Dr. Ron Towner from the Laboratory of Tree-Ring Research at the University of Arizona explains the principles behind dendrochronology and why this dating method is valuable to archaeologists. Ron demonstrates how to accurately count tree-rings, and discusses the importance of patterns and master chronologies.
Dendrochronology: How Tree-Ring Dating Reveals Human Roots
Trees are often used to make analogies about the past. Family trees, the tree of life, getting back to your roots…. But beyond the powerful imagery that trees give us to represent our history, what can trees actually tell us about the past?
Dendrochronology is the scientific method of tree-ring dating. Americans first developed it in the early 20th century and now "dendro" is a common method of chronology that is used by scientists all over the world. Dendrochronology has become a fundamental tool in science, for reinforcing and expanding on the timelines of historical and ecological events in the past.
Dendrochronology operates on the principle that in temperate climates, like the southwestern United States, trees grow one ring every year. In the springtime when moisture surges, the cells of a tree expand quickly. Over the course of the summer as the ground becomes more dry, the cells begin to shrink. This change in cell size is visible in tree-rings, or growth-rings. The variation in ring width is based on the amount of water a tree absorbed in a given year. Rainier years are marked by wider rings, drier years by narrow ones.
So, dendrochronology in its simplest form is a matter of counting rings. One ring = one year. Fifty-five rings on a stump = a fifty-five year old tree. But, it's not always that simple. In fact, only about 40% of tree samples are successfully dated by dendrochronologists, says Ron Towner, Associate Professor of Dendroarchaeology at the Laboratory of Tree-Ring Research. Natural tree variation, sudden climate changes... or if a tree is planted near a creek or a river, for example, it may get so much water (and water is what makes those little tree cells expand) that the rings no longer equate to each year elapsed.
But for the 40% that are datable, counting the rings on a sample tells dendrochronologists how old the tree was when it was cut down. However, counting alone does not tell dendrochronologists what time period the tree is from. To find that out, scientists must focus on the pattern of rings rather than number of them.
Say you walk into an old forest and you find the stump of a thousand year old tree, explains Towner. Say you also find a piece of wood from a different tree in that same forest, and it has 500 rings on it. If you were to lay a cross section of that wood on top of the older stump, you would find that somewhere inside the rings of the older tree, 500 of them would match the pattern of the piece from the younger tree.
In other words, tree rings develop in the same pattern (e.g., wide ring, wide ring, narrow ring, wide, narrow, etc.) in all trees across the same climate or region. If it rains a lot in that old forest mentioned earlier, then all of the trees get lots of water and all of them grow a wider ring that year. Summer drought; all of the affected trees show a narrower ring. But, tree ring patterns never repeat themselves either, which is what makes them identifiable in time and place.
Dendrochronologists identify these patterns by laying a strip of graph paper across a sample, and marking only the narrow rings. This is called skeleton plotting. So rather than measuring or marking every single growth-ring, this technique highlights only the seasons of drought (those little narrow rings).
This method works because of the human brain's aptitude for recognizing patterns. Humans are actually, "much better at that than computers are," Towner says.
Once all of the narrow rings have been plotted, it's time to bring in the master chronology. Remember how trees share growth-ring patterns? Well, dendrochronologists sample hundreds and thousands of trees (don't worry! they use drills rather than cutting down all those pretty trees) from the same region or climate to create one great master pattern, called a master chronology.
This is a graphical representation, drawn or printed on graph paper, of the average tree ring pattern in a given area over time. So, dendrochronologists simply slide their new skeleton plot along side the master chronology until all of those little lines marking the narrow rings match up... and voilá! The new sample is dated.
Currently there are chronologies dating as far back as 11,000. In the southwest, like at the Dillard site where Time Team America excavated, the master chronology goes back to 322 B.C. But if most trees only live for a hundred years or so, how can a master chronology go so far back in time?
By taking a younger wood sample with a known date, and then matching the rings inside the pattern of an older sample, you can count backwards on the tree rings to determine how much older it is... essentially. Once that age is confirmed, the longer pattern can now be used to date an even older sample, and so on and so forth.
So, now we know how trees are dated, but what does that tell us exactly? "Virtually all societies used wood in some fashion, and so, that provides us with materials that we can tree-ring date, and that will give us precise years for when sites were occupied," says Towner.
Dendrochronology can also reveal the origin of the wood on a site, and by knowing when and where human activity occurred, archaeologists have a much better context for trying to understand the past.
Just like the rings in a tree grow every year, scientists' knowledge of the past grows with every addition to their master chronologies. Eventually, trees may tell a history even more ancient than our own. But for now, this timeline gives shape to an extensive portion of our shared human past, without which we'd be like trees without their roots.