Written in the wood

With its stout orange trunk and long, graceful needles, the tree looks like any other ponderosa pine growing on Mount Bigelow. But a sliver of its wood, taken amid Earth’s warmest year on record, shows that this tree has a story to tell — and a warning to offer.

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Bigelow 224 germinated nearly 200 years ago — a spindly sprout rooted in meager soil. Yet the pine has grown taller and wider each year by adding another ring to its trunk.

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Amid the balmy temperatures and lengthening days of the spring and summer, the newly formed tissue — known as earlywood — was the pale gold of morning sunlight. When autumn arrived, the tree switched to denser and darker latewood, signaling the beginning of the end of the growing season.

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All that the tree experienced — the winds that shook its branches, the rain that soaked its roots — was recorded in the rings. An extra-wide band attested to the prime growing conditions of 1856.

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A thin line bisecting the following year’s ring is the relic of a springtime drought that ended with the arrival of a summer monsoon.

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For centuries, Bigelow 224 stretched sunward while history unfolded below. The tree witnessed the rise of industrialization and the devastation of Native communities. It watched Arizona become the nation’s 48th state in 1912. Generations lived and died, wars were lost and won, humans walked on the moon and transformed Earth. Still, the tree has survived.

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But then came 2023, the hottest year that humanity — and Bigelow 224 — had ever seen. All around the planet, temperature records fell like dominoes. Up on Mount Bigelow, an unrelenting heat wave made the air feel like an oven and sucked moisture from the thin soil.

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The toll of those unprecedented conditions was etched into Bigelow 224’s trunk. Scorched by relentless heat and parched by a delayed monsoon, it appeared to stop growing midway through the season. The ring for this year is barely a dozen cells wide.

It is a silent distress signal sent by one of Earth’s most enduring organisms. A warning written in wood.

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For decades, scientists have used rings from long-lived trees like ponderosa pines to uncover clues about ancient climates. Their analyses helped prove that human-made greenhouse gas emissions are warming the planet and demonstrated that modern climate extremes are unprecedented in the last 1,000 years.

Now, cutting-edge techniques are allowing researchers to observe how the rings form in real time. Studies of Bigelow 224 and its neighbors show how recent disasters leave an indelible imprint on tree cells — information that will sharpen researchers’ view into the past and help them predict the fate of forests in a fast-warming world.

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Unearthing the future

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A series exploring how clues from Earth's past can help humanity confront modern climate change.

Part 1

Hidden beneath the surface

Part 2

Buried under the ice

Part 3

Ancient warning of a rising sea

Part 4

Written in the wood

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“The power of tree rings … is they have the context of what we’re experiencing now,” said Kiyomi Morino, a tree ring scientist at the University of Arizona who has spent much of the past decade studying the trees of Mount Bigelow. “They can give us some perspective.”

“And in many cases these days,” Morino added, “it’s perspective on how bad things really are.”

Morino had made the drive up Mount Bigelow so many times, they knew the route by heart. Heading east out of Tucson, Morino passed first through a dusty landscape studded with saguaros and spindly ocotillo cactuses. Then the road angled upward, and the desert gave way to grasslands and thickets of feathery mesquite.

With every foot of elevation gain, the temperature dropped and the trees grew taller. Morino’s van trundled through bizarre rock formations surrounded by chaparral, then into the dappled sunshine of an oak forest, until finally reaching their destination. This isolated mountain range was known as a sky island, because its high altitude and relatively low temperatures allowed forests to grow above a sea of rock and scrub and sand.

Morino stepped out of their van and breathed the sharp, late November air. Honey-colored light streamed between the trunks of ponderosa pines and Douglas firs; yellowed grass crunched underfoot.

Just ahead, one of the pines bore a small metal tag that read “224.” Morino smiled at the familiar tree.

“I feel at home,” they said.

Ever since arriving at the University of Arizona’s Laboratory of Tree-Ring Research in 1992, Morino has felt that same sense of belonging — a surety that this is the work they were meant to do and the place they were meant to be. When Morino discovered a few years ago that their first name could be translated from Japanese as “tree reader,” it seemed a little bit like fate.

