By Whit Bronaugh

Annual growth rings in a cross-section of a bristlecone pine. (Credit: Nicholas Turland)

New Englanders had Paul Revere to warn them of the coming British forces, but on May 19, 1780, there was no early morning ride with the cry, “The Darkness is coming! The Darkness is coming!” The dawn was cloudy with a reddish tinge, but by noon, it was dark as night from New Jersey to Maine. Night birds started to sing. Flowers closed their petals. A record number of people said, “What the — ?!” Within a day or so, the light returned, but it would be 228 years before any was shed on the mystery of the New England Dark Day, although the evidence was there all along. Finally, in 2008, scientists found the culprit: smoke from widespread forest fires, particularly in the Algonquin Highlands of Ontario, had blocked the sun. And the smoking gun? Tree rings.

Dendro-chron-ology (tree — time — study of) would seem, at face value, to be a tiny field hardly deserving an –ology at all. You just take a cross-section and count the rings to see how old the tree is, right? Well, apparently, that’s like saying a musician just plays scales. Although the field is less than a century old, there are now scientists who study dendroclimatology, dendrovolcanology, dendropyrochronology, dendrochemistry, dendroarchaeology, dendrotempestology, dendro — well, you get the idea.

In more familiar terms, these scientists use tree rings to study the history of climate, geology, ecology, and our ancestors. Dendrochronologists have documented the fall of Rome, recorded the eruption of volcanoes no one witnessed, settled a boundary dispute between Oklahoma and Texas, established the year of death of a murder victim (by the age of a root that grew over the body), indicated the strength of the 1908 Tunguska meteoroid impact that knocked over 80 million trees in Russia, dated the Stradivari Messiah violin, and provided a calibration for radiocarbon dating that literally changed history. Who knew?

It all started when a gargantuan asteroid collided with Earth about 4.5 billion years ago. That singular event gave Earth its tilt, which gave us the seasons, which cause the familiar growth rings in trees. It’s appropriate, then, albeit surprising, that the field of dendrochronology was founded by an astronomer.

In 1901, Andrew Ellicott Douglass was interested in establishing a long-term record of sunspot activity. Reasoning that sunspots affect the sun’s output, which affects our climate, he got the eyebrow-raising idea to look at growth patterns in tree rings for evidence of past sunspot activity. Soon, Douglass noticed that a pattern of small rings in the years 1899, 1902, and 1904 was consistent among ponderosa pines near Flagstaff, Arizona. Eventually, he documented the pattern, now called the Flagstaff Signature, in trees throughout the southwestern states. He went on to show that tree rings recorded rainfall and climatic cycles, but he was never able to convince most scientists that tree rings could record sunspot activity.

Dating on a fire-scarred ponderosa pine from the Mount Rushmore National Memorial. (Credit: Dr. Peter M. Brown/Rocky Mountain Tree-ring Research)

Douglass’ problem with the sunspot record is but one example of why dendrochronology is far more than a child’s science project. Tree rings are like life-and-times autobiographies; they record the history of an individual tree as well as the ecological history of that tree’s surroundings. Examining the sawn end of an old log, you and I might notice that some rings are thicker than others, indicating good growing years. But dendrochronologists have deciphered a far more complex language that conveys stories, not just dates, otherwise hidden by the ravages of time.

Small rings can indicate slow growth due to an insect outbreak, pollution, or the tree putting most of its resources into a large fruit crop. Large rings may signal a burst of growth after the end of a drought, the fall of nearby canopy trees that had been hoarding the light, or the fertilizing action of a recent fire or volcanic ash fall. Rings that are large on only one side of the tree indicate reaction wood growth to reinforce the tree’s strength against gravity after a hurricane or landslide gave it the old gangster lean.

A dendrochronologist must be able to read microrings just two cells wide, false rings embedded in a single annual ring, fuzzy ring boundaries in tropical trees, missing rings, pinched rings that are missing part of their circumference, frost rings formed from freezing temperatures during the growing season, and fire scars that lack telltale charcoal. Embedded in the text of annual rings are subtle motifs that are the signatures of events that occur regularly, like El Niño, or randomly, like major volcanic eruptions.

As with historians, some trees are better than others at recording past events. Many tropical trees that grow in areas without distinctive wet and dry seasons do not have rings. The vast majority of tree species do not live more than a couple hundred years. Still, dendrochronologists have found use for nearly 700 different species of trees and shrubs. Like any good handyman, they look for the right tool for the job. Often, that means whatever is growing in the place of interest. But for those interested in ancient history and long-term patterns, it is often the ancient trees that are read. Douglass himself established a giant sequoia tree-ring record going back more than 3,000 years, but it was one of his students, Edmund Schulman, who discovered the oldest trees in the world, the now famous bristlecone pines.

Great Basin bristlecone pine (Credit: Whit Bronaugh)

Great Basin bristlecone pines live at the timberline in their namesake region, where, in the words of Schulman, they achieve “longevity under adversity.” The cold and dry windswept environment, thinness of the groves, and lack of vegetative cover keep insects, fungi, disease, and fire at bay, while the short growing season may pack 140 rings into an inch of wood. Many of the older trees are reduced to a few living twigs sustained by a narrow strip of bark and living cambium clinging to a weathered skeleton of twisted wood. The oldest living tree Schulman found, which he called Methuselah, still thrives two miles high in the White Mountains of California and just celebrated its 4,842nd germination day. It sprouted just a century or so after the invention of writing, was more than 200 years old when the Great Pyramid was built, and was already 3,000 years old at the height of the Roman Empire. And scientists have found that 4,700-year-old bristlecone pines have the health, vitality, and seed-making ability of 200-year-old youngsters. On the harshest growing sites, bristlecone pines are practically immortal and will likely survive until the climate changes on the scale of an ice age, or the very ground beneath them wears away.

