Lore of the rings

Listen to this essay 22 minute listen We were a team of five: Jan, Paul, Jonas, Claudia, and me. Four dendrochronologists and one son of a dendrochronologist. Jan Esper flew in from Germany. Paul Krusic drove his Land Rover from his home in Stockholm down to southern Italy and then on the ferry across to Greece. He had brought his son Jonas, a precocious 12-year-old Viking. Claudia Hartl and I drove in from neighbouring Albania, where we had already spent 10 days sampling. We all met up in Samarina, a small Greek village at the foot of Mount Smolikas, the highest peak in the Pindos mountains. The reason we were there was to core trees called Bosnian pines (Pinus heldreichii), which we suspected could be old. What we didn’t know that morning was quite how old. Nor just how much these trees would tell us about the centuries of events and circumstances they had experienced in their lifetime. Trees hold stories: they are witnesses of time and history. Of course, we can read a tree’s age and local climate from its rings, but in recent years we’ve discovered so much more. Deep inside a tree’s trunk is an archive waiting to be found: tales of planetary shifts, cosmic events, and even turning points in human history. And the oldest trees have seen it all. Paul had been the first to suspect that the Bosnian pines, growing on the rugged mountain slopes of Greece’s heartland, might be very long lived. While reading an obscure dissertation, he had stumbled upon a photograph of the species on Mount Smolikas. As a tree connoisseur, he could see that the pines displayed all the stereotypical features of an ancient tree: an untapered, columnar stem with heavy branches; large, exposed roots; a flattened crown, not unlike a bonsai tree; and a dead top. The photograph showed the pines growing in a steep, rocky landscape, inhospitable to most organisms and processes that kill trees. In places with few insects, fires or humans, trees can grow for centuries if not millennia. But to find out the pines’ exact age, we would have to climb Mount Smolikas to collect samples. In the morning, we set off on our two-hour hike, along a trail mostly used by shepherds. It would lead us to the upper treeline. Along with our lunches and raingear, we carried corers and notebooks in our backpacks. And Paul brought a chainsaw to cut cross-sections from the sun-bleached dead logs littering the landscape. These were the relic trunks of pine ancestors that had died hundreds of years earlier. We hoped that they would tell us about the history of Mount Smolikas even further back in time than the lifespan of the living trees, perhaps over a millennium or more. Once we arrived on the mountain ridge, the sight of ancient trees in an utterly barren landscape awaited us. We were hungry to drive our corers into the worn stems of the old trees, to hear their creaking once the sharp edge of the corer cut into their wood, to smell their sap once we removed a pencil-sized wood sample from their stems, to count their rings. To core living trees, we use a hollow increment borer to drill manually into the trunk, aiming to reach the oldest part: the pith. In theory we do this at breast height, but in practice, with boulders and fire scars and low branches and steep slopes, we core where we can and where it is safe to do so. Coring does not harm or kill a tree. After all, the only living part of a tree trunk is a wafer-thin layer of cambium cells inbetween the wood and the bark. This cambium layer is merely punctured by our corers, which are typically only a quarter of an inch in diameter, and the tree barely notices it. The person doing the coring, on the other hand, will notice it, because coring is hard – especially after hours of hiking on rugged terrain and at high elevation. Luckily, on our Mount Smolikas field campaign, the weather was sunny and mild, the forest was free of mosquitoes, and the feral dogs and bears were absent. Also, we were in Greece. After each long day, we replaced our boots with flip flops and sat down for grilled lamb and retsina. Then rinse and repeat the next day. Not only had we sampled a tree so long-lived, we’d found it in one of the oldest civilisations in Europe At the end of 10 halcyon Smolikas days, we had cored and cut cross-sections from hundreds of trees and logs. Yet we would not find out just how old the Smolikas trees and logs were until much later, until we had counted, measured and crossdated all the rings in the lab. After another Smolikas expedition with a different party a year later, we discovered that one of the Bosnian pines on the mountain is Europe’s oldest-known living tree dated with tree rings: more than 1,075 years old. While some have claimed to find older trees, those are often ‘heritage’ trees: they have unique cultural or historical value, but have not been dendrochronologically dated. Their purported old age is based on estimates or unreliable historical records. Others have used carbon-14 isotope analysis to estimate the age of trees, but there’s no technique as precise as crossdating tree rings. We named the oldest Bosnian pine Adonis, after the Greek god for beauty and desire. We were astounded. Not only to have sampled a tree so long-lived, but to have found it in Greece, the country with one of the oldest civilisations in Europe, where people have