The Botanist Daring to Ask: What If Plants Have Personalities?
(Bloomberg) -- The Valentine ecological study area in Mammoth Lakes, Calif., is situated in the caldera of an ancient volcano 8,000 feet above sea level. There’s no fence to keep tourists out of the 156-acre reserve owned by University of California at Santa Barbara, just a sign warning that trespassing won’t be tolerated. Most wouldn’t even know where to look; the entrance is a perimeter of scraggly pine forest with no trail through it—unappealing, compared to the ski area practically next door.
But immediately beyond the hedge of trees, the land opens into a rise that, in July, is covered with frosty green sagebrush and glossy manzanita crowns. Giant Jeffreys pines, armored in scales of rust-orange, vanilla-scented bark, stand above the low plants. Corn lily, pale pink phlox, white rein orchid, mules ears, serviceberry, and orange tufts of semi-parasitic desert paintbrush emerge from the bone-dry, gravelly ground. Two deer, young bucks with nubs for antlers, leap away as I walk. So do grasshoppers. Above the ground-level drama rise the jagged tops of Sierra Nevada peaks, still smudged with snow in places, despite the July sun.
And then there’s Rick Karban, bent over a sagebrush bush, plucking off tiny, black beetles with tweezers.
Karban, a professor at the University of California at Davis, is among the foremost researchers of plant signaling and communication. A lithe beanpole of a man with arrow-straight posture and a tuft of white hair, he hands me a pair of tweezers and a paper pint container—the kind used for ice cream—and tells me to start collecting the bugs, which he’ll reuse in future experiments. (The paper lid has air holes punched in it.) He placed them on the bush the night before; whether they’re still there will tell him how hard the plant had tried to get rid of the perceived predators.
But beetles have predators, too.
“Ah, a ladybug is eating one,” Karban says, momentarily disappointed at the lost data point. “Ah, okay. It’s real life!”
In the past 15 years or so, thanks to advances in plant genetics and a new openness toward plant research once considered fringe, botanists such as Karban have found that plants produce and respond to complex chemical signals. They can detect the slightest touch. They know when they’re shaded by a cloud or a fellow plant, and whether that plant is related to them. Several species can recognize their genetic kin and rearrange their bodies to avoid competing with siblings. They can manipulate predators to do their bidding and transmit electrical signals among their roots. One paper suggests that some may perform arithmetic division to keep from starving at night, when they can’t perform photosynthesis. At least one Chilean vine appears capable of impersonating the leaf morphology—down to the vein pattern and texture—of up to four nearby plants.
Karban’s research has shown how chemicals wafting off sagebrush can be interpreted, even by nearby wild tobacco, and how that same wild tobacco, when it begins to be damaged, can summon predators to eat caterpillars that feed on it. He’s also found that sagebrush are more responsive to cues from their genetic kin.
His latest experiments, however, concern a facet of behavior that until this point was thought to reserved exclusively for humans: personality.
Such a research question turns the past 100 years in the field of plant biology—and, for the most part, animal biology—on its head. Up to this point in contemporary botany, individual plants within a species have been seen as replicants. No individual trait has mattered, and only the average of the population would count. Any individual variation that fell outside a trend line was considered noise.
Personality research, however, treats that noise as valuable data, seeing a spectrum of behavior where traditional botany sees only a mean and median.
To be sure, this is the outer edge of research into plant behavior. Karban hasn’t published anything on it yet, but he’s a respected scientist with 40 years of scientific research under his belt, and his dedicated interest is an indication that this is thought experiment’s time has come. If his results are compelling, they could have enormous implications—well beyond the tiny world of plant researchers.
The way Karban imagines plant personalities function is like how humans behave—say, during a pandemic. “If you have variation in how anal people are about washing their hands, you might have some individuals who are hyper hygienic, and under certain conditions”—such as the ones we’re living under right now—“they might have an advantage over individuals who are really cavalier,” he says. But the same trait may not always be the winning strategy. “Under other kinds of conditions, being that person would be selected against,” Karban says. An excessive focus on hygiene is also connected to certain psychological disorders; at the population level, it is linked to allergies.
A diversity of human responses to the environment, one might argue, makes us more resilient as a whole. The same may be true of plants. “Animals and plants are really different, clearly,” Karban says. “But animals and plants face similar kinds of selective pressures. Things want to eat them. They need to find food, they need to find mates. If we know more about animals, and animals have solved a problem in a particular way, it’s not unreasonable I think to ask: Huh, I wonder if plants have done something analogous.”
