Zach St. George
The first and only time Steve Jackson spoke to Bill Critchfield was in the late 1980s. Critchfield, an authority on the conifers of North America, was at home recovering from a heart attack. Jackson, then a postdoctoral researcher at Brown University, had called looking for advice on how to tell jack pine from Virginia pine.
Jackson was also curious about something the elder botanist had mentioned in a recent paper: mysterious spruce fossils from the American Southeast. The fossils dated to the end of the Pleistocene ice age, about 18,000 years ago, and had been found across the region, including in Louisiana’s Tunica Hills. Scientists had usually identified the fossils as white spruce, a species that now lives far to the north, but they’d been arguing for decades about what its presence said about the region’s ice age climate. Some held that white spruce pointed to a climate similar to modern Canada or Alaska. Others argued that the climate had been milder than that, and suggested that the spruce fossils had been carried south from somewhere else.
In his paper, though, Critchfield had suggested a third possibility. “Critchfield said, ‘You really need to follow up on this,’” Jackson, now director of the Southwest and South Central Climate Adaptation Science Centers at the U.S. Geological Survey, recalls. “‘I don’t think those are white spruce.’”
So begins the mystery of a tree at the center of the so-called Quaternary conundrum, the apparent mismatch between how scientists project a changing climate will affect living things in the future and how it seems to have affected them in the past. Specifically, the disagreement is between mathematical models and the fossil record. Scientists use both to peer into the future, assuming that the way the world worked today and yesterday is also how it will work tomorrow.
The models and the fossils agree on the broad outline: When the climate changes, the arrangement of the world’s species changes, too. They shift upslope and down, toward the poles and away. Some species grow more common, others rarer.
As our current climate warms, this rearrangement is again underway. Around the world, scientists have observed species shifting their ranges, including plants, seemingly the slowest, least mobile of lifeforms. Seed by seed, plants are following the climatic conditions that suit them. While it is so far subtle, this movement could be one of climate change’s most profound effects. Plants are the basis of life on Earth, providing food and habitat for countless other species — humans included. And the migration of plants is likely to drive the migration of people.
There is also the question of the long-term effects of climate change on the diversity of life. For decades, scientists have worried — and the models have suggested — that many species would be unable to keep up with the rate of change, and could face extinction. Plants would be especially at risk. As one scientist summed it up in the journal Climate Change in 1989: “The speed with which the climate is expected to change in the next century presents a problem: Can vegetation track climatic changes that occur so rapidly?”
On the one hand, the broad fossil record seemed to suggest that it could. As the ice age began to fade roughly 18,000 years ago, temperatures spiked, seas rose, and species raced to follow the conditions that suited them. But few seem to have gone extinct. Those that did were mostly large mammals, including mammoths, mastodons, and giant ground sloths — all highly mobile. There didn’t seem to be any similar cluster of extinctions among plants. At the time of Jackson’s phone conversation with Critchfield, in fact, no plants were known to have gone extinct in the late Pleistocene in North America.
But the spruce cones of Louisiana’s Tunica Hills hinted at another, less hopeful possibility: that many plants had gone extinct, and scientists simply hadn’t noticed.
Scientists debate the possibilities
Today, the Tunica Hills are covered in oak and loblolly pine, sweetgum and pawpaw, Osage orange and flowering magnolia, along with many other plants. The flora is typical of southern uplands, suggestive of heat and humidity. But other scenes lie buried not far below. It was here, in the 1930s, that a botanist found fossil cones of what he believed to be white spruce, beginning a decades-long scientific debate.
The cones dated to the late Pleistocene. At that time, Canada lay under a mass of ice that, at its largest, was as big as the one now covering Antarctica. Ice sheets held so much water that the sea was 400 feet lower than today. Scientists of the mid-1900s knew that the edge of the ice cut across the Midwest, reaching as far south as Ohio. They thought that few trees were likely to have survived farther north. But they disagreed about what had happened to forests south of the ice.
On one side of the debate was University of Cincinnati plant ecologist Emma Lucy Braun, who thought that the effects of the glaciation on plant communities were relatively mild. In a series of papers published in the 1940s and 1950s, she argued that the plants of the modern boreal forest had migrated south in response to cooling temperatures, mingling with less cold-tolerant temperate forest species, but that farther south, the species-rich forests of the Appalachians and the American Southeast had maintained roughly their present arrangement. White spruce fossils from along the Gulf Coast seemed to contradict this view. Straight-trunked, with steepled boughs, prickly needles, and oblong, scaled cones, white spruce can now be found from Newfoundland all the way to the Alaskan Arctic. Just as a palm fossil signals tropical heat, a white spruce fossil signals cold. But Braun dismissed the problem, writing that the spruce cones may have floated south down the Mississippi.