continuously been using wood and cutting trees for more than 3,000 years. Adonis sprouted when most of Europe was still deeply entrenched in feudalism, it grew continuously for 40 human generations, and is still growing today. Slowly but steadily, it is adding ring after ring on the outskirts of its stem, right under the bark, recording history, year after year. What studying Adonis and its ilk would go on to show us, though, was more than simply its old age. Such ancient trees, we now know, can tell us a great deal more about their life experiences within a world that has transformed as they’ve aged. Adonis is not the only tree with stories to tell, nor is it the oldest. Many of the world’s longest-living trees grow in the Americas. In Chile, El Gran Abuelo, an alerce (Fitzroya cupressoides), was tree-ring dated in 1993 to be 3,622 years old. In the Sierra Nevada mountains of California, the giant sequoias (Sequoiadendron giganteum) can live to be more than 3,200 years old. The world’s oldest known trees, the bristlecone pines (Pinus longaeva), grow just east of there, high up in California’s White Mountains. The stunted and twisted bristlecones are perseverance personified. Methuselah, the oldest-known living bristlecone, aptly named after the legendarily long-lived biblical patriarch, germinated in 2833 BCE. It predates the Pyramid of Djoser, the oldest Egyptian pyramid, by two centuries. Like the Smolikas landscape, the dry, exposed slopes in the White Mountains empower bristlecone pines to persevere for millennia, but they also warrant the preservation of wood after the trees eventually expire. The barren landscape is inhospitable to wood-decaying fungi and insects, and the plant groundcover is too scarce to sustain wildfires. The resinous remains of bristlecone trees can lie on the limestone rocks for millennia. To know the exact year when a tree died, whether it was 7,999 or 8,000 years ago, we crossdate the tree-ring pattern we find in its wood against the pattern in a reference chronology, which is anchored in the present by living trees. Crossdating is pattern-matching. We match the pattern of centuries of wide and narrow rings in one tree with the pattern in another tree. It works because trees that grow in the same area at the same time all experience the same conditions – the same dry years and the same wet years, the same hot summers and the same cool summers – and, as a result, they all form the same sequence of wide rings and narrow rings. All the trees on Mount Smolikas formed a narrow ring in 2024, the driest summer in Greece on record. The giant sequoias in California all show a notoriously narrow ring in the dry year of 1580, making that the most reliable marker ring to date any sequoia sample. In the bristlecone pines, a sequence of narrow rings in the 16th century BCE, caused by a sequence of remarkably cool summers following a sequence of volcanic eruptions more than 3,500 years ago, marks time in any piece of bristlecone wood that is old enough. Based on millennia of exactly dated tree-ring records, we can now date the first Neolithic settlements Because wood cells are dead and wood structure does not change once its cells are formed, the pattern of narrow and wide rings laid down over the centuries does not change once a tree dies. It is preserved in standing snags, in long-fallen and eroded tree trunks, even in subfossil wood preserved in riverbeds and bogs, and in charcoal. Because the tree-ring pattern is preserved, we can crossdate ever-older wood against our reference tree-ring chronology and extend it further and further back in time. By adding the tree-ring sequences found in remnant wood, the bristlecone pine tree-ring chronology now extends back to 6827 BCE, almost 4,000 years before Methuselah, the oldest living tree, grew its first ring. This timespan involves so much of human civilisation – the development of agriculture, of technology and science, of religion, language, writing and culture – that it is hard to grasp. Yet, its length is superseded by the central European oak-pine chronology, which extends back almost 12,500 years without skipping a single year. The majority of the oak and pine trees that contribute to this chronology lived for less than 300 years before they died. But combined and crossdated, they span the entire Holocene, the relatively warm and climate-stable epoch that started about 11,700 years ago. Through crossdating, we can determine the exact year in which each ring in a piece of remnant wood was formed, as long as the wood contains sufficient rings to form a distinguishable pattern. The piece of remnant wood can be a snag or a log in a forest, or a tree that toppled into a bog or a lake hundreds of years ago, but it can also be a human-crafted wood remnant. A beam in a historic building, the wood lining of a Roman water well, the oak panel on which a Flemish Primitive master painted his art, or a Stradivarius violin. Based on millennia of exactly dated tree-ring records, we can now date, to the year, even some of the first settlements that Neolithic humans built on the edge of lakes. For instance, a team of dendrochronologists recently discovered that a Neolithic settlement on Lake Kastoria, about 100 km north of Mount Smolikas, was built in phases between 5328 BCE and 5140 BCE. That is more than 7,000 years ago, at