Great Basin bristlecone pines are not the only trees that count their rings by the thousands. A Patagonian cypress in Chile that was logged in 1975 germinated 3,613 years earlier, around the time the cat was first domesticated, the woolly mammoth became extinct, and alphabetic writing was invented. The oldest-recorded giant sequoia sprouted about 350 years later. Only five other tree species have exceeded 2,000 years: western juniper, coast redwood, and foxtail pine in California; Colorado bristlecone pine in Colorado; and sacred fig in Sri Lanka. Eight conifers from North America, and one from Tasmania, have lived more than 1,000 years.

Early in the development of dendrochronology, Douglass realized there was a way to extend the tree-ring record back before the oldest living trees laid down their first ring. Douglass called it crossdating. He matched patterns of tree rings, like the Flagstaff Signature, from trees that have overlapping lifespans. Let’s say that you use a Swedish increment borer, a tool that removes a pencil-like section that cuts across the tree rings, to take a bore sample from a living ponderosa pine in Arizona. You find that it sprouted in 1890, so its inner rings include the Flagstaff Signature of small rings in 1899, 1902, and 1904. Next, you take a bore sample from a nearby fallen Rocky Mountain juniper that obviously died many years ago, but you find the Flagstaff Signature among the outer rings. Now, you can count the rings of the living ponderosa pine back to the 1899 ring of the Flagstaff Signature, cross over to the same ring on the fallen juniper, and continue counting rings until you know the year the juniper sprouted. As long as you can keep finding logs with overlapping lifespans and matching signatures, you can keep extending the timeline. In this way, Douglass established a timeline of ponderosa pine tree rings going back to the year 1,286 CE (Common Era).

Meanwhile, archaeologists studying the Anasazi Indian culture of the Southwest asked Douglass to apply his crossdating method to the wooden beams in ruins well preserved under overhanging cliffs in the dry desert air. Douglass quickly determined that the Pueblo Bonito ruin predated the Aztec Ruins (both in New Mexico) by 40-45 years. Eventually, he established a “floating” chronology of some 500 years that gave the order of construction for Anasazi structures, but no dates. “Floating” because it was not connected to the present; there was a gap in the timeline between the chronology for the ruins and the ponderosa pine tree-ring sequence. Finally, in 1929, Douglass sampled a beam from the Show Low site in Arizona that bridged the gap. He called the beam HH-39, but compared it to the Rosetta Stone because it allowed exact dating for all Southwestern ruins constructed since 700 CE.

Black cottonwood tree rings (Credit: Whit Bronaugh)

With crossdating, dendrochronologists can extend the timelines for different trees or locations far beyond the oldest living trees. Based on pieces of wood that have been lying on the ground for thousands of years, the bristlecone pine chronology goes back to 6716 BCE, around the time the first cattle were domesticated. Combining subfossil pine and oak logs from river deposits in Germany, we now have an unbroken record that goes back to 12,460 years ago, when all humans were Stone Age hunter-gatherers, the dog was man’s only domesticated friend, and most of Canada was still a giant hockey rink.

For wood artifacts that are older, or do not contain enough rings for confident crossdating, archaeologists often turn to radiocarbon dating. But even that is now based on calibration with tree rings because the level of carbon-14 in the atmosphere has not been constant over time. Bristlecone pine tree rings showed that many earlier, uncorrected carbon-14 dates were centuries too old. Among other historical corrections, bristlecones proved that some European artifacts were actually older than their supposed Mediterranean progenitors.

The history recorded in tree rings is interesting in its own right, but it also helps us understand the present and prepare for the future. Scientific records of how the world works go back only a relatively short time. We didn’t even recognize El Niño events until the 1970s, but tree rings record them at least back to 1699. Using tree-ring signals that correlated with the North Atlantic Oscillation, we now have a record of sea-surface temperature for the North Atlantic back to 1713. We understand the risk of extreme floods better because flood debris has left scars on the upriver side of bur oak tree rings back to 1648. Reaction wood caused by high winds has been used to establish a record of typhoons back to 473 CE using Japanese cedar. Giant sequoias have recorded fires for the last 3,000 years. Bristlecone and foxtail pines show the frequency of volcanic eruptions big enough to affect climate for the last 5,000 years. Tree-ring chronologies have revealed past snow avalanches, landslides, changes in the water table, flood heights, earthquakes, and insect outbreaks.

Reading between the rings, dendrochronologists can take advantage of a tree’s excellent chemical memory because chemicals rarely move across the rings. Fancy techniques, like X-ray fluorescence or laser-induced breakdown spectroscopy, can measure elements or stable isotopes in any given year of the tree’s life. This can give a record of past tropical cyclones (which produce less oxygen-18 than regular thunderstorms), air temperature, pollution levels, soil composition, and the intensity of interstellar cosmic rays, which affect solar activity and the earth’s magnetic field. Recent radiocarbon analysis of 11,000 years of tree rings has even confirmed that trees have recorded the history of sunspot activity. Finally, Douglass, the astronomer who studied trees to find answers about the sun but instead found answers about human history, was proven right.

We now know that the people of New England had nothing to fear on that Dark Day in 1780, but the unknown was enough to send them cowering. With vulnerability to floods, volcanic eruptions, earthquakes, hurricanes, global climate change, and other impacts of our own activities, we too face an unknown future. But with dendrochronology as our guide, we are far better equipped to understand and deal with it. All we have to do is read the history written by trees.

Whit Brounaugh writes from Eugene, Oregon

For more information on tree rings, visit the Laboratory of Tree Ring Research, founded by A.E. Douglass himself, at the University of Arizona at and The Ultimate Tree-Ring Web Pages at