What to expect:

Three weeks in a row! You guys are probably exhausted from all of these videos. But that ain’t stopping me. Like last week’s, this one is also a little different….

Many of you have probably heard that if you count the rings of a tree, you can determine its age. Well, there’s some truth to that. Known as dendrochronology, we can study tree rings to help us understand our past. In this video, we dive into tree-ring dating. We look at its discovery and history, the science behind it, and its applications in archaeology.

The three archaeological sites we explore include:

• Pueblo Bonito, New Mexico.

• Roskilde Fjord, Denmark.

• The Merchants and Drovers Tavern, New Jersey.

I hope you find this stuff as fascinating as I do!

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How Trees SECRETLY Keep Track of Time

What if I told you that this Redwood tree was 3,000 years old and that it’s been silently recording history all along? Each tree holds snapshots of the past - revealing everything from ancient droughts to the rise and fall of civilizations. This natural timekeeper might just be the most accurate historian we have.

You’ve probably heard that each ring in a tree represents one year, and by counting them, we can determine the tree’s age. But there’s so much more to the story. In this video, we’ll explore how people first discovered this fascinating and powerful method of dating, how its accuracy has evolved over time, and finally, how archaeologists use it to unlock the secrets of our past.

So… how can we read history through the rings of a tree?

The Discovery

Dendrochronology, or tree-ring dating, has an interesting backstory that stretches over centuries. It all started with some ancient observations. Around 2,000 years ago, the Greek botanist Theophrastus was the first to note that the cross-sections of trees expose concentric rings. But it wasn’t until the Renaissance that the true nature of these rings began to be understood.

In the 15th century, none other than Leonardo da Vinci noticed that trees grow a new ring every year, and that the thickness of these rings depended on the environmental conditions in which they grew. For example, trees that grew during wet, favorable years produced wide rings, while those that experienced drought or harsh conditions formed much thinner rings. Da Vinci’s observations laid the groundwork for what would become dendrochronology.

Fast forward to the 18th century, and two French scientists whose names I definitely can’t pronounce, Henri-Louis Duhamel du Monceau and Georges-Louis Leclerc de Buffon, took this further. They studied the effects of different growing conditions on tree rings and discovered something interesting. A severe winter in 1709 left a distinct, dark ring in European trees, providing a reference point for naturalists to compare with other trees across Europe. This was one of the first instances where tree rings were linked to a specific historical event.

Then, in the early 19th century, the idea of using tree rings to study climate patterns emerged. Alexander Catlin Twining, an American scientist, suggested that the patterns in tree rings could be used to reconstruct past climates over entire regions. Not long after, the English polymath Charles Babbage proposed using tree-ring dating to study trees buried in peat bogs or even in geological layers. These early pioneers set the stage for what would later become one of archaeology’s most important dating methods.

Finally, dendrochronology truly came into its own in the early 20th century, when astronomer Andrew Ellicott Douglass perfected the technique. He began using tree rings to study sunspot cycles, but soon realized their potential to date ancient wooden structures. Thanks to his work, dendrochronology became an essential tool for archaeologists, giving us precise dates for ancient events and helping unlock secrets of the past that might otherwise have been lost. This made it the first form of “numerical dating”. Meaning that it could be applied to calendar years. This is in contrast to relative dating, like stratigraphy, which mostly just tells us when things are older or younger than each other without exact dates.

Now, let’s dive into the science of tree-ring dating.

The Science 

Every year, a tree adds a new layer of growth between its bark and the previous year’s wood. This layer becomes a ring, and each one is like a fingerprint of the conditions that year. In good years - when there’s plenty of rain, sunshine, and nutrients - the tree grows more, producing a thicker ring. But in tough years, like during droughts or harsh winters, the growth slows down, resulting in a much thinner ring. Additionally, the rings generally get smaller as the tree ages. By looking at the pattern of thick and thin rings, scientists can not only figure out the age of the tree but also reconstruct the climate during its lifetime.

