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One of the most essential aspects of archaeology is dating objects from the past, and one of the most critical tools in dating historic objects is dendrochronology. Dendrochronology, also called tree ring dating, is a scientific method used to determine the age of wood and to reconstruct past environmental conditions by analyzing growth rings in trees. However, it isn't just a matter of counting tree rings, there's a science to it that has allowed us to understand a great deal about our past. Learn more about dendrochronology, how it works, and how it's used on this episode of Everything Everywhere Daily. This episode is sponsored by Fiji Water. You've probably heard of Fiji Water and have seen it in stores. Well, Fiji Water really is from the islands of Fiji. Drop by drop, Fiji Water is filtered through volcanic rock 1,600 miles away from the nearest continent and all its pollution protected and preserved naturally from external elements. 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There are multiple ways to date things, and depending on what's being dated, researchers will use a different technique. In one of the very early episodes of this podcast, I covered the topic of radiometric dating. Radiometric dating involves using the known decay rate of radioactive isotopes to estimate the ages of things like rocks. Uranium, potassium, and other elements can be used for such dating. Carbon 14 is another type of dating that doesn't work with rocks but can be used for organic matter going back about 60,000 years. But one of the best methods for dating more recent objects, things which go back hundreds or a few thousand years, is using tree rings, or dendrochronology. Long before dendrochronology became a formal science, people noticed that trees contained rings that seemed to correspond to years of growth. The earliest written reference we have comes from the ancient Greek philosopher Theophrastus, a student of Aristotle, who wrote around 300 B.C. about the annual nature of tree ring growth. He observed that trees added layers every year. Although he didn't fully grasp the potential of using these rings to understand past conditions, ancient civilizations likely understood this connection intuitively. Indigenous people across North America, for instance, used tree age estimates for practical purposes long before European contact. They could judge the age of trees for construction purposes and understood that larger, older trees had lived through more seasons. However, these early observations remained practical rather than scientific, lacking the systematic approach that would later define dendrochronology. During the Renaissance, Leonardo da Vinci made remarkably prescent observations about tree rings in his notebooks around the year 1500. He noticed that rings were thicker in wet years and thinner in dry years, essentially identifying the fundamental principle that would later make dendrochronology possible. The French scientist Henri Louis Duhamel Dumontce conducted some of the first controlled experiments on tree growth in the 1740s. He wounded trees and observed how they healed, proving that trees added new layers of wood each year from the outside. His experiments provided the first rigorous proof that each ring indeed represented one year of growth, a fact that had been assumed but never scientifically demonstrated. During this period, European foresters began developing more systematic approaches to understanding tree age. German forestry schools, which were among the most advanced in the world, started teaching students to count rings to determine timber age for practical forest management. This represented the first institutionalized use of tree ring counting, although it still remained focused on practical rather than historical applications. The 19th century brought increasingly sophisticated observations that set the stage for modern dendrochronology. Charles Babbage, better known for his work on mechanical computers, made important observations about tree rings and climate in the 1830s. He suggested that tree rings might preserve records of past climatic conditions. The German American scientist Jacob Kushler and the Russian scientist Fedor Shevdov also made major advances in the understanding of tree rings in the 19th century. The transformation of dendrochronology into a formal science is largely credited to the American astronomer Andrew Douglas. Douglas, who had worked at the Lowell Observatory in Arizona, was interested in the relationship between sunspot cycles and climate. Around 1901, he began studying tree rings in the American Southwest to see whether they could record fluctuations in rainfall and temperature. His meticulous observations showed that tree rings reflected not only annual growth, but but also long term environmental cycles. By comparing samples from living and dead trees, he developed the technique of cross dating, which allowed him to extend chronologies far beyond the lifespan of any single tree. Douglas work culminated in the Beam Expeditions, a series of efforts to collect beams from Puebloan ruins across Arizona and New Mexico. Archaeologists had long struggled to date these structures precisely, relying on relative dating methods like pottery styles. Using dendrochronology, Douglass was able to assign exact calendar years to major sites such as Pueblo Bonito in Chaco Canyon and Batacan in Navajo National Monument. The breakthrough came in 1929 at the Batacan Ruin in Arizona. Douglass identified a piece of charcoal that bridged the gap between his modern and archaeological chronologies. The single sample, dubbed HH39, allowed him to date hundreds of archaeological sites across the Southwest with unprecedented precision. The moment of this discovery was so significant that it's known in archaeological circles as the Day that Time Stood still, the day when absolute dates became available for southwestern prehistory. In 1937, Douglas founded the Laboratory of Tree Ring Research at the University of Arizona, which became the Global center for Dendrochronology. The lab formalized methods of preparing samples, cross dating, and building master chronologies. During this period, Denbrough chronology expanded beyond the Southwest to Europe, where scholars like Bruno Herber in Germany and later researchers in Scandinavia and the British Isles applied it to historical buildings and climate studies. By mid century, continuous chronologies covering Thousands of years had been established in places like Germany and Ireland. From the 1960s onward, dendrochronology became a fully interdisciplinary tool in archaeology. It was used to date Viking ships, medieval churches, and historical timber frame buildings across Europe. In climatology and environmental science, researchers used ring