A Story About Once and Future California, Written in the Rings of Redwoods

February 8, 2017

The core sample from the oldest coast redwood tree ever found is a thin rod of wood the width of a pencil, a couple feet long, and densely scored at irregular intervals by dark brown stripes. In this compact package, exposed to the air and sitting on a table in front of me at the Humboldt State University Institute for Redwood Ecology, lies 1,500 years of history from a tree dating back to 328 CE.

Dendrochronologist Allyson Carroll hand-counted each of the 1,498 minuscule rings in this core. And not just this sample, but hundreds more; in all, Carroll has counted more than a quarter of a million coast redwood tree rings.

She does it because these little rings have immense power. In the labs of scientists, cores like this will have another life. Rings will be counted, measured, and combusted into vapor, and their elemental origins will be tallied, all in an effort to peer into the deep history of the earth.

Coast redwoods (Sequoia sempervirens) are famous for being the tallest trees in the world. Redwoods shoot up hundreds of feet into the air and hail from another age, their ancestors arriving in the fossil record over 200 million years ago. Yet the tiny spaces between the rings – spaces so small scientists peer at them with microscopes – tell big stories about the past that no human alive has seen, and a future we have yet to understand.

a redwood core with rings
Dendrochronologist Allyson Carroll points to “wedging” in a redwood slab, where rings pinch together and seem to disappear. (Photo by Lucy Kang)

From the Bay Area, the route to the redwoods is straightforward. Highway 101, appropriately known as the Redwood Highway, heads to the heart of redwood country, punctuated by roadside attractions (“World Famous Grandfather Tree!” “Trees of Mystery!”) that testify to the longevity and size of redwoods.

Humboldt State sits tucked between the northern California coast and the redwood groves of Arcata Community Forest. It’s here that Allyson Carroll strung together the information from hundreds of thousands of tree rings to build a coast redwood timeline, a critical key to deciphering the historical climate information contained inside Sequoia sempervirens. Hers is the longest coast redwood chronology established to date, made through three years of work and funding from Save the Redwoods League’s Redwoods and Climate Change Initiative, a groundbreaking long-term research project exploring how climate change will impact redwoods for the foreseeable future.

Humboldt State University dendrochronologist Allyson Carroll, sitting at a microscope with coast redwood core samples in the Institute for Redwood Ecology. (Photo by Lucy Kang)
Humboldt State University dendrochronologist Allyson Carroll, sitting at a microscope with coast redwood core samples in the Institute for Redwood Ecology. (Photo by Lucy Kang)

The Institute for Redwood Ecology operates out of a sunny office next to a lab reminiscent of high school wood shop, with tool-lined walls and stacks of circular wood slabs arranged like wedding cakes. Photos of living redwoods adorn the walls, while remains of dead trees lay scattered across the room. A massive slab, maybe three or four feet in diameter, sits on a table, like a specimen waiting to be dissected. A lone graduate student supervised by Carroll is in the lab over spring break in an otherwise deserted campus, measuring redwood samples that will eventually be dated using the chronology Carroll constructed.

In show-and-tell style, Carroll holds up a blue and silver increment borer, a tool that extracts tree cores from living trees. She turns it to show me the sharpened metal tip that’s twisted into a trunk to extract the core, like a comically large corkscrew handle. This simple hand tool was the first step in rewriting the climate history of northern California.

“It’s a tried and true way to get the sample,” Carroll says. “It’s simple and elegant.”

The tools of tree ring study, like the increment borer, haven’t changed much since the beginning of dendrochronology in the early 20th century.

At heart, dendrochronology (from the Greek for “tree” and “the study of time”) is a down-to-earth science, grounded in things anyone can observe without the use of scientifically obscure machines.

It’s common knowledge that each alternating light and dark band in a tree’s cross-section is a testament to a year in its life, a record of the wood’s natural growth during each season. Leonardo da Vinci reputedly observed as much in a painting treatise in the 15th century. But the so-called “grandfather of dendrochronology” A.E. Douglass developed a science of cross-dating, or a systematic search for patterns of variation of tree rings over specific years. The size of tree rings shrink and grow depending on things like how much water or light the tree is getting.

