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The Strange Quest to Save North America’s Most Elusive Oak Tree

How one 90-year ecological mystery reveals the complexities of conservation—and the problems with the “endangered species” approach.

There was perhaps no one better than Cornelius H. Muller—one of twentieth-century America’s most notable oak fanatics—to document the continent’s most mysterious oak trees. In July 1932, the horseback-riding botanist first encountered Quercus tardifolia while collecting samples in the steep-cut canyons of Texas’s Big Bend National Park. Muller jotted down details of the twigs (slender, somewhat fluted), buds (hairy at the tip), leaves (dull blue-green), and branches (short, stiff). He anointed it with a Latin name that referenced the tree’s late-season leaf development. This process of scientific description is a kind of sacrament, a communion between scientist and species: Muller saw Q. tardifolia, and so it was.

Then, as suddenly as it blinked into taxonomic existence, the species vanished from sight. 

In Muller’s wake, a succession of ecologists have scoured Big Bend for more of the species, trying to prove that it exists. None have succeeded. Ecologists only ever found one possible Q. tardifolia tree—which may or may not have been Muller’s original specimen; it died around 2011, before it could be genetically analyzed or botanically cultivated for conservation. Today, Q. tardifolia has grown about as lonely as a species can get, with an estimated population in the double digits and a known range of one location. 

“All we’ve known is one tree,” Wes Knapp, chief botanist at the conservation nonprofit NatureServe, told me. But in early September, he and three other botanists set out to try again. Knapp, an extinction expert, specializes in plants known to single sites. Morton Arboretum herbarium director and plant systematist Andrew Hipp would lead an audit of the molecular makeup of any samples from the trip before slotting them into their rightful branch on the tree of life. Adam Black, a botanical and horticultural consultant in Texas, would offer his encyclopedic knowledge of the state’s wildlife. Michael Eason, director of rare plant conservation at the San Antonio Botanical Garden, who wrote the book on wildflowers in the Lone Star state, would do the same.

The journey ahead was a quixotic one: They would search for a tree or two in an 880,000-acre national park. Muller’s 1936 description would serve as the closest stand-in to a treasure map—even though rainfall and cloud coverage dramatically transforms the appearance of foliage from year to year. If Q. tardifolia is, indeed, out there, drought, fire, or flood could wipe it out in hours. Every passing year, climate change makes this outcome increasingly likely. By demonstrating the oak’s existence, the botanist quartet could pave the way for conservation projects to protect it from disappearing again. 

In this sense, the hunt for Q. tardifolia—which might at first appear a frivolous exercise with an infinitesimal chance of success—points to a paradox at the heart of modern conservation, for this and other species. “You can’t really prove a hypothesis like ‘extinct,’” Knapp said. On the other hand, he said, “Something’s not worth our time and effort if it doesn’t exist.” We can’t understand how much we’re destroying if we don’t accurately account for nature’s baseline. We can’t plan for what to save if we don’t know what’s left. And if we can figure out what is still around, we might notice when it’s not. The four botanists hoped to replicate Muller’s sacrament: By seeing Q. tardifolia with their own eyes, they just might will it back into existence.

When the crew agreed to let me tag along and document their search, I knew the chances of actually encountering Q. tardifolia were slim. Three months post-excursion, the molecular work is ongoing, its results still taking their shape through analysis. But what I found, in three days of scouring Big Bend’s canyons and asking the botanists about their work, was that enigmas like Q. tardifolia are as much a resident of nature’s landscape as the common plants recognizable to you and me. And conservation—the desperate attempt to preserve the planet’s biodiversity as the climate crisis accelerates—is even harder than it already sounds. Even seemingly practical, concrete questions of what species to focus on and how much money to spend are difficult to untangle from intractable philosophical debates about the value of unknown entities. And to engage in those debates to begin with, researchers have to cover a lot of ground. Literally.

In his muddy, full-size pickup—a modern field vehicle upgrade from Muller’s steed—Adam Black and I began the 490-mile drive from Austin’s airport to Terlingua, a town of 110 residents and the group’s home base for the week.

Black had already explored Big Bend’s flora many times over; he’d never seen anything that neatly matched Muller’s Q. tardifolia description. In the Trans-Pecos region where we were headed, a chain of mountain ranges provides humid and elevated refuges in the otherwise arid and flat Chihuahuan Desert. A whopping 22 oak species (almost one-quarter of the country’s total oak diversity) dapple those sky islands. In the resulting swarm of biodiversity, certainty about any one tree’s identity is a luxury, Black told me.

