Dendrochronology

Podcast Transcript

One of the most essential aspects of archeology 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 reconstruct past environmental conditions by analyzing growth rings in trees.

However, it isn’t just a matter of counting tree rings; there is 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 is used on this episode of Everything Everywhere Daily.

Dating events and items from the past is an important part of science. Scientists have provided dates for everything from the age of the universe to items that have been found in ancient tombs.

However, there is no one way to date an object. There are multiple ways to date things, and depending on what is 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 dating is another type of dating that doesn’t work with rocks, but can be used for organic matter going back about 60,000 years.

One of the best dating methods for more recent objects, going back hundreds or a few thousand years, is using tree rings, or dendochronology.

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 BC about the annual nature of tree growth rings.

He observed that trees added layers each year, though he didn’t fully grasp the potential for using these rings to understand past conditions.

Ancient civilizations likely understood this connection intuitively. Indigenous peoples 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 prescient observations about tree rings in his notebooks around 1500. He noted 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 du Monceau 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, though it 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 Kuechler and the Russian scientist Fedor Shvedov 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 Ellicott Douglass. Douglass, 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 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.

Douglass’s 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 Betatakin in Navajo National Monument.

The breakthrough came in 1929 at the Betatakin ruin in Arizona. Douglass identified a piece of charcoal that bridged the gap between his modern and archaeological chronologies. This single sample, dubbed “HH-39,” 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, Douglass founded the Laboratory of Tree-Ring Research at the University of Arizona, which became the global center for dendrochronology. The lab formalized methods for preparing samples, cross-dating, and building master chronologies.

During this period, dendrochronology expanded beyond the Southwest to Europe, where scholars like Bruno Huber in Germany and later researchers in Scandinavia and the British Isles applied it to historical buildings and climate studies.

By mid-century, long continuous chronologies covering thousands of years had been established in places like the German oak and Irish bog oak sequences.

From the 1960s onward, dendrochronology became a fully interdisciplinary tool. In archaeology, it was used to date Viking ships, medieval churches, and historical timber-framed buildings across Europe.

In climatology and environmental science, researchers use ring-width and density variations to reconstruct past droughts, floods, volcanic eruptions, and temperature changes.

So, dendochronology 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 sheath of living cells between the wood or xylem and the bark or phloem.

This cambium generates new xylem cells inward and new phloem cells outward, and the way it produces these cells 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. This latewood appears darker and more compact.

The transition from light earlywood to dark latewood creates the visible ring boundary. One cycle of earlywood plus latewood represents one year of growth in most climates.

The amount of growth in a ring will be dependent on 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, with no clear seasons, you don’t see clear rings inside trees.

If you cut down a tree, you could just count the rings and go back in time to when the tree sprouted.

However, the real key to dendochronology is cross-dating.

Without cross-dating, the furthest we could go back with tree ring dating is the oldest tree 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 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, 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, sometimes down to the exact season if the bark edge is also preserved.

In principle, dendrochronology can provide exact year-by-year dating as long as you have a continuous, unbroken sequence of overlapping samples. The method is not limited by the lifespan of individual trees; instead, it is limited by how many old logs, timbers, or preserved trees can be linked together in sequence.

There is, of course, a catch to all this. There isn’t a single, universal master sequence that can be used for everything. One region may have a drought in the same year that another region is wet. So, you have to create multiple master sequences unique to each area.

This means that the dating for each area depends on what archaeological wood can be found and what trees exist in each region.

For example, the original master chronology developed by Andrew Douglass, based on ponderosa pine and Douglas-fir, extends about 2,000 years.

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.

The Irish oak chronology runs back nearly 7,400 years, and the German oak chronology has been extended to about 12,000 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 dendochronology can provide a high degree of accuracy down to a given year, it has 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, ingested by living organisms, and then slowly decays over time.

However, the amount of Carbon-14 that is created annually is not constant. Events called Miyake events are spikes in Carbon-14 production, which can be found in the dendochronology 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 help better 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 worldwide.

The establishment of the International Tree-Ring Data Bank 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 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 the past…..so long as it is made out of wood.