“Picture it like a bar code,” Carroll says. She shows me the tiny pencil marks that cross the landscape of a tree core. Like bar codes that store and track information, patterns of wide and narrow tree rings store and track climate data. Pieced together and aggregated, cores can form a tree ring chronology, a map of the average width of tree rings for each year that can be mapped against known climate data and then extended into the far past.

Cross-dating only works if you can pinpoint which rings come from which years. It’s simple with trees that predictably grow a ring every year, like giant sequoias, for which Douglass developed a 3,000-year tree ring chronology. But it took decades for the coast redwood chronology to be pushed back further than a millennium, let alone two.

redwood rings under a microscope
Annual rings of a coast redwood core sample, as seen under a microscope. (Photo by Lucy Kang)

Blame the unruly growth of coast redwood tree rings. Coast redwoods are notoriously tricky to cross-date, with plenty of skipped years, false rings, and wedging (“under the microscope, it almost looks like someone smushed them all together,” Carroll says) to throw off all but the most diligent researcher. The oldest tree ring Carroll sampled skipped a whopping 186 rings. It is only by matching any given sample against a larger, more robust group of cores that dendrochronologists can tell whether rings have been skipped.

It may be meticulous work, but it’s relatively straightforward. “All we use is like a dollar’s worth of sandpaper,” Carroll says. “A lot of tools of modern science are very technical, very expensive. But the simple increment borer and some glue and sandpaper, some core mounts, and a microscope, and you can really start going.”

Several core samples are extracted from each tree at multiple heights, then taped and glued to wooden mounts for stability and sanded by hand. Carroll lays a dozen samples down on the frosted glass table to illustrate. In one sample, the rings disappear into a dark block. These are scars from fires, retained by trees and written into the physical record of their bodies. Carroll is looking for clues like these when she examines each ring in the samples, methodically marking out the years with a pencil and logging the data into a computer program.

“See that dot there?” she says, pointing to a pencil marking on the side of a core mount. “Each dot is a decade in pencil, every pencil.”

Dot by dot, Carroll counted her way into the deep past. “I set out a grid of marks, just using a number 2 pencil.” In the end, she pieced these dots together into a coast redwood chronology stretching back 1,685 years that can now be used to date other trees and samples.

It gets boring, she agrees. “And you can’t force it,” she says. “Like if you try to dig a hole, you can just go faster, just do it. But if you’re trying to cross-date a tree, you just have to take your time and do it methodically to make sure that you’re doing it right.”

And yet the tedium, the kind of persevering research so little mentioned when we talk about science, pays off. In the case of dendrochronology, cross-dating has been used variously to date a Stradivarius violin, aid in the manhunt for Ted Bundy, and unravel the nearly 300 year old mystery of a Japanese “orphan tsunami” in 1700 that was, finally, traced to a magnitude-9.0 earthquake in the Pacific Northwest.

But this is only part of the story told by the tree rings. To continue the story of deep time of Earth’s climate history, we have to zoom down into world invisible to human eyes. The thread continues in ever smaller spaces contained within these giant trees, past even the resolution of microscopes, at the level of atoms and isotopes.

Coast redwood core samples examined by HSU dendrochronologist Allyson Carroll. The four central samples are from, in order of left to right: 1) the oldest coast redwood tree sampled, dating to 328 CE, and tightly packed with rings; 2) a young redwood tree sample, with wide rings; 3) a sample with visible black scars, where the tree was damaged but not killed by fire; and 4) a sample with pencil marks that aid in the tree ring counting process. (Photo by Lucy Kang)
Coast redwood core samples examined by HSU dendrochronologist Allyson Carroll. The four central samples are from, in order of left to right: 1) the oldest coast redwood tree sampled, dating to 328 CE, and tightly packed with rings; 2) a young redwood tree sample, with wide rings; 3) a sample with visible black scars, where the tree was damaged but not killed by fire; and 4) a sample with pencil marks that aid in the tree ring counting process. (Photo by Lucy Kang)

Just as dendrochronology deciphers the climatic history of our world by reading the cross-sections of its oldest living inhabitants, isotopic analysis does so by peering deep into their atomic structures.

Todd Dawson has done a lot to advance this science. He’s enjoyed a decades-long professional love affair with redwoods, and his work, like Carroll’s, has been funded under the Redwoods and Climate Change Initiative.