Still, he felt hopeful. Back in May, Black and Knapp visited Sul Ross State University, in Alpine, Texas, to lay eyes on the Q. tardifolia specimens there. The pair were impressed by what they saw: thick, cardboard-like leaves fuzzed over with a dense white mat on the underside. The leaves were distinctive enough, both agreed, to stand out among the swarm. But knowing what to look for is only half the equation, Black told me from the wheel of his car. “You never really know what’s going to show itself.”  

Oaks have been around for about 56 million years, scientists estimate from fossilized pollen grains found near what is now Salzburg, Austria. Along their wind-pollinated and acorn-borne evolution, the genus Quercus has become one of the northern hemisphere’s most diverse, with 450-plus species globally growing on five continents, in dense forests and open grasslands, in bone-dry savannas and waterlogged wetlands, in soil both alkaline and acidic. Across the planet, oaks serve as an ecological anchor that cleans air; filters water; sequesters carbon dioxide; and feeds or hosts fungi, insects, birds, mammals, and more.

More than 60 percent of the genus’s diversity lives in the Americas, in a swath from Canada’s southern border to Colombia’s northern Andes. They dominate in both biomass and diversity, making up between one-fifth and almost one-third of the aboveground biomass in continental U.S. and Mexican forestland, respectively. Millions of years ago, when the continent’s forests were shoved up against its coasts and you could find palm trees in the Rocky Mountains, the oaks began to assemble in the Arctic, before moving south as temperatures cooled. “They were young upstarts. They pushed out things that were being squeezed into extinction,” Hipp said. In the grand history of North America’s plant communities, “oaks won.”

Oaks display some special qualities that may help them weather the mess we have made of the planet; they are generally drought-tolerant, fire-resistant, and quick to migrate. They also have a penchant for hybridizing, which may allow them to adapt more quickly to changing conditions such as extreme heat and new diseases.

This frequent hybridization can also blur oaks into a genetic smear like the one swarming across Big Bend. In a conservation system built on the worship of individual species—like ours—that taxonomic trickiness becomes a practical problem. Before we save something, we usually like to name it. (Conversely, if we can’t, we probably won’t bother to conserve it.) This desire to define nature is distinctively human. We have attempted to tame wilderness with the plow of categories, dredging the space between species with Latin names. We map the ranges of life. We sequence the mosaic of its many genomes. Nature, however, often defies the human drive for order. “Classification,” Hipp, the evolutionary biologist, told me, “is to benefit us.” And so the distinctive quality that helps oaks survive tough circumstances makes us less likely to help them.

In Q. tardifolia’s case, long-standing debate over whether the tree is a distinct species or the genetic gray area between two oak parents threatens to undercut future conservation efforts. “Tardifolia is like this mystical thing,” said Béatrice Chassé, co-founder of Arboretum des Pouyouleix in France and editor of the journal International Oaks. Chassé has twice traveled to Big Bend looking for it, unsuccessfully, and told me she has yet to encounter any convincing evidence that it’s a species in its own right. “I can’t shake away the feeling,” she said, that what Muller saw was an evolutionary blip. “It’s just my opinion.”

And so the conundrums facing Knapp, Hipp, Eason, and Black—Does Q. tardifolia still exist? And has it ever existed?—span the full aperture of biological existence. The tree might be extinct, or too fragile to save from that eventual fate. It might be thriving, quietly, off the beaten track. It could be a mutant disguised as a taxonomic error. “I think it’s a great mystery,” Hipp said.

On Day One of botanizing, I asked Knapp how he was feeling about our odds. “I put our chances at extremely low,” he said. Improbable but not impossible. “I wouldn’t be here if I had zero hope,” he added. Knapp has spent much of his career cataloging endangered plants, rediscovering presumedly lost ones, and studying species that persist in last, lonely locations. (His ongoing dissertation work at the University of North Carolina at Chapel Hill focuses on identifying and preventing extinction events.) Even on the best outings, he said, “You don’t start your day hoping, ‘I think I’ll rediscover an extinct species today.’”

Soon after we headed down Big Bend’s Window Trail, the week’s first assembly line snapped into action. Having identified a tree worth sampling, Hipp dropped to the ground and opened up his forest-green portable field press (which he would continuously carry, bookbag-like, on one shoulder, even as we ascended above 7,000 feet in elevation). He opened a circa-2018 Houston Chronicle sports section, carefully labeled it in permanent marker, and placed a leafed twig between its pages. Knapp read off an exact GPS location and elevation. Black plucked individual leaves from the tree—for later molecular analysis—and dropped them into a plastic baggie, then stashed it in the cooler lunch box strapped to his backpack. “Rock and roll,” Hipp said, stepping up from his knees. “Stellar.” The whole process clocked in at about 90 seconds per sample. They repeated it every few yards, with ever-improving efficiency. 

