eastern U.S., ring widths don’t vary much.
Another problem is that not all tree species generate ring patterns that are both clear and consistent enough to be used for dating. Certain varieties of trees, such as willows, can have erratic ring patterns that confuse researchers. By contrast, oaks are among the most reliable species for dating.
Not all tree species produce clear, distinctive ring patterns that are suitable for dating. Tree Rings by Bill Kasman. Public Domain.
Since tree rings are sensitive to both climate and species, master sequences have only been produced for a few regions and types of trees. These include an Irish oak chronology that extends to 5300 BC, an oak chronology in Germany that stretches to 8500 BC, and a California bristlecone pine chronology in the Southwestern U.S. that dates to 6700 BC.
Despite the above limitations, dendrochronology is critical worldwide for its ability to calibrate radiocarbon results.
Dendrochronology for Radiocarbon Calibration
Recall that radiocarbon dating works by measuring the amount of carbon-14 (14C) left in an organic sample, and then checking this against the background 14C level in the earth’s atmosphere.
Unfortunately, the 14C content in the earth’s atmosphere has fluctuated over time, and scientists need to be able to account for these changes to calibrate radiocarbon results. The data for such calibration comes from tree rings.
Tree rings preserve 14C well. When working with a tree that has been dated, scientists also know the year that corresponds with each ring. This means that if they measure the amount of 14C left in a dated tree ring, they can use the known decay rate of 14C to figure out how much of it was in the earth’s atmosphere during that year.
An example of a radiocarbon calibration curve. The blue line shows the “raw” radiocarbon results, and the red shows calibrated results that account for carbon-14 fluctuations over time. Image found on Wikimedia. Public Domain.
Tree rings are powerful calibration tools up to 12,600 years ago, beyond which dendrochronologists haven’t established master sequences. Scientists need to use other dating methods to check radiocarbon results that exceed 12,600 years old, which we’ll discuss in the future.
- In temperate climates that don’t get too much rain, tree rings can date archaeological remains to exact calendar years.
- Dendrochronologists use crossdating to organize tree rings into master sequences that can cover thousands of years.
- Even in regions where dendrochronology doesn’t work as an absolute dating method, tree rings are the main tools for calibrating radiocarbon results up to 12,600 years old.
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