Redwoods loom large in Dawson’s imagination. When he’s around them, it’s hard not to conjure up a comparison between human and near-mythic giant. “You just feel so small compared to this tree that stands hundreds of feet above the ground and has lived for so many hundreds of years,” he says. “They mean a lot to me. They’ve gotten under my skin, in my blood.”

Dawson’s intellectual fascination with redwoods has spanned a career – closer to a lifetime, actually. The first spark of interest ignited as an undergrad at UC Santa Cruz, a campus home to dozens of majestic, towering trees that Dawson passed every day on his way to class.

Add to that bucolic setting the tinder of a pivotal ecology class led by legendary botanist Jean Langenheim, who didn’t lecture merely about what was currently known about redwoods. She took one step further and explained all the gaps in knowledge that remained from a full scientific understanding of these stately giants.

“I thought, gosh, we don’t know very much about what fog does, or what their physiology is, things like that,” Dawson says.

After he received his PhD Dawson returned to California as a professor at UC Berkeley, and to the subject that had sparked his interest decades ago, to build a research program around redwood ecology and particularly isotopic analysis of redwoods.

As we sit in his office in Berkeley Dawson pores over a computer screen filled with turquoise and peacock swirls of real-time air currents over North America. Currents like these create the great foggy wall that creeps into San Francisco during the summer months. “You walk into a redwood forest in the summertime, and it’s freezing. The trees are actually creating their own kind of climatic conditions out there.”

Fifteen years ago, Dawson realized that redwoods absorb water directly through their leaves – a scientific “game changer,” as he calls it. In the summer, redwoods can get up to 45 percent of their water from fog in the absence of rain, as their needles collect and condense the moisture. But other species also benefit from these foggy interactions, like sword ferns, some of which get all their water from the dripping leaves of their looming neighbors.

“You walk into a redwood forest in the summertime, and it’s freezing,” Dawson says. “The trees are actually creating their own kind of climatic conditions out there.”

Dawson can pinpoint exactly how much fog coast redwoods have absorbed in a given year by measuring the isotopes in their tree rings. Isotopes are atoms of the same element that have different numbers of neutrons. Take carbon, for example, the building block of life on our planet. The most common isotope, carbon-12, carries 6 protons and 6 neutrons. But naturally occurring carbon-13 has an extra neutron that makes it slightly heavier. Trees that are stressed will have more of the heavier carbon-13. Likewise, fog water has higher levels of the oxygen-18 isotope. It’s a small difference in weight, but an important one.

In a recent, yet to be published research project, Dawson paired up with Utah State University’s Steve Voelker and Southern Oregon University’s John Roden to reconstruct past climate based on coast redwood tree ring widths and isotopic analysis.

Voelker, a dendrochronologist, cut slabs from fallen trees along Highway 101 and cross-dated them. Barred from using a chainsaw because of the breeding season of the threatened marbled murrelet, he used an old cross saw salvaged from a defunct redwood sawmill – one likely used to log hundreds of redwoods some hundred years ago, now pressed into the service of redwood conservation.

With these slabs and another from HSU’s Institute for Redwood Ecology by way of Allyson Carroll, Voelker compiled a dendrochronology that went back to the year 900 CE, which, paired with isotopic analysis, gave more clues – and a surprising revelation – to our current story of past climate.

“We can go way, way, way back in time,” Dawson says.

Todd Dawson in his lab
UC Berkeley Professor Todd Dawson standing in front of a water extraction line, used in studies to determine whether redwoods use fog water, in the Center for Stable Isotope Biogeochemistry. (Photo by Lucy Kang)

One thousand, one hundred some years is both a long time, and not.

By one measure, this doesn’t even begin to touch geologic time, the scale against which the births and deaths of mountains are counted – known as deep time. Scottish geologist James Hutton fathered the concept when he intuited that striations in rocks pointed to a much longer history of the earth than previously accepted.

Deep time is on the scale of millions of years. Yet to humans, with our inherent century-long expiration dates, such a time becomes nearly mythological, on the scale of Noah’s 950 years in the Book of Genesis.

But where it gets interesting is where one thousand, one hundred some years overlaps with our time scale. This amount of time can be thought of in terms of generations lived, empires risen and fallen, or knowledge gained and lost.

Or the lives of old redwoods.

