Another post from the What’s Up, Archaeology? blog!  Here’s the full text:

This week I’m teaching my students about the different methods archaeologists use to figure out how old an artifact and/or site could be, like radiocarbon dating to thermoluminescence.  One of my favorite kinds of methods is dendrochronology, also known as ‘tree-ring dating.’ Trees typically produce a distinct thin or thick growth ring per year, depending on the weather.  A wet year will produce a thick ring, while a dry year will produce a thin ring.  Why is that important?  Well, those rings can help date an archaeological site (something specific, like 735 AD!)!  On top of that, tree rings can help us understand past and present environmental processes and conditions (UA Laboratory of Tree-Ring Research), like climate change.

Trees that are sensitive to wet and dry cycles create distinct thick and thin rings, while complacent trees are boring.

In archaeology, dendrochronologists use a sample of wood with distinct tree-rings from an archaeological site (i.e. a wooden structure or a roof beam) to get the age of the tree when it was cut down—that’s the important thing to remember, the final ring is when the tree was cut and then likely used at the site.  It’s all about counting the rings on that cross section of wood.  The pattern of thick and thin rings from the sample is matched up to a much larger set of tree-ring samples or sequences from areas with similar environmental conditions.  By studying samples from “California bristlecone pine, European oaks, and other trees, tree-ring experts have developed a master chronology over 8,000 years into the past” (Fagan and Durrani 2016:115).

So, if I had a sample from a wooden beam from Cliff Palace at Mesa Verde National Park, I would try to match the pattern of rings with established tree-ring sequences in the Southwest.  That would give me the date of when the tree was cut and then used for that structure!

A rundown of how to match a sample from a site to a larger master sequence of tree-rings.

But here’s the hitch (there just had to be one), we have to keep in mind that that trees may have been cut earlier, used at a structure, and then reused at a different building where it was discovered by archaeologists.  Despite this issue (important thing to consider!), dendrochronology remains a fundamental tool in figuring out the age of archaeological sites.  You can see where samples have been taken from a variety of archaeological sites throughout the Southwest, like at Chaco Canyon National Historic Park.

The arrow points to where a tree-ring sample was removed from a wooden beam at Hungo Pavi, Chaco Canyon National Historical Park, NM.

For more information, watch this great video on dendrochronology from Time Team America:

http://www.pbs.org/time-team/experience-archaeology/dendrochronology/

References

Fagan, Brian M. and Nadia Durrani

2016 Archaeology: A Brief Introduction. 12^th Edition. Routledge, New York.

University of Arizona Laboratory of Tree-Ring Research, electronic resource, https://ltrr.arizona.edu/about/treerings