Karban’s hypothesis is an alluringly logical way to explain differences in individual plant responses. Under his schema, a distress signal from a cavalier plant would mean the danger is more likely to be real and worth marshaling precious resources in response.
But if plants do indeed have personalities, that would have significant implications for all areas of plant research. Plant biologists might come to understand why certain individuals survive pest infestations better than others. It could also allow for more clarity about what climate change will do to the plant kingdom.
Karban and his team have found that communication among sagebrush plants is most effective early in the growing season, when the plants are growing actively and have the most access to water. As droughts become more severe and more common, plants may not be able to communicate as effectively and may be less able to defend themselves as a result.
Extrapolate that research to food crops such as corn and soy, and the stakes become more obvious. Warmer winters in the northern hemisphere will allow agricultural pests to breed and feed more. A 2018 analysis projected that insects would eat 50% more wheat and 30% more corn under 2C of global average warming than they do now. At the same time, crop yields may plummet with increased heat stress.
Climate change will bring “things that we haven’t thought about, that we and other organisms have no evolutionary history of dealing with,” Karban says. Having an array of different threat responses may make it less likely that a single one—whether that’s a new fungus or a swarm of locusts—would wipe out the whole population at once.
Karban’s office at UC Davis is a small rectangle set off from a large, open-plan etymology lab, where plastic Tupperware of tiny, dead butterflies litter a bench. What’s a botanist doing in a bug lab? He shrugs. “I started out in cicadas,” he says.
Cicadas lay their eggs in trees. When the larvae hatch, they drop to the ground, burrow into the tree’s roots, and stay there for 17 years, sucking its sap. As a young scientist, Karban studied how some trees grow calluses around the eggs, trying to crush them to death before they can hatch. That got him interested in plant self-defense.
After a long career studying sagebrush, Karban is finely attuned to variations in their distress response—one metric, he says, of personality. Some might be natural-born scaredy-cats and signal wildly at the slightest disturbance, in which case even their kin might not respond with any defense mechanisms of their own. But when other, more risk-tolerant individuals signal distress, their fellow plants might respond immediately, pumping out volatile chemicals, too, and boosting their defenses.
In 2017, Charline Couchoux, a behavioral ecologist at the University of Québec at Montréal, emailed Karban to propose that he needed a framework, a methodology to identify individual behavioral differences in plants. Couchoux had developed one—for animals. She’d spent thousands of hours studying chipmunk personality in the woods at the Vermont-Québec border. When she controlled for sex, social status, and age, certain chipmunks were clearly more skittish than others, and that remained the case over their entire lives.
Chipmunks make certain calls when they’re distressed. “Some guys would be eating seeds, and a leaf falls on the ground. They panic and they make a call,” Couchoux says. Those are the meek ones. “Some guys just keep foraging.”
From an evolutionary, survival-of-the-fittest perspective, one might assume the meeker chipmunks are doomed, but Couchoux found that this wasn’t the case. A shyer, less aggressive chipmunk might take fewer risks, eat less, and have fewer babies each year—say, one per year after the first year. But meekness has its virtues: Less risky behavior means fewer chances to be gobbled by an eagle. So that chipmunk lives a longer life and produces more babies. A really bold chipmunk might have three babies in a single year but die sooner. In the end, both chipmunks have three babies. “It’s basically equivalent,” she says.
Karban and Couchoux are now working together on a paper that they expect will define the study of personality in plants, and Karban plans to release a series of papers on his sagebrush personality work after that. This, they hope, will finally make plant personalities a real—if not accepted—part of the scientific conversation.
That is far from guaranteed, however. Even among botanists, the idea of seeing plants as individuals in the way Karban is trying to do in his work is likely to be controversial. Ascribing individual proclivities to animals was anathema as recently as the 1980s. Donald Griffin, the zoologist who in 1944 discovered that bats navigate by echolocation, spent his career urging scientists to consider the matter of animal subjectivity. He saw that bats had the ability to change their behavior as external circumstances change—a hallmark of intelligence. Animal thought and reason ought to be legitimately studied, he argued. Could they not be considered individuals with volition, or even possibly with consciousness?
Griffin was roundly rebuked for even broaching the topic in a 1976 paper. Now, just four decades on, it isn’t heresy to talk about animal cognition, to study the behaviors of individual animals, or to ascribe personalities to them. Everywhere researchers look, it seems, there is much more to the inner lives of animals than we ever thought possible.