On the other side was Yale ecologist Edward Deevey. He thought the glaciation had sweeping effects on North America’s plants. The cold had likely pushed the temperate forests of the modern United States “south of the Rio Grande and deep into peninsular Florida,” he wrote in a 1949 paper titled “Biogeography of the Pleistocene”; the boreal forests of Canada and Alaska, meanwhile, had retreated all the way to the Gulf of Mexico. When Deevey looked around him, he saw a floral community shaped by the ice ages. “No distribution is taken to be old unless it can be proved not to be young,” he wrote. For him, the white spruce fossils were evidence.
The debate remained unsettled by the early 1970s, when Louisiana State University graduate students Hazel and Paul Delcourt visited the Tunica Hills. They waded up the Little Bayou Sara to the bluff where the spruce cones were discovered decades earlier. “At first I saw nothing more than a pile of leaves, twigs, and other plant debris,” Hazel Delcourt wrote in her 2002 book “Forests in Peril.” But as they dug into the bank, they soon found spruce fossils.
These fossils didn’t seem to support Braun’s hypothesis that spruce bits had floated down the Mississippi, Delcourt wrote — even the most delicate parts were intact. But they didn’t quite support Deevey’s hypothesis, either. They were mixed in with fossils of other species that painted a picture of a milder climate. Spruce, Delcourt wrote, “just didn’t fit in.” Then there was the oddness of the fossils themselves. The cones were twice the length of modern white spruce cones, and the wood was “peculiar,” the growth rings unusually narrow. But the Delcourts didn’t pursue the question, and the spruce fossils’ true identity remained a mystery.
An extinct plant species?
Years later, in the early 1990s, prompted by the phone call with Critchfield, Jackson visited the Tunica Hills, in the company of a Louisiana-based geologist. He still remembers the feeling when he broke open a piece of sediment and saw a spruce cone sticking out of it. “‘Wow, I don’t think this is white spruce,’” he remembers thinking. “It was thrilling,” he says. “‘This is something that no longer grows on the Earth.” Back at his lab at Northern Arizona University, he set about investigating whether his hunch was right.
Although he knew the fossils dated to the late Pleistocene and weren’t that old, Jackson says he set that aside. This was to avoid a possible trap, one that seems to have snared many of his predecessors. As Charles Darwin noted in “On the Origin of Species,” the farther back in time you go, the less fossilized creatures resemble modern creatures. By the late Pleistocene — practically yesterday in geological time — nearly all of the fossils are of species that still exist. Earlier botanists had assumed that the fossils were white spruce, because that was the modern spruce that the cones most resembled.
But Jackson tried to forget his preconceptions. He pretended to be “in some deep, distant, far-off period of time in which I could no longer depend on there being modern species,” he says. He focused only on the fossils’ physical traits. By the size of the cones, the dimensions of the seeds, the anatomy of the needles, and other, more arcane measures, he determined that the tree really was something different. The reason that white spruce fossils seemed to contradict their settings was because they did — the fossils weren’t white spruce. In a 1999 paper with his graduate student Chengyu Weng, now a paleoecologist at Tongji University in Shanghai, Jackson announced the discovery, naming the tree after Bill Critchfield, who had died months after their phone call. The mysterious spruce became Picea critchfieldii.
One mystery was solved, but another remained — one with implications for other plant species, including those that humans rely on for food and more: Why did the tree go extinct?
What causes plant extinctions?
Extinction is complicated, involving not just individuals, but whole populations, not just death, but also failure to reproduce. Sometimes it happens suddenly, but more often it follows a long decline. Sometimes it has a single cause, but more often there are many. Understanding why Critchfield’s spruce went extinct could help scientists know what other species may face in the future. “It may or may not be representative,” Jackson says. “Each extinction is different. But even knowing the details of a single extinction can be instructive in telling us, ‘These are the sorts of things that can happen.’”
In a broad sense, the potential culprits in the tree’s extinction — and all extinctions — fall into three categories. The first is changing physical conditions. During the millennia before the tree went extinct, the world experienced several climatic shifts. First, beginning around 20,000 years ago, it rapidly warmed; around 13,000 years ago, it suddenly cooled, before warming again after another thousand years. Perhaps at some point during the climatic seesawing of the late Pleistocene, Jackson says, places with the physical conditions Critchfield’s spruce needed to survive disappeared.
The second potential cause of the tree’s disappearance is other living things. Maybe the changing environment tipped the odds of competition to another species of tree or plant, or new physical stress left the tree vulnerable to some insect or fungus. Humans could even have played a role. At the time of the tree’s disappearance, they were spreading across the Americas. They could have carried a new pest with them, like the deadly invasive beetles and fungi that killed off American elms and chestnuts in the 1900s.