Now, here’s where it gets really interesting. It’s not just a single tree that tells the story. Researchers compare tree-ring patterns from different trees in the same area to build something called a “tree-ring chronology.” This is like a master calendar that stretches back in time, sometimes thousands of years. Even when trees die or are cut down, their wood can be matched to this chronology. 

For instance, when scientists find old wood samples, whether from a forest or even a historic structure, they can compare the unique ring patterns to the master timeline. This allows them to pinpoint exactly when that tree stopped growing, revealing key information about its age and the environment at that time.

And dendrochronology is incredibly precise - down to the exact year, sometimes even the season! That’s because no two years produce the same ring pattern, and over long periods of time, these patterns form a kind of barcode. Once scientists crack that code, they can match wood samples from archaeological sites, historical buildings, or even ancient shipwrecks to a specific point in history.

But it doesn’t stop there. The rings also capture information about volcanoes, floods, and even solar activity. Volcanic eruptions, for example, can leave traces in the form of unusually narrow rings due to the ash clouds blocking out sunlight, reducing the tree’s growth. Floods may create distinct rings as the waterlogged soil impacts the tree’s health. Periods of intense solar radiation can cause shifts in climate, which show up as changes in ring growth patterns. 

In a way, the tree has been quietly recording everything happening around it - capturing a detailed history of its surroundings, year by year. So, while the process sounds simple - just counting some rings - it’s much more complex and provides an incredibly accurate timeline of the past. This is why dendrochronology has become one of the most powerful tools in archaeology and climate science.

It’s also used to calibrate radiocarbon dates. Calibration is necessary because the amount of Carbon-14 in the atmosphere has not been constant due to factors like solar activity, volcanic eruptions, and changes in Earth's magnetic field. Without calibration, radiocarbon dating would yield ages that could be off by hundreds or even thousands of years. 

Scientists measure the Carbon-14 content in individual rings and compare it to the expected levels based on the tree's actual age. This creates a calibration curve that accounts for variations in atmospheric Carbon-14 over time. Scientists can then achieve more accurate and reliable results when dating ancient artifacts and fossils.

Despite its efficacy, tree-ring dating also has some limitations. So let’s explore a few of these challenges before seeing how it’s been applied to some truly fascinating archaeological sites. 

First, it’s geographically restricted. Dendrochronology only works in regions where trees produce distinct annual growth rings. This means it’s highly effective in temperate climates, where trees experience clear growing and dormant seasons. But in tropical regions, where trees may grow year-round without distinct seasons, the rings are either absent or very hard to distinguish. So, dendrochronology isn't a global solution for dating events.

Second, the method depends on having a well-preserved sample of wood. Wood can degrade, rot, or burn over time, making it unusable for tree-ring analysis. This means that finding suitable wood samples can be a challenge in many archaeological or historical sites. In some cases, wood may be present, but if it's too damaged, it can’t be accurately dated.

Another limitation is the need for an established reference or “master” chronology. To date an ancient piece of wood, scientists need to match its rings to an already-established timeline. If a region hasn’t developed such a timeline, or if the sample predates the earliest trees in that chronology, it becomes difficult to place the sample in a specific time frame. For instance, while dendrochronology can stretch back thousands of years, it’s not as useful for dating beyond the lifespan of the oldest trees in a given region.

Finally, environmental factors can complicate the story. Trees in the same region may show similar growth patterns, but local factors like soil quality, competition for resources, or microclimate effects can sometimes create irregularities in ring patterns. This can make it difficult to line up tree-ring sequences perfectly across different samples.

While dendrochronology is a powerful tool, these limitations remind us that no dating method is perfect. Still, when conditions are right, tree-ring analysis can provide stunningly accurate snapshots of the past. So how have these snapshots supported archaeological research? 