The lab is known as the birthplace of dendrochronology — the science of telling time through trees. Down in its archives, Morino had access to the world’s largest collection of tree samples: massive cross sections of giant sequoias, pencil-shaped cores taken from stalwart Alaskan pines, roof beams of historic buildings, charcoal from fires that burned out long ago.

And up in the mountains, just an hour from their Tucson office, Morino was surrounded by some of nature’s best recorders of environmental change.

The conifers that grow in this parched and punishing landscape are so sensitive to fluctuations in temperature and moisture that even tiny shifts in weather are reflected in the width of their rings, Morino said. But the trees are also hardy, capable of producing powerful resins to trap insect pests and growing thick bark as a buffer against wildfire flames.

This means they can preserve evidence of events that no human remembers — or ever saw. Though the National Weather Service’s records go back only until the 1890s, and thermometers have existed for just over 300 years, ponderosa pines and Douglas firs are capable of living at least twice as long. Farther west, in California’s White Mountains, scientists have discovered bristlecone pines that are as ancient as Egypt’s pyramids.

“Some of the oldest trees grow in the harshest environments,” Morino said. “There’s benefits in adversity.”

The more time Morino spent immersed in the science of tree rings, the deeper they wanted to go. How did a tree create the cells that eventually became its wood, Morino wondered. What might the shape and size of a cell say about the climate at the specific moment it was formed?

Morino was especially intrigued by a feature called a false ring — a line of denser wood that formed in the middle of an otherwise normal year. False rings were known to occur in monsoon climates like Arizona’s, where long months of dryness are punctuated by a violent burst of rainfall partway through the summer. But what exactly happened during ring development to make the wood turn out this way?

These questions couldn’t be answered by looking at rings from the past. So Morino launched an intensive experiment in 2014 to study how Bigelow 224 and its neighbors developed their rings in real time — a process known as xylogenesis, from the Greek words for wood and birth.

“Being able to observe and understand the mechanisms, the nitty-gritty of how a tree ring forms — for me, that just opens up our ability to interpret rings that were formed hundreds of years ago,” they said.

Week after week, year after year, Morino drove to the mountaintop field site to check on the few dozen firs and pines they had chosen to follow. At each tree, Morino would extract some of the thin layer of tissue that exists just beneath the bark. Viewed under a microscope, those samples allowed them to examine each stage of development for that year’s ring.

The growing process always began around mid-March, when increasing sunlight and melting snows woke the forest from its winter slumber. It was time to build.

First, the tree’s living tissue — called the cambium — generated a ring of tiny cells. Then the tree sucked up water from its roots and pumped the liquid into the new cells until they inflated like balloons. Over the course of several weeks, if enough moisture was available, the cells could grow up to 10 times their initial size.

Next, the tree reinforced the cell walls using long chains of cellulose, the fibrous sugar that gives fruits and vegetables their crunch. Lignin — another tough chemical compound — was added to make the tissue rigid and strong. By locking up carbon in these long-lived molecules, the tree could keep it out of the atmosphere for hundreds of years.

Finally, to make the wood functional, the cells themselves had to die. Chemical cleanup crews rid the cells of their contents, leaving only the empty cases of the cellulose and lignin-reinforced walls.

That didn’t mean the tree stopped growing. As long as it had enough water and light, the cambium kept creating new layers of cells, gradually increasing the width of that year’s ring.

But as conditions changed — when shifting seasons brought shorter days and drier soils — the trees stopped producing new cells and focused their resources on building up the walls of ones they already had. Gradually, the large, light cells that made up spring and summer’s earlywood gave way to smaller, thicker latewood cells in the fall — creating the dark, dense line that would help distinguish that year’s ring from the next.

By the time Morino returned to Mount Bigelow in November, most of the trees were finished growing for the season. But tissue samples could still reveal how the forest had fared this year.

Standing before Bigelow 224, Morino pressed a metal coring device into the trunk and lightly tapped it with a rubber hammer. Everything was silent but for the soft thump of the hammer and the whispering of the wind among branches.