Other scientists think people could have played a more direct role. University of Arizona paleontologist Paul Martin suggested that the tree may have succumbed to human-lit fires. Martin, who died in 2010, is best known for theorizing that human hunters caused the extinctions of North America’s mammoths, giant ground sloths, cave bears, and many other large mammals at the end of the Pleistocene. Their fires might have killed off Critchfield’s spruce, too. In a 2005 book, Martin wrote of the spruce, “It should not surprise us if an occasional plant species were drawn into the overkill vortex.”
The third potential reason the spruce went extinct is the simplest: It got stuck. As the climate changed, and places with suitable physical and biological conditions shifted, the spruce didn’t keep up. “It couldn’t migrate fast enough,” Jackson says, “or something bad happened along the way.”
Jackson intended to follow his discovery of Critchfield’s spruce with more fieldwork in the Southeast. He wanted to gather more data about the tree’s range and abundance in the time leading up to its disappearance, to begin to work out why it went extinct — and what, if anything, its extinction might say about future extinctions. “All we’ve got are these little fragments of information,” he says. But his applications for funding were declined, and then the geologist who was his guide in the Tunica Hills died. At that point, Jackson says, he gave up. The trail went cold.
What can plant fossils tell us about future climate change?
Today, most paleobotanists favour Deevey’s take from the late 1940s: Even small amounts of climate change can have drastic effects on the arrangement of the world’s plants. Since the late 1800s, the world’s average surface temperature has risen by roughly 2 degrees Fahrenheit, an increase that scientists have in recent years tied to lengthening fire seasons, growing numbers of drought-killed trees and insect outbreaks, and sudden transformations of ecosystems — from forests to grassland, from grassland to desert, from tundra to scrubland. In one recent study, a group of researchers at the University of Miami and the National University of Colombia at Medellín analyzed millions of plant records from across North, Central, and South America. They found that from 1970 to 2011, warmth-loving plants have become more common nearly everywhere they looked, from the Amazon to the Arctic. That kind of rearrangement isn’t necessarily bad, from a plant’s perspective, says Jacquelyn Gill, a paleoecologist at the University of Maine. “That’s a sign of resilience.”
The question is whether plant species will be resilient enough. For decades, scientists have worried that many plants wouldn’t be able to move fast enough to keep up with the changing climate. The predicted rate of climate change, “is likely to outstrip many species’ ability to stay within appropriate climatic ranges,” wrote one scientist in a 1991 paper, in a typical example. “Ninety-five percent of the rare plants in the southeastern United States may be vulnerable to extinction as a result.”
Often, these worries were backed up by models. Models are attempts to approximate, mathematically or otherwise, how a slice of the world behaves, especially one that is difficult to directly observe. One type of model attempts to project where a species might be able to survive in the future. Such models are based on the climate in the species’ current range. Once the model can reliably project where the species lives right now, it can project its range in the future.
What many of the models showed was dire. In one influential 2004 study in the journal Nature, researchers examined the extinction risk to species living in regions comprising roughly a fifth of Earth’s land. Based on a series of models, they projected that between 15 and 37 percent of the species in those areas would be “committed to extinction” by 2050, due to climate change.
But there seemed to be a mismatch between the grim forecasts of the models and the recent fossil record, as another group of researchers pointed out in a 2007 review article. While the fossil record showed that climate change caused upheaval, it didn’t clearly show that it also caused extinctions. “We note a Quaternary conundrum,” the authors wrote — although the modeling data suggested that many species could be at risk from climate change, they wrote, “during the recent ice ages surprisingly few species became extinct.” Among plants, extinctions were even rarer. In North America, the only plant known to have gone extinct in the late Pleistocene was Critchfield’s spruce.
Are plant extinctions really rare?
There are a couple of possible explanations for the discrepancy. One, as the modelers themselves often point out, is that the models are underestimating the range of conditions in which species might survive. The real range of a species might be shaped not only by climate, but also by soil composition, drought frequency, the presence or absence of competitors or predators, among a host of other factors, as well as the species’s past and present success in reaching otherwise suitable places. The current absence of a species from a place doesn’t necessarily mean it couldn’t survive there.
The other possibility is that more species went extinct in the recent past than scientists think. At first glance, the pattern from the Pleistocene seems to hold into the deeper past: Plants do seem to be more resistant to extinction than animals. Roots make plants individually tough, and seeds make plant species collectively tough. Luke Mander, a paleobotanist at the Open University in the United Kingdom, describes this idea by comparing the hardiness of plants in a botanical garden to animals in a zoo. If you hit all the animals on the head with a hammer, you’ll probably be left without a zoo. But if you do the same to the plants, he says, you’ll still have a botanical garden the next day.