the very start of the spread of agriculture and the construction of settlements from the Balkans throughout Europe. The wooden piles of the settlement have been so well preserved in the anaerobic conditions of Lake Kastoria that we not only know the exact year when the trees were felled, but even that it happened in winter. To fix the date of the waterlogged wooden posts, the researchers measured the radiocarbon content of their individual rings, looking for signature ‘Miyake’ rings, in which the radiocarbon level can be 20 times that of the long-term average. Such radiocarbon spikes – named Miyake events after the Japanese researcher Fusa Miyake who first discovered them as a PhD student – are caused by powerful solar flares, during which the Sun ejects massive amounts of radiation in Earth’s direction. This causes atmospheric radiocarbon to suddenly peak. If such a massive solar storm were to occur today, it could potentially deplete Earth’s ozone layer, disrupt its geomagnetic field, and seriously mess with our technology and telecommunication systems. But luckily such superflares are rare, happening only every few centuries or even millennia, with the most recent superflare recorded worldwide as a radiocarbon peak in tree rings of the year 994 CE. It was the 5259 BCE Miyake event that was recorded in the wood samples from Lake Kastoria. Our understanding of human history through dendrochronology goes much further than dating alone. By comparing the tree-ring pattern in a piece of historical or archaeological wood not just to one reference chronology but to a network of chronologies, we can not only determine the year when a tree was cut, but also where the tree grew. For instance, just earlier this year, an international team of dendrochronologists used the tree-ring series measured on the soundboards of 284 violins made by Antonio Stradivari in late 17th and early 18th centuries to trace the origin of his preferred spruce wood to the high-elevation forests of Val di Fiemme in northern Italy. Not only do the widths of the rings in historical wood tell us about the trials and tribulations that the trees witnessed during their lifetime, the dates when the trees were felled also tell a story. In times when ancient civilisations thrived, construction thrived, which required the felling of many trees and widespread deforestation. The reverse was true as well: in harsh times for human civilisations, forests would thrive. A 2,500-year-long compilation of archaeological and historical wood from central Europe shows felling dates and thus building booms in Roman times and in medieval times, but a lull in building activity during the Völkerwanderung, a period of lasting political and social turmoil that was marked by the Fall of Rome in 476 CE. The felling-date record also shows a building hiatus in the mid-14th century, coinciding with the time of the Black Death. Whereas the bubonic plague outbreak was catastrophic for Europe’s human population, it gave Europe’s trees a breather from relentless deforestation. Trees in California will grumble when there’s a drought, and manifest their discontent through narrow rings Then there are the trials and tribulations that the trees not only witness, but experience themselves over their lifetime. The good years and the bad years, the storms and the floods, the droughts and the fires. All of these climatic conditions can affect whether a tree flourishes or withers, and are stored in its wood. By reading the rings, dendrochronologists can study past climate long before we started measuring weather conditions with instruments, and long before we started interfering with the climate with the Industrial Revolution and fossil fuel emissions. Like people, most trees enjoy talking about the weather. But different trees tell different stories. Trees in dry regions, like California or the Balkans, will grumble when there’s a drought and will manifest their discontent through narrow rings. Their counterparts at high elevations or high latitudes, in alpine and boreal forests, will be more displeased by cold summers than by drought, and will record lackluster summer temperatures in their narrow rings. Dendrochronologists make use of these differing stories by sampling trees in dry regions to study past droughts and in cold regions to study past temperatures. And that brings us back to Adonis, and his story. In the Mediterranean climate of Greece, the width of the tree rings in Adonis and his fellow Bosnian pines serve as a proxy for past droughts in the Balkan region. But to read the temperature history, we also turned to a different characteristic of wood: its density. In warm summers, the pines have a lot of energy to build thick wood-cell walls and dense wood. The reverse is true in cold summers, which are common on Mount Smolikas due to its altitude. Using the tree rings in our Mount Smolikas pines, we assembled drought and temperature proxy records spanning almost 1,300 years. Yet the climate history that the Bosnian pines record over their lifespan extends much further than their local or regional environment. By matching the patterns we find in the Bosnian pines to those of ancient trees in other, climatically connected regions, we can create a much broader-scale view of our climate history. When I first looked at the Mount Smolikas temperature