When they pulled out the instrument, it revealed a delicate microcore no bigger than a matchstick. The entire story of this growing season was contained right at the end, in a piece of pale wood almost invisible to the naked eye.

Morino loved this process — the way it immersed them in the forest’s quiet rhythms, pulling their thoughts out of the complicated past and worrisome future, anchoring them in the now.

In a way, coring helped Morino experience time like a tree. Despite their long memories, ponderosa pines were attuned to the present moment — always altering their growth in response to shifting environmental constraints.

For example, if the early summer weather became too hot and dry, the trees would close the pores on their needles through which they admitted air for photosynthesis. They became less choosy about the carbon they consumed — opting to use a heavier, less nutritious form of the element that was already available in their tissue, rather than risk losing moisture through open pores. Put simply, Morino said, the trees starved themselves to avoid dying of thirst.

Meanwhile, cell production slowed down, and the cells themselves were made smaller. The emerging wood looked dark and dense, almost like the latewood that usually appeared later in the year.

This, Morino’s research showed, is what created the false rings seen in so many southwestern tree cores. The arrival of the summer monsoon usually allowed the trees to resume regular cell production for a few more months. But the thin lines of drought-formed cells remained in the wood — the signatures of a species acutely adapted to its environment, permanent evidence of the trees’ commitment to careful growth.

“In some ways, that sensitivity really helps them,” Morino said. “The trees are sensing it’s getting drier and warmer and slowing down cell production and waiting for summer rains … versus kind of going gung-ho and using up all your resources, and then you’re screwed.”

Yet even the most stalwart southwestern trees struggle to cope with climate change’s escalating toll. Looking across the slopes of Mount Bigelow, Morino spotted several pines with skimpy canopies of rust-brown needles — telltale signs of an organism under fatal stress.

“These trees were fine two or three or four years ago,” Morino said.

Those trees will be counted among the many casualties of Earth’s hottest year in recorded history. Global temperatures were 1.4 degrees Celsius (2.5 degrees Fahrenheit) above the preindustrial average. In Canada, some 5.2 million acres of forest were scorched by wildfires this summer. The amount of sea ice around Antarctica dwindled to an unprecedented low. A historic drought turned swaths of the once-mighty Amazon into a wasteland of dead fish and cracked mud.

Amid a record-setting 53 straight days of temperatures above 100 degrees Fahrenheit, the Tucson region saw 176 heat-related deaths this year. And up on Mount Bigelow, July temperatures were a stunning 3.6 degrees Celsius (6.5 degrees Fahrenheit) above normal. The hotter air acted like a sponge, sucking every drop of moisture from the soil. But the first monsoon rains — which usually arrive in mid-June — came more than a month late, leaving parched plants gasping for a drink.

Bigelow 224 seemed relatively healthy when Morino visited in late November. But they knew from experience that a tree’s outward appearance doesn’t always reflect the hardships it has endured.

“You can’t really tell anything until you look under the scope,” Morino said. “That’s where the discovery happens.”

Back in the lab, Morino sliced the microcores they had collected into even thinner pieces. Each was placed on a glass slide, then stained purple to reveal the shape of its cell walls.

Then Morino switched on a digital microscope and placed 224’s slide beneath the lens.

Bigelow 224’s most recent growth appeared in vivid, violet detail on an adjacent computer screen. The rings from 2021 and 2022 looked relatively normal: Each was dozens of cells wide, with a robust strip of dense latewood.

But this year’s ring was less than a third of the size of the others, and it hardly contained any latewood.

“That’s a pretty narrow ring,” Morino said, frowning. “That’s really interesting.”

They pointed to the thin band of latewood squished up against the pinkish cells of the tree’s bark. It looked as though the tree had started to form a false ring, but then never resumed its normal cell production after the monsoon’s late arrival. But that would mean the tree’s last cells formed in July — months before the usual end of the growing season.