The Archaeology

Dendrochronology first entered the field of archaeology in the early 20th century, thanks to A.E. Douglass, the astronomer who initially studied tree rings to track solar cycles. By comparing the rings of ancient and modern trees, Douglass developed the first continuous tree-ring chronology in the American Southwest. Archaeologists quickly realized the potential for using these strategies to date wooden beams from prehistoric Pueblo homes, revolutionizing the way they could precisely date ancient structures. Since then, dendrochronology has been applied worldwide to date everything from ancient temples to Viking ships.

Tree ring dating, like most aspects of archaeology, is similar to solving a puzzle. By comparing the ring patterns of wood samples from ancient beams, artifacts, or dwellings with an established master chronology, scientists can pinpoint the exact year the tree was cut down.

The process begins by extracting a small core sample or examining a cross-section of the wood. The ring patterns are then compared to local or regional tree-ring chronologies, built from both living and dead trees, to find a match. Once a match is found, the year that tree was cut can be established. That date then marks the earliest possible date that the artifact or structure was created. This makes it an invaluable tool for reconstructing the history of many sites.

The worst archaeological sites for preserving organic material like trees are in environments that fluctuate often. The dramatic changes between wet and dry or hot and cold speed up the decaying process. Some of the best are in environments that are consistently arid or totally submerged in saturated sediment. We’ll start by looking at an example of the former. 

Pueblo Bonito

The ruins of Pueblo Bonito can be found in Chaco Canyon National Park, New Mexico. Here, Ancestral Puebloans of the American Southwest constructed a settlement that spans 3 acres and contains around 800 rooms. There are tiered structures in some sections, reaching up to four or five stories. The builders employed core-and-veneer techniques to construct many of these multi-story dwellings, whereby two parallel stone walls were built with rubble thrown in between to fill the gap. This resulted in massive masonry walls up to 3 feet thick. An architectural analysis of the site suggests that the population at its peak was around 70 individuals [1], although it is uncertain whether it was a place for living or a place for ritual. 

This was the site that Douglas illuminated with his novel dendrochronology [2]. “By translating the story told by tree rings,” he said, “we have pushed back the horizons of history in the United States for nearly eight centuries before Columbus reached the shores of the New World,” 

Expeditions of Pueblo Bonito and other ruins around the region were conducted by the National Geographic Society in the 1920s. The researchers took samples from wood ceiling beams at various locations. Based on artifacts recovered, they already knew that some sites were relatively older than others, but no dates had been assigned. At first, Douglas’ analysis of the wood samples provided one date range for the sites outside of Pueblo Bonito, like Mesa Verda, the Citadel Ruins, and Mummy Cave. The tree rings tell us that these sites fit within a roughly 180-year period. 

However, the samples collected directly from Pueblo Bonito did not match. Douglas was left with two chronological periods separated by a gap of unknown length. Then, by comparing the latest tree rings from Pueblo Bonito with some newly acquired pine and juniper wood samples, the two prehistoric periods were joined. The last piece of the puzzle was to connect this 580-year period to a more recent chronology. After scouring villages all around the southwest, they eventually found Beam HH39 at Whipple Ruin in Arizona. This 39th tree-ring specimen of their research bridged the gap between prehistoric and more modern tree chronologies. It was concluded that  Pueblo Bonito dated to around 700 AD, which is actually more recent than its previously suspected age of 2,000 years.

As said so graciously by Douglass himself, “A tree is not a mechanical robot; it is a living thing, and its food supply and adventures through life all enter into its diary.”

Now let’s move on to a site that’s been sitting in saturated soils, preserving some ancient ships.

Viking Ships

In the 1960s, archaeologists uncovered the remains of five Viking ships in Denmark's Roskilde Fjord [3]. The ships were submerged under water and sediment. This discovery provided significant insights into Viking shipbuilding techniques. The vessels, which dated back to the 11th century, appeared to have been intentionally sunk to create a blockade in the fjord. They varied in size. Some were small warships while others were larger trading vessels.

