With fall coming to a close, there is no better time to talk about tree rings and their use in archaeology.

You probably know that trees have rings (which you can see and count when you look at a stump after a tree has been cut), but did you know that the rings of a tree let you know how old it is? Tree ring dating allows archaeologists to date when a tree was cut. The method was developed in the early 20^th century by A.E. Douglass.  Douglass was an astronomer who worked at archaeological sites in the Southwestern United States.  By the 1960’s, tree ring dating spread to Europe.  Soon, with the rise of computers and statistical methods, scientists, like archaeologists, were able to create long series of tree ring dates that could be used to help figure out how old things are

Dendrochronology, or tree ring dating, examines the rings produced by trees each year.  The thickness of the ring changes each year based on the growing season, changes in the climate in the weather, illnesses, and things like that.  For example, if there is a drought in the area the tree might produce a very narrow ring, but if it is warm and sunny, with just enough rain, the ring might be thicker.  The size of the rings can also depend on the age of the tree, because as a tree gets older it produces narrower rings.

So, how do archaeologists use this information? Dendrochronology has two uses in archaeology: it can be used to calibrate (correct) radiocardon dates, and it can be used to date things all on its own.  Archaeologists look at other trees of the same species in the area because they have the same ring patterns.  Together older trees and younger trees are used to create long, chronological growth sequences that can help us date artifacts and archaeological sites that are hundreds, even thousands of years old. The sequence is created by overlapping the tree rings so a long series of rings can be seen.

When done correctly, dendrochronology can be used by archaeologists to date the cutting of a tree to within a year.

Tree ring dating isn’t without its limits though.  Dendrochronology can only be used effectively in places with distinct seasons because the change in season is what causes distinct tree rings to be produced.  It also can only be used when a master sequence has already been created using the same types of trees people used long ago.  The types of trees that grow or flourish in an area changes over time, which means a dating sequence might not be able to be created.

When scientists and archaeologists use tree rings for dating it is always a good idea to use multiple samples because the wood can sometimes be older or younger than the purpose or structure they were used for.  Older pieces of wood were often reused, while new pieces of wood were often used to mend things.  This means that single pieces of wood may not give you the correct date, even using dendrochronology, so it is very important to look at more than one set of tree rings.

Maybe you’ve heard of carbon dating, and are wondering, “Why do archaeologists use tree-ring dating at all? Couldn’t you just carbon date the tree?”

Yes, you could, but carbon dating (which our final blog post in the series will be about next week) always has an error range of as many as 50-100 years, meaning that we can only have a general idea of how old something is. Tree-ring dating lets us find out the exact year that a tree was cut down! It can be a very accurate method of dating. It is too bad that we do not find wood more often in Ontario!

Want to learn more about dating methods? Be sure to check out the series introduction or follow the links below!

Sources
Henri Grissino-Mayer, “Principles of Dendrochronology.” Department of Geography, University of Tennessee. 2015.
http://web.utk.edu/~grissino/principles.htm
Paul Bahn, and Colin Renfrew. Archaeology Theories, Methods, and Practice. 5^th ed. London: Thames & Hudson. 2008.