Morino pulled the slide from the microscope and swapped in a new sample taken from a ponderosa pine just a few yards away. It, too, bore a strangely narrow ring with just a few rows of denser latewood cells.

They switched it for another sample. Then another. Each sliver of wood showed the same pattern.

“Wow,” Morino said. Sitting back in their chair, they puzzled through what the narrow rings might indicate. Could the record heat and delayed rainfall have stressed the trees so much that they stopped growing in the middle of the year?

“Maybe the trees were like, ‘Okay, the monsoon is not coming, and this is costing us a lot to keep forming this wood, so we’re going to sort of call it a day,’” Morino said.

They would need to conduct more analyses — and sample many more trees — to know for sure why the thin rings formed. But Morino’s mind was already racing with new questions. Had this pattern appeared in previous years? Could it be used to uncover droughts and delayed monsoons from the past?

Even more important were the implications for the future. What would happen to the trees — and the carbon they pulled out of the atmosphere — if the planet was truly getting too hot for them to grow?

Tree rings are often compared to ancient texts in an obscure language — each line saying something distinct, depending on its width.

But the more scientists learn about the cells that make up tree rings, the more they realize they have been missing huge parts of the story. Trees aren’t just documenting the average temperature and rainfall of a given year, Morino said. They are taking detailed notes about the events that shaped their daily lives. Written into the shapes and sizes of their cells are accounts of spring droughts and summer monsoons, heat waves and avalanches, the rare peaceful moment in the sun.

It is a whole new dialect for scientists to master. But if they can learn to read this cellular language, they will be able to learn more from tree rings than ever before.

In an office just down the hall from Morino’s, climate scientist Kevin Anchukaitis hopes xylogenesis research will enhance scientists’ view of the past, enabling them to compare modern climate disasters with events that occurred long before humans began tracking the weather. If he and his colleagues can identify individual heat waves, floods and storms in the tree ring record, they will know whether today’s extremes are truly without precedent — and how much more frequent such events could eventually become.

Another of Morino’s colleagues, ecophysiologist Jia Hu, says cellular analyses will help illuminate the fates of forests themselves. By tracking the forms of carbon that ponderosa pines pack into their cell walls, she has found that western trees are showing more signs of stress amid drought than they did just a few decades ago. That’s probably a consequence of the fact that dry conditions are increasingly exacerbated by scorching temperatures, making it harder for trees to cope.

“We can see that there’s a tipping point,” Hu said. “We’re pushing these trees kind of beyond where they’re adapted.”

Even if warming doesn’t completely kill the trees, Morino’s findings on Mount Bigelow hint at how it might harm one of their most essential functions: storing carbon. If water shortages force trees to form smaller cells and fewer ring layers, they will have less space to put the carbon that they pull out of the air.

Research suggests that forests absorb about 20 percent of the greenhouse gases people release into the atmosphere each year. If that number dwindles, Hu said, humanity will be losing one of its greatest allies in the fight against climate change.

In the days after their visit to Mount Bigelow, Morino’s thoughts lingered among the trees they left behind. Bigelow 224 had survived nearly two centuries by respecting the constraints of its environment. In curtailing its cell production this summer, the tree may have given itself a chance to live another year. But not even the hardiest ponderosa pine possesses endless stores of resilience, Morino knew. The forest couldn’t tolerate many more summers like this one.

Then Morino thought about their home city of Tucson, with its glass buildings and gleaming green golf courses, much of it sustained by water transported from hundreds of miles away. They contrasted the careful growth of pine trees with the behavior of a species that would erect a city in the middle of the desert. That would divert the entire contents of rivers to farm fields and factories. That would continue to pollute the atmosphere even as their communities flooded and baked and burned.

This extreme year should be a wake-up call, Morino said. Just like the forests, people now face a climate unlike anything previously experienced — a climate that will test humanity’s exceptional capacity to endure.

But, Morino added, people can also change. They can learn to live within the limits of the planet, to heed the warnings written in wood.

“If there’s any lesson to be learned from trees by us, by people, by societies … it’s this,” Morino said. “That’s how we’re going to survive. That’s how the trees survive.”