That morning, after collecting a super-rare oak, Q. graciliformis, a different tree 15 yards down caught the group’s collective eye. Out with the plant press, the newspaper, the GPS, the baggie, the cooler. Five yards past that, they deemed yet another tree collectionworthy. Again, the press was unbound, the pages stuffed, the coordinates declared, the baggies stored.

We were moving at a rate slower than half a mile per hour (a “botany crawl,” Knapp joked). This is the pace of reverence—the slow speed necessary for noticing. And there was a lot to notice: prickly pears and rock trumpets, tumbleweeds and butterfly swarms, hummingbird bushes and the hornets flitting though them. Woodpeckers. Road runners. Alligator lizards. Scorpions. Stevia. 

Even with their gaze limited to just oaks, the botanists marveled at Big Bend’s panorama of variation. Leaves could be strongly toothed or prickly or scalloped or luxuriant. There were veins to count and stubble to inspect under hand lens. Leaves close to the ground might be broad, thin, and fleshy (optimized for slurping up nutrients), while leaves bejeweling the same tree’s crown might be teaspoon-size, thick, spiky, and dark (optimized for resisting drought and herbivory). “We’re looking at a single tree wondering what species is this,” Hipp mused aloud. “It depends on what part of the tree you’re looking at.”

On the way to Cattail Falls, the day’s last stop, Knapp paused to point out a Viguiera stenoloba, the skeleton-leaf goldeneye (or a “DYC”—a damn yellow composite, to coarser botanists who scoff at the Asteraceae genus’s 20,000-plus daisy-like species). 

Holding a hand lens to its bloom, Knapp showed me how this DYC was not a single blossom but a bouquet. Its dark buttoned center contained concentric rings of minuscule disk flowers. Along the edges, each yellow petal corresponded to its own individual ray flower. In all: too many individual flowers to count by hand. The common sunflower, a close relative, can contain more than 1,000 disc flowers, plus another 13 to 30 ray flowers. What we mistakenly see as one flower is actually a colony unto itself.

This subversion of expectation, Knapp said, is why nature is worth examining with humility, and up close. “Every name is a hypothesis,” he went on, having discarded the V. stenoloba earlier on the trail, “that needs to be scrutinized.”

At 10:30 p.m., hiking boots 11 miles wearier and dinner finally eaten, the group sat around the dining table and talked the day over. I asked about the trip’s central questions: Does it seem like Q. tardifolia is a species after all? Does anyone think it’s out there?

“I was a little skeptical,” Hipp said. “I’m still a little skeptical.” Black, however, had grown more optimistic. We hadn’t seen anything like the distinctively fuzzy and thick-leaved herbarium specimens. If Muller’s Q. tardifolia was the offspring of two more prominent oak species, Black reasoned, its siblings would be among the hybrid swarm. “We saw a lot of variation,” Black said. “But not like tardifolia.

The discussion quickly turned to what should happen if they did stumble upon Q. tardifolia in the coming days. The United States Botanic Garden provided $60,000 to fund their project in partnership with the Morton Arboretum; that money will cover several trips to Big Bend and the molecular work that will follow this one. But in all likelihood, shielding Q. tardifolia from extinction—only possible following a miraculous rediscovery—would demand a domino of greater expenses: careful propagation and cultivation over several years, transplants back into the wild, absolute maximum site protections. And it might not work, in the end. What, then, would be worse: diverting precious conservation resources to a doomed species, or sacrificing it prematurely? “I’m not saying I would put this thing out of existence,” Hipp said at the dinner table. But, he continued, “you can’t pour all of our resources into a species that can’t be saved.” Eason jumped in: “It’s not one species,” he said. “It’s millions of years of evolution.” A lineage that extensive deserves as much attention as we can afford it—right?

The field of botany often faces difficult decisions about how to spend precious financial and human resources. While more than half the species listed under the federal Endangered Species Act are plants, in recent years flora have received less than 5 percent of total funding for endangered species recovery. And as compared to animals, plants are protected and studied by a thinning field of human custodians. In 2009, a major survey of the field found that 40 percent of federal botanists and about 30 percent of academics in the field planned on retiring by 2019. It is easy to make a utilitarian argument for plant conservation: Plants protect watersheds, make oxygen, help us discover and produce medicines, breed resilient crops, and provide shelter and food for countless other life-forms. Wherever they’re found, oaks are “disproportionately important” in this utility, Hipp said, given the diversity of life they sustain from their roots up through their trunks, and in their canopies. And yet we cannot muster the same resources for them we regularly dole out for charismatic megafauna (for which an emotional appeal is often sufficient). “Whereas individual species of animals will have teams to take care of them,” Emily Brin Roberson, a scientist with the Native Plant Conservation Campaign told me, “plants and the people who take care of them are often left to themselves—to conserve, by themselves.” 