history plot, there was one year that stood out to me: 1976. The summer of 1976 was one of the coldest summers in the Balkans over the past 1,300 years, and that was counterintuitive to me. Where I grew up in Belgium, the summer of 1976 was one of the hottest and driest summers on record. Until the Europe-wide heatwave of summer 2018 (followed by the ones in 2022, 2023, 2024 and 2025), the summer of 1976 had been the heatwave with which all others were compared. We linked subcellular measurements at Earth’s surface to a hemispheric-scale pattern that occurs 10 km above Earth’s surface. And it worked. Everything is connected As it turns out, the summer of 1976 is representative of a seesaw in summer temperatures between the southeast and the northwest of Europe – something we saw when we compared our Mount Smolikas tree-ring record with a 700-year-long tree-ring record from Scotland. Relatively cool summers in the Balkans coincided with relatively warm summers in Scotland, and vice versa. That meant that we could use these two tree-ring records to study the history of the mechanism driving the summer temperature seesaw, namely the jet stream – the band of eastward-flowing winds that circle the entire Northern Hemisphere at the altitude at which airplanes fly. Even though the jet stream is located high above Earth’s surface, its wavy pattern can create dramatic climate extremes at our planet’s surface. The jet stream doesn’t move across the Northern Hemisphere in a straight line, but rather meanders like a snake. At times when it buckles, undulating in big northward and southward waves, it can create heatwaves in some locations that coincide with cold spells in others. Such big curves slow down the jet stream, meaning that it stays in position longer, setting the stage for weather extremes: eg, week-long heat domes instead of a hot summer day; or floods instead of a rainy day. The jet stream can create temperature seesaws, such as the one between the Balkans and Scotland. By combining the tree-ring records from the two nodes of this seesaw, we succeeded in reconstructing year-to-year changes in summer jet stream location over the past 700 years. Let me rephrase: we linked subcellular-level wood measurements at Earth’s surface to a hemispheric-scale pattern that occurs 10 km above Earth’s surface. And it worked. Everything is connected. What makes such tree-ring records so powerful is that they predate human-made climate change. To disentangle the contribution of climate change from jet stream patterns in the climate extremes that wreak havoc to our societies, we need a jet stream record that extends back further than the Industrial Revolution. With 700 years of European jet stream history, we can also spot relations with events as described in historical documents. For instance, starting in the 5th century, with the arrival of Christianity in Ireland, Irish monks faithfully recorded and described notable societal events in the Irish Annals. This more than 1,000-year long historical predecessor to the Game of Thrones includes detailed descriptions of wars, political intrigues and the plague, but also reports on storms, droughts and other extreme weather events. In Italy and Greece, scribes have been writing about floods and heatwaves since the early 16th century, and about grape harvest and wine quality in the Balkans since the 17th century. Sure enough, the jet stream seesaw we saw in tree-ring records is reflected in these historical documents. In summers when the jet stream was further south than normal, the British Isles would suffer from heatwaves, while floods and storms would pummel the Balkans, where the wet and cold summers would delay grape harvests, lower grape yields and diminish wine quality. And vice versa. Over the past centuries, most wildfires in the Balkans happened when the jet stream was further north than normal and Balkan summers were even hotter and drier than normal. To our surprise, we also found that the summer jet stream may even have orchestrated the spread of the plague. The Black Death thrived in cool and wet summers, which the jet stream brought to the Balkans when it moved south and to the British Isles when it moved north. As a case in point, the Irish Annals report that the plague devastated Ireland from 1348 to 1350, when the jet stream was positioned far further north than normal. By reading the rings of the Mount Smolikas pines, we found that a jet stream-driven seesaw has controlled not only European summer weather extremes since the Middle Ages, but also societal extremes, such as harvest failures, epidemics and human mortality. Our findings are a harbinger for what to expect when not only the jet stream itself becomes more erratic under anthropogenic climate change, but when jet stream-driven climate extremes are exacerbated by it. Trees are witnesses. Witnesses of our forest history, of our climate history, of our human history. But they are also witnesses of our present. The man-made climatic extremes the world is experiencing today will all be recorded in their wood – as will whatever we do next. Centuries from now, this story will be there for future dendrochronologists to core and to read, just as we did with Adonis and his neighbours on the slopes of Mount Smolikas. So, let’s make sure we give the trees something good to talk about.