The Skuldelev ships featured clinker-built construction, where overlapping planks were used - a hallmark of Viking ship design. Each ship had a single mast, double-ended hulls, and curved stems. The timber used originated from places as distant as Norway and Ireland. The preserved planks were later reconstructed at the Roskilde Viking Ship Museum.

During the mid-1990s, an extension to the museum took place. Workers encountered remnants of a ship and excavations quickly ensued. This led to the discovery of nine additional ships. While some dated back to the Middle Ages, one was a Viking warship, known as Roskilde 6. It measured 36 meters in length, making it the longest Viking ship ever found. Dendrochronology was used to date the ship's construction to around AD 1025. The timbers are currently undergoing conservation to ensure that these ships, like the Skuldelev vessels, serve as valuable records of Viking history.

So the longest Viking ship ever discovered was dated using dendrochronology. 

Old American Architecture

The last example of tree-ring dating I want to share with you comes from a graduate school professor of mine, Dr. Rich Veit. Veit is one of the leading archaeologists in the state of New Jersey. He focuses primarily on historical archaeology, vernacular architecture, and military sites. 

One site he worked on was the Merchants and Drovers Tavern in Rahway, New Jersey. It’s the oldest tavern in the town that’s still standing, and its architecture is representative of early America. It encompasses three-and-one-half stories, a gambrel roof (indicative of early Dutch influences), and four dormers on the front. A chimney rises at each end of the building, and its clear symmetry is something you see in many of the colonial and post-colonial era structures of the United States. 

According to the New Jersey Historic Trust [4], “The building was located along one of the main routes between Philadelphia and New York and soon became a regular stagecoach stop for travelers.” It also sits adjacent to Rahway Cemetery, which contains the remains of some notable historical figures, such as Civil War Officer Walter Bramhall and Colonel John Cladek. 

However, some breaks in its foundation indicate that it didn’t always have the symmetry and size we see today [5]. Debates about the year of construction ensued. Some believed that it was built in the early 1700s, others in the early 1800s. Others yet speculated that there were multiple phases of construction to this historical building, which would better explain the break delineations in the foundation. 

In 1999, Dr. Veit collected tree-ring samples from 13 different locations in the building and sent them off to Columbia University for examination [6]. The scientists at Columbia concurred with the multiple phases of the construction hypothesis. They found that samples of the first construction phase dated to between 1795-1796. The second phase of construction dates to between 1818-1819. It was initially just two and one half stories tall, with the third story added in the second phase.

These examples show us how trees can be used to enlighten our understanding of our past. I plan on making more videos about other dating methods used in archaeology, but for now, check out this video on the prehistoric archaeology of New Jersey - where I work as an archaeologist. 

References:

[1] Bernardini, Wesley. 1999. “Reassessing the scale of social action at Pueblo Bonito, Chaco Canyon, New Mexico”. Kiva, the Journal of Southwest Anthropology and History. 64 (4): 447–470.

[2] Douglas, A. 1929. “The Secret of the Southwest Solved by Talkative Tree Rings.” The National Geographic Magazine

[3] https://www.bbc.co.uk/history/ancient/vikings/dig_reports_01.shtml#six 

[4] https://www.nj.gov/dca/njht/funded/sitedetails/merchantsdroverstavern.shtml 

[5] National Register of Historic Places, Merchants and Drovers Tavern, Rahway, NJ.

[6] Veit, R. 1999. Dendrochronological Study of Merchants and Drovers Tavern, Rahway, NJ. Report prepared for the Rahway Historical Society.

Song Suggestion

Music is a human universal. It’s found in every culture, at every corner of the globe. The Evolve.2 song suggestions are hand-picked by yours truly. I love all types of music, from cool jazz to death metal. So, at the end of every newsletter, I leave a little something I want to share with you all - something I’ve been jammin’ to lately.