The next morning, Hipp handed me a magazine dog-eared to a poem on page 194, “Naming,” by Jason Allen-Paissant. “A name / is reassurance / a comfort in the flesh / to hold,” Allen-Paissant writes. “I have started to see that nothing is itself / everything turns to something else.”

Charles Darwin himself referred to species—the building blocks of taxonomy and the foundation of his own work—as “merely artificial combinations made for convenience.” In On the Origin of Species, he both argued for species as a biological reality and against the “vain search” for an objective metric for identifying them.

Other biologists went on to propose many frameworks for distinguishing the smallest pixels in life’s vast portraiture. Among them, the biological species concept reigns. (If you’ve ever taken a college-level biology course, it would have been the one included in your textbook.) Within this framework, members of a species are bound together, and set apart from others, by an ability to produce offspring.

While popular, the biological species concept is imperfect. Its reliance on sexual reproduction immediately excludes asexual lifeforms. When it comes to extinct species, it demands information we may not be able to glean from fossils. And hybridization—which occurs in about one-fourth of flowering plants and one-tenth of animals—pokes a sizable hole in the framework’s fabric.

Scientists interested in how hybrids tangle the tree of life’s branches often turn to oaks as an example. If specieshood erects solid reproductive barriers, in the Quercus genus, these walls are littered with peepholes, windows, and doors. In 1991, when Washington University researchers studied DNA drawn from the buds and leaves of 128 white oak trees hailing from 18 populations across Mexico and Central and Eastern United States, they made a stunning discovery. They found evidence of genetic exchange not only between similar trees living near each other but also in distinct, distantly-related species. Whatever space existed between oak species, the scientists concluded, was even murkier than they’d already imagined.

Conservationist and philosopher Aldo Leopold referred to ecologists as living “alone in a world of wounds.” But during September’s outing, four Q. tardifolia–hungry botanists showed me another way through the sad muck of extinction: sheer silliness.

At the gate to Big Bend on Day Two, for example, a national park employee eyeing our research permit asked what they were studying. “Oaks,” Eason told him. “We were here yesterday, so were the guys behind us.” He paused for a semantic wink. “Don’t let them in,” he added before speeding off. Eason earned the nickname “Mr. 30 Minutes,” after insisting the group was half an hour away from the trucks—for several hours, including one spent racing downhill in a sudden downpour. Resting for a snack, Black announced he kept his copy of Mr. 30 Minutes’ field guide, Wildflowers of Texas, by the toilet. “I think of it as a compliment,” Black said. During one car ride, Knapp recounted a time in 2006 that he rediscovered an ultrarare orchid in Maryland. By the time he proudly returned to the spot with two botanist friends, a few days later, every last blossom was gone. “Fucking deer,” Knapp concluded.

On the way up to Big Bend’s 7,300-foot-tall peak, Casa Grande, a more sober Hipp laid out another possibility for Q. tardifolia’s future.

If it turns out to be an extinct species, he said, the genes that made it unique may not be permanently scrubbed from the natural world. Oaks’ propensity for hybridization means that somewhere out there, a visibly indistinguishable Q. tardifolia hybrid could be invisibly harboring its ancestor’s genetic code. The possibility struck me as beautiful: The tree might have vanished, but its molecular buds could lay dormant in the branches all around us.

Once he returned to the Morton Arboretum, Hipp and his colleagues would sequence small regions of DNA scattered across the genome (at thousands of sites, a fraction of each oak’s many millions of base pairs). They would do the same for several Q. tardifolia herbarium samples—including one of Muller’s type specimens. Through comparison, the hope was that they’d be able to determine whether Q. tardifolia is genetically distinct from its Big Bend oak kin. The process—highly technical and intensely minute in scale—will help uncloak Q. tardifolia’s molecular identity. While the molecular work would increase Hipp’s confidence about what Q. tardifolia is, it couldn’t answer the question of whether a tree like Muller’s is still out there.

“People think of science as an enterprise of certainty,” Hipp said. That couldn’t be further from the truth. He is completely unfazed by the prospect of a world that is uncertain to its roots, of one that is incompletely knowable. By revealing how tenuous our grasp of nature is, Hipp went on, science makes the natural world more precious—not less. Up ahead on the trail, Eason interrupted the speech. “I think you’re going to want to collect this,” he yelled, pinching an oak leaf. “Oh shit!” Hipp exclaimed, before scampering up the trail. “I was giving a sermon!”