width and density variations to reconstruct past routes, floods, volcanic eruptions, and temperature changes. So dendrochronology is important, but how exactly does it work? There is more to it than just counting the rings on trees. We have to start with the rings themselves. Trees produce rings because of how their growth responds to seasonal and environmental changes in temperate and some subtropical regions. The ring pattern is the result of cambial activity that is the work of the cambium layer, a thin sheaf of living cells between the wood, or xylem, and the bark, or phloem. This cambium generates new xylem shells inward and new phloem shells outward. And the way it produces these shells varies over the course of a year. When the growing season begins in spring and early summer, water and nutrient availability are usually high, and trees prioritize rapid transport of water to support new leaves. The cambium produces large diameter xylem cells with thin walls. These cells conduct water efficiently but are structurally weaker. Under a microscope, this earlywood appears as a lighter band. As the season progresses and growth slows, the cambium produces smaller diameter xylem cells with thicker walls. These are denser, stronger, and less efficient for water transport, but they provide mechanical support for the tree. This late wood appears darker and more compact. The transition from light early wood to dark late wood creates the visible ring boundary. One cycle of early wood plus late wood represents one year of growth. In most climates, the amount of growth in a ring will be dependent upon the climate conditions in any given year, which is why some rings are thicker and some are thinner. In some parts of the world, such as the tropics, where they don't have clear seasons yields, you do not see clear rings inside of trees. If you were to cut down a tree, you could just count the rings to go back in time to when the tree sprouted. However, the real key to dendrochronology is cross dating. Without cross dating, the furthest we could go back with tree ring dating is the oldest tree that we could cut down. Because tree ring widths are determined by climate, all trees from all species in the same area should have a similar pattern of thick and thin rings. You can also make comparisons going back in time. Suppose one living tree grew from 1800 to 1900 and another dead log spans from 1700 to 1820. If both show the same sequence of narrow and wide rings from 1800 to 1820, then the two chronologies can be overlapped, extending the record back to 1700. By repeating this with many overlapping specimens, dendrochronologists can construct a continuous master sequence spanning thousands of years. The master chronology can then be used as a reference. Any piece of wood, say, from a beam from an archaeological site, can be cross dated by aligning its ring pattern against the master sequence. This gives a precise felling year, and sometimes down to the exact season, if bark edges are also preserved. In principle, dendrochronology can provide exact year by year dating, as long as you have a continuous unbroken sequence of overlapping samples. This method is not limited by the lifespan of individual trees. Instead, it's limited by how many logs, timbers, or preserved trees can be linked together in a sequence. There is, of course, a catch to all of this. There isn't a single universal master sequence that can be used for everything in the world. One region might have a drought in the same year that another region is wet. So you have to create multiple master sequences unique to each area. And this means that the dating for each area depends on what archaeological wood can be found and what trees exist in that region. For example, the original master chronology developed by Andrew Douglas based on ponderosa pine and Douglas firs, extends about to 2000 years. However, in California, individual living bristlecone pines in the White mountains are nearly 5,000 years old. And dead wood preserved in the same environment has allowed researchers to build a chronology of about 9,000 years. In Europe, Irish oak and German oak pine sequences are among the longest continuous chronologies we have. The Irish oak chronology runs back about 7,400 years, and the German oak chronology has been extended to about 12,000 years years, covering much of the period since the last ice age. At the start of the episode, I mentioned that there were multiple ways to date objects. Many of these techniques can be used to help calibrate the other. Because dendrochronology can provide a high degree of accuracy down to a given year, it's been used to calibrate carbon 14 dating. I'll cover carbon 14 dating more comprehensively in a future episode. But carbon 14 is created in the upper atmosphere by cosmic rays. It's then ingested by living organisms and then slowly decays over time. However, the amount of carbon 14 that's created annually is not constant. Events, called Myack events are spikes in carbon 14 production, which can be found in the dendrochronological record. There have been five such events recorded in 7176 BC, 5259 BC, 660 BC 774 and 993. It is believed that these events were the result of massive solar flares that hit the earth. Knowing these spikes in carbon 14 can then better help calibrate tree ring records. Modern dendrochronology has become truly international, with active research programs on every continent except Antarctica. International organizations like the Tree Ring society, founded in 1974, have helped standardize techniques and facilitate collaboration between researchers all over the world. The establishment of the International Tree Ring Databank has created a global repository of regional tree ring chronologies, making data available to researchers worldwide and enabling large scale comparative studies. This collaborative approach has revealed global patterns in climate variability and has helped scientists understand how different regions respond to major climate events. Dendrochronology is a vital part of our ability to understand the past. It has its limitations, but despite those, it is perhaps the most powerful tool we have for being able to date and understand our past. So long. Of course, as it's made out of wood the executive producer of Everything Everywhere Daily is Charles Daniel. The associate producers are Austin Otkin and Cameron Kiefer. My big thanks go to everyone who supports the show over on Patreon. Your support helps make this podcast possible, and I also want to remind everyone about the community groups on Facebook and Discord. That's where everything happens that's outside the podcast, and links to those are available in the show notes. As always, if you leave a review on any major podcast app or in the above community groups, you too can have it read on the show.