On the other hand, Mander says, the story of Critchfield’s spruce suggests scientists may simply have missed many floral extinctions. Due to the difficulty of identifying the small, scattered fragments of the fossil record to the level of species, scientists who study extinction in the past often work in higher taxonomic rankings — at the level of “spruce,” for example, rather than of “white spruce” or “red spruce.” This means that, if a plant goes extinct but has many surviving relatives, as Critchfield’s spruce does, it could be easy for scientists to miss its disappearance from the fossil record.
For now, it remains unclear whether the spruce is a hint that more extinctions remain undiscovered — that the dire predictions of the models are right — or whether it is the exception that proves the rule of floral resilience. “Did I happen to stumble on the one major tree species that bought it at the end of the Pleistocene?” Jackson says. “Or is it the tip of an iceberg we can’t access because we haven’t applied the tools, or we lack the tools? I think that’s still an open question.”
Unravelling the mysteries of Critchfield’s spruce
Two decades after Jackson and Weng announced their discovery of Critchfield’s spruce, the tree remains an enigma. It often gets mentioned at scientific conferences, Gill says, held up as a curiosity. “I think it could be a bit of a Rorschach test, where we can project on this tree sort of what we want to see,” she says. One could say that extinction is natural, and that we’re fortunate that only this species, and not other species of spruce, went extinct, she says. “Or you could look and say, ‘It’s a warning, a canary in the coal mine. As the climate changes, we need to make sure we’re helping species not end up like this example,’” she says.
But scientists may still arrive at a better understanding of the tree itself. In the early 2010s, a group of researchers at the University of Illinois Urbana-Champaign and the Illinois State Museum developed a method that could potentially be used to more easily track the tree’s whereabouts with pollen. Pollen can last for thousands of years in low-oxygen environments like peat bogs and lake beds, and is far more widespread than larger fossils like the needles and cones that Jackson used to identify Critchfield’s spruce. The problem is that the pollen of different species of spruce, shaped like a half-deflated Mickey Mouse head, is hard to tell apart by eye. The researchers took a statistical approach, using the minute differences in average dimensions of the pollen grains of the various spruce species to train a computer to sort between white and black spruce.
In a 2014 paper, the research group showed that pollen of Critchfield’s spruce has a distinct morphology from other spruces’ pollen, meaning a computer may be able to learn to pick it out as well. The main purpose of the work was to be able to investigate the dynamics of the population over time, according to Mander, the study’s lead author. Being able to reliably identify Critchfield’s spruce pollen could show the tree’s whereabouts and abundance through time, and help pinpoint when it went extinct.
Eventually, pollen could provide an even closer look at the tree. In the early 2000s, forest geneticist Laura Parducci, now of the Sapienza University of Rome, showed that it was possible to analyze DNA from ancient pollen grains. Recently, Tim Temizyürek, a graduate student in her lab, tried to develop a way to automate this process and get a more complete picture of the grains’ genomes. This would provide a vast new trove of information, Parducci says, and could allow researchers to confirm whether Critchfield’s spruce is indeed its own species, and if so, how it is related to other spruces. Significant hurdles remain. At the moment, the success rate in extracting DNA from 10,000-year-old pollen grains is only 1 or 2 percent, or even less in older samples. And the work of preparing samples manually, as Parducci did in the early 2000s, is so painstaking as to be prohibitive, while samples of ancient pollen seem to be too contaminated for the automated method Temizyürek tested. But if researchers can find a way around the contamination issue, Parducci says, there’s much to be learned from analyzing these ancient grains.
That, for now, is where the story of Critchfield’s spruce ends. As Gill points out, it has a shape-shifting quality. While it contains all the classic elements of a mystery — a sudden disappearance, an investigation, suspects and red herrings and scattered clues — it remains a mystery unsolved, neither a clear sign of hope nor an obvious harbinger of problems to come, for plants or the people who depend on them.
But Parducci suggests another possible ending, unlikely but not so unlikely that it should be ignored: Maybe, she says, the tree is still out there somewhere. “Maybe it’s somewhere else.”
Jackson, for his part, thinks the tree is extinct. Botanists have examined the trees of North America pretty closely, he says. “People would’ve seen it.” Still, he doesn’t completely rule out the possibility. In the 1980s, he notes, a botanist found an unusual tree hidden deep in the mountains of Northern Mexico. Now known as the Martinez spruce, it was new to science. It’s not impossible, Jackson says, that Critchfield’s spruce could have a similar story — maybe, it too, remains somewhere in the remote mountains of Mexico. Perhaps it is just a single stand of trees, straight-trunked with steepled boughs, prickly needles and oblong cones, seen but unnoticed, hidden in plain sight.
Zach St. George is a freelance reporter and the author of “The Journeys of Trees: A Story about Forests, People, and the Future.” He has written for The Atlantic, Scientific American, and many other publications.
UPDATE: A previous version of this piece omitted the current title of Steve Jackson. He is director of the Southwest and South Central Climate Adaptation Science Centers at the U.S. Geological Survey.