Around 4 p.m. on Day Two, as the temperature peaked at about 101 degrees, we arrived at Boot Spring. It was here, almost 90 years ago, that Muller first came across Q. tardifolia. It’s also the clearing where, in the 1990s, local botanists took herbarium samples of the last remaining known Q. tardifolia individual. It was graced with a good-sized crown and drooping branches, according to botanist and Sul Ross herbarium director Mike Powell, who enjoyed lunches with his wife under the putative Q. tardifolia tree. “The leaves were right there, in your eyes,” Powell told me when I called him up after the trip. “It was distinctive. It matched the description.” I told him it sounded like he had no doubt about its identity. “Exactly,” he confirmed. 

Jackie Poole, a botanist with Texas Parks and Wildlife who co-wrote a field guide to the state’s rare plants, identified that tree in 1996. Poole, who knew Muller personally and asked him about the location, had spent years searching for Q. tardifolia. “I never found anything that I thought was even close to it,” she said. “I kind of gave it up for a while.” Then, on a hike, Poole and some friends stopped for lunch under a tree. “I happened to look up at it and thought, ‘Wow that looks like Q. tardifolia,’” she said. It was a moment of genuine amazement.  “I walked past it dozens of times, probably sat under it,” she said. “It’s been there the whole time.”

Sadly, the tree died in 2011, before anyone tried to cultivate it or sequence its genetic material. “That gets me angry,” Knapp said in September, as he scanned the Boot Spring’s open woods in vain. “Why didn’t we do anything while we could? 

Of all the spots on our scouting trip, Boot Canyon seemed the likeliest location for another Q. tardifolia holdout. But on arrival, it was clear that a wildfire had swept through the site in spring; charred remnants of lost vegetation littered the canyon. For the first time during the trip, I felt disheartened. Despite the scale of the loss it describes, extinction is an anticlimactic affair—an ever-dimming twinkling of light blinking out into darkness. We can’t find something anymore, so we stop looking. Either way, it disappears from sight. Extinction has no dramatic flourish—no final explosion or last gasp—to signal a lost cause’s conclusion. We just have to listen until silence itself says something meaningful. In Boot Canyon, the group didn’t hear or see anything promising.

By the last day of botanizing, the group was exhausted. Three days of scrambling up and down Big Bend’s deep, sheltered canyons had left my legs covered in mosquito bites, cactus scratches, and rashes of an undetermined origin that took a week to recede. It had also filled the plant press with more than 90 specimens. 

Before we headed back to Terlingua one last time, Black pointed a few dozen miles away, across the Mexican border, the Sierra del Carmen mountains stood. Like Big Bend, the Sierra del Carmen sky island ecosystem is home to exemplary oak diversity. Perhaps, Black mused, a transnational wind or bird had carried stray Q. tardifolia pollen or acorns from one mountain range to the other, dropping them in a site completely inaccessible to humans. “Maybe it’s not lost,” he said. “Maybe it’s where it wants to be.”

That night, we lay belly-up outdoors. Thanks to the sparsely populated expanse surrounding the park, Big Bend enjoys the least light pollution of any national park in the lower 48. Above us, the night sky lived up to its reputation for offering unparalleled stargazing. Each star in its uncountable bouquet extended a petal of light across trillions of miles and into our puny lines of vision.

“It’s sad that …” I started to say. “That we’ve lost this?” Knapp finished for me. “Yes,” I said. The four of us—Knapp in Asheville; Hipp in the Chicago suburbs; Black in Navasota, Texas; and me in Brooklyn—each have no regular access to this pristine a version of the sky at home.

In our silence, I felt as insignificant as a tree in a million-acre forest. We had spent the week discussing the order humans have tried to impose on wilderness—urgent and desperate efforts spiraling it further into an oblivion of our making. In return, awe is the sentiment that nature imposes on us. It is an embodiment of existence’s magnitude, the overwhelming scale of nature and the underwhelming supremacy of us. “We got more than we deserved,” Hipp said, as he considered the deep reaches above us. “The universe is enough.”

The next day, we set out for the eightish-hour drive from Terlingua to Austin. (Who’s to say that along the way, we didn’t pass by Muller’s final resting site—the “field of bull-thistles somewhere in Texas” where he requested that his ashes be scattered?) Early in the drive, the search party took one last reverent lap for oak samples. Hipp crouched down by a tiny sapling, a constellation of molecules yawning a few inches out from the soil bed. “It’s a rough life,” he told it. “You’re doing good.”