Old Giants: The Last Days of Oakland’s Redwoods

In the hills east of Oakland, a pocket of ancient and enormous redwoods thrived just where the ocean fog reached and rested, in a small band spanning the ridge from present-day Redwood and Leona parks east to Moraga. It took little more than fifteen years, from 1845 to 1860, for crews of loggers to level the entire forest. Read more

Old trees perform a magic trick of walking between two worlds. They are a bridge between everyday, human time and the incomprehensibility of deep time. Artist Rachel Sussman, who documented the world’s oldest living organisms over the span of a decade, calls them “living palimpsests.”

“Trees contain layers of history within themselves – their own individual histories, as well as the larger natural and human-generated events from weather to fire to pollution,” Sussman said in an email. “Their stories are waiting to be told, and dendrochronology is their storyteller, unlocking narratives that we could not otherwise bear witness to in a single, human lifespan.”

Those stories contain valuable, yet vulnerable information about the earth’s past. The death of old trees is like the fire of the Library of Alexandria, writes Ross Anderson in Aeon, “an event horizon, a boundary in time across which information cannot flow.”

It is thanks to the longevity of redwoods, both in lifespan and in physical building material, that such a metaphoric library even exists. The rich auburn heartwood that gives redwoods their name is notoriously resistant to the rot and decay of the descending years, even after the tree has died. This is why coast redwoods make such excellent records, giving scientists the ability to peer back thousands of years in time while weather records in California stretch back only a century and a half.

Californians sometimes hold onto an illusory assurance of knowing what “normal” is. Unfortunately, periodic drought, flood and fire may actually be hallmarks of the natural landscape, says urban theorist Mike Davis. Short-sightedness and lack of long-term data, Davis writes in The Ecology of Fear, coupled with a desire to “get on with things,” spell periodic and yet inevitable disaster for Southern California and a public unprepared for the intervals of severe climate events.

The rest of the state shares in this fate, as Dawson and Voelker’s team would find out.

Berkeley combustion chamber
A combustion chamber in an elemental analyzer at UC Berkeley’s Center for Stable Isotope Biogeochemistry, used to determine the isotope ratio of coast redwood samples. (Photo by Lucy Kang)

Californians are too familiar with the state’s recent five-year exceptional drought. But the redwood tree records seem to be saying that this is the new normal. Dawson and Voelker uncovered evidence of long-term climate cycles that unfold over decades or even centuries. The longest of these occurs on a period greater than the average human lifespan, yet touches the lives of all Californians today.

“This long record is really now painting a new picture that drought is probably more common than we thought in the tree ring records,” Dawson says. “So the redwoods are telling us a new drought story.”

Embedded in the tree ring data is evidence of not one but three distinct cycles ranging from the yearly to the centennial. The most familiar, the El Niño-La Niña cycle, takes place over a few years and involves dramatic changes to the pattern of winds and water temperatures across the Equatorial Pacific. The Pacific Decadal Oscillation is a decades-long cycle in the Northern Pacific in which the waters off the West Coast fluctuate between warmer and cooler surface temperatures. The longest cycle the team found spans over a century and potentially involves wind patterns that connect the North Pacific with the North Atlantic.

Dawson says the redwood cores hold the first record of all three cycles. The tree ring records show that California is caught in the middle of climatic cycles that are so long, we’ve been ignorant of them until recently. What it also means is that the worst drought we’ve seen may actually be the norm when taking the long perspective.

“This drought, which we think of as very severe, definitely was not as severe as some of the past droughts that were recorded in the redwood tree ring records,” Dawson says. “We have at least eleven droughts we know of that go back in time that were either as severe or more severe than the one that we’re currently going through.”

The recent drought, says Dawson, seems to line up perfectly within the century-long drought cycle his team found. And they also have evidence of a twenty-year long drought in the mid-8th century, when they suspect that all three drought cycles may have converged at the same time. Drought is looking like an unavoidable part of the landscape. And that’s a problem for Californians.

Paleoclimatologist Lynn Ingram believes that the state is currently overdrawn on its water reserves because of a fundamental overestimation of water reserves, arising from the partial weather records collected by the state that go back only to the year 1850.

“Imagine a performance of a Beethoven symphony with only the first chords, played over and over again,” she and co-author Frances Maud write in The West Without Water. “If that were all we heard of the symphony, we would believe it was beautiful – but simple. We could hum along and predict what would come after the initial chord sequence. However, as we know, Beethoven wrote complex and intricate symphonies, often full of surprises.”

A long view of the climate record is full of surprises. Using records like tree rings, lake sediments and other forms of proxy data, paleoclimatologists discovered centuries-long droughts in the past. The Medieval Climate Anomaly from around 900 CE to 1300 CE saw two such megadroughts that offer a bleak possible vision of the future. Yet, the recent past saw relatively benign climate – a mixed blessing that led to rapid development of California’s water resources and growth in population.

“We’ve developed all of our surface waters, and we’re pumping all our groundwater,” Ingram told me. “And we’ve kind of maxed out. We’re using the most water we can use, so if suddenly we went into average 40 percent less water, which is what it was like during the medieval time for one to two centuries, we’d have to really adapt to that.”

What will happen in the future, when these long-term climate cycles meet man-made climate change, is anybody’s guess. It’s not just that climate change may be causing severe droughts. Rather, it’s that we didn’t even understand our climate baseline to begin with. And now humans are changing it in ways we simply don’t understand.

The world has recently crossed a new and dubious threshold, one where CO2 levels in the atmosphere have passed and are likely to stay past 400 parts per million (ppm), more than the 350 ppm level deemed safe by some. Until last year, the world hadn’t seen these levels since 4 million years ago.

Humanity’s mark on the earth is undeniable. Sitting up in an airplane, thousands of feet in the air, Dawson can’t help but notice all the landscape changes humans have wrought – swaths of crop fields, wrapped by snaking lines of roads.

“We are now moving into a climate changing world,” Dawson says, “at a pace that the earth has never experienced in all of the records that we’ve ever collected anywhere, and throughout all of our fossils, our tree rings, our corals.”

coast redwood grove
(Photo by Evan Spiler,

At Carroll’s suggestion, I drive to the redwood groves at Bull Creek Flats in Humboldt Redwoods State Park. Old growth forests like this one call to mind not just to the past, but the future too. According to research from the Redwoods and Climate Change Initiative, old growth redwood forests are the biggest sequesters of carbon, storing three times as much as other forests.

I’m thinking about all this when I pull up into the nearly deserted parking lot off a dirt road. The trail I follow meanders through redwoods of all sizes and their attendant ferns, ubiquitous moss and hanging lichen – an excessive display of all the different greens nature can devise, from chartreuse and viridian to deep emerald.

My walk is an exercise in trying to pick out signs of deep time. I breathe in, deliberately trying to attune myself to a different time scale. It helps that there’s not a sound to be heard, except when I’m near the creek. I press myself to discern some sort of primordial significance lingering in the air, but mostly I just sense the heavy humidity and fog drippings collecting on the moist earth. Unlike Vonnegut’s Tralfamadorians, I’m stubbornly stuck in the present.

That’s why I find the idea of the “deep present,” coined by Oakland geologist Andrew Alden, so appealing. Like a type of geologic mindfulness, it combines the wisdom of the deep past with an appreciation of exactly where we are in time.

“The deep present,” writes Alden in an email, “is the awareness that what’s around us today is a snapshot that superimposes many thousands of years of history onto this moment. Anything in the geological record that’s happened since the glaciers left – giant quakes, thousand-year floods, centuries-long droughts ­– could happen again tomorrow.”

If the impeding threat of climate change is, at least partially, a problem of compressed time and myopic vision – when the breakneck pace of greed outstrips our means to understand ­– could old redwood trees serve not only as a source of scientific data, but as a reminder of the deep present? I think so.

Old growth redwood forests are archives of collective time, accruing in certain protected pockets of northern California. The lifetimes of the trees here could encompass dozens of ours and tell us much in an era when geologic time and human time are colliding catastrophically in climate change. The story told in coast redwood tree rings tell of a nature that is far older and bigger than us.

The deep present involves placing the current era inside that story, within the great span of geology and long cycles of climate. It’s seeing the past, reconstructed using tree rings and other paleoclimate records, as an active part of the present, in the hopes of adapting to a future we’re in the midst of creating, but can never fully see.

About the Author

Lucy Kang is a writer, radio producer and sometimes artist based in San Francisco.