Efforts to do the same for plants have begun something of a war within the field of botany. Researchers are divided between those who are willing to use language such as “plant intelligence” and those who think it’s preposterous; after all, no plant brain has been located. The war plays out in botany journals: Researchers pen colorful response papers, flinging venom tempered only by the constraints of formal scientific discourse. Karban prefers not to place himself in either camp. But he knows that putting forth a term such as “plant personality” is bound to provoke the passions of the anti-plant-intelligence side. For anyone in plant research, this is the field’s high drama.
Depending on how Karban’s work is received—and whether others can ultimately replicate it, a crucial step for a new idea to gain validity in science—it could be part of a sea change in how we understand and interact with the plant world. In 1840, when a German chemist named Baron Justus von Liebig published a monograph breaking down the three main elements plants needed for growth, it demystified soil fertility, which had long been an enigma. Within a few decades, those three elements—nitrogen, phosphorus, and potassium—became the basis for the modern synthetic fertilizer revolution, which permanently changed the practice of farming.
Since then, however, we’ve come to understand that plant health is far more complex and that the relentless use of synthetic fertilizers can, in fact, do indelible harm to ecosystems and soil fertility in the long run. New layers of complexity have more recently come into focus, involving interspecies relationships among untold numbers of microbes and fungi.
Plant personality could be yet a further level of that complexity. Right now, individual variation in plants’ responses to pests is a mostly inexplicable phenomenon, much as the basics of soil fertility once were. Understanding that not all plants are the same—and the ways they are different—could give researchers an inroad to understanding plants’ distinctive behavior and perhaps lead to the development of more resilient agricultural crops.
Respecting such individuality, however, will be a greater challenge. Agricultural researchers have warned of the dangers of monocultures—planting a single variety of crop over large swaths of land—ever since the mid-19th century, when a microbe known as potato blight proved particularly deadly to the Irish Lumper, a staple food crop in Ireland at the time. The devastation of the potato harvest caused mass hunger and around 1 million deaths. Yet, thanks largely to the economics of modern agriculture, which values yield above all else, many of the world’s food staples continue to be grown in vast, undifferentiated fields. As Karban and Couchoux’s initial findings illustrate, wild populations rely on both the meek and the bold to stay alive.
Back at Mammoth Lakes, now fully lying in the dry gravel dust in nylon khakis to get a bug’s-eye view, Karban is counting beetles. His floppy field hat pokes above the sagebrush bush he’s buried his face in.
As I sit on the ground nearby, I inhale wafts of sagebrush’s signature camphoric smell, which is herbal and slightly spicy. These are a bouquet of a few of the plant’s many volatile chemicals—signals they emit in response to stimuli, which fellow sagebrush can eavesdrop and respond to. That, Karban thinks, may be their version of “expressive” or “quiet,” if we can only learn how to listen.
It would be a major feat to put forth a compelling argument that plant personality exists and should be studied further—and then perhaps develop a system for measuring it. Much as in humans, where the mind is studied by inference (what a person does) rather than neurological mechanisms, Karban is looking for patterns in behavior. “I’m a big fan of using what decades of psychology has learned—their methods—and asking if they apply to plants,” he says. “In some cases, that’s no, and that’s fine.”
But he’s found one method to be particularly compelling. It separates behavior into two processes. The first is judgment, or the perception of raw information; the second is decision-making, or “weighing the costs and benefits of different actions you might take and then taking an action.” This, he says, “applies perfectly well to plants.” How different plants might weigh the threat of predators and then take action against them—by, say, making themselves chemically unattractive, or in the case of tobacco, chemically summoning predators that will eat whatever is eating them—could be a strong signal of individual personality.
I ask Karban if his work has changed his perception of plants. “I think so,” he says. “I grew up with the idea that plants are barely alive. Now I’m consistently impressed with what they can do.”
As we walk out of the field site, we descend from the dry moonscape and into a shaded ravine with a stream running through it. Everything is intensely green. Karban points out a wild tiger lily, a cow parsnip. He spots a tuft of yellow beaked monkey flowers. “You can trick them with a blade of grass,” he says. “When they think they’ve been pollinated, the stigma closes up. If it’s really pollen, it stays closed. Like, ‘Okay, I’ve gotten what I’m after.’ If you trick them with a blade of grass, it will close, but then in half hour or so it will be like, ‘Oh, that’s not right,’ and open back up.”
We keep walking. Quaking aspen, forget-me-nots, alders.
“People ask me: Do plants feel pain?” But the question misses the point, Karban says. “Plants know they’re being eaten. They probably experience it very differently than we do. They’re very aware of their environment, they’re very sensitive organisms. And the things they care about are very different from what we care about,” he says. “They know when I am bending over them and casting a shadow. It’s ridiculous to think they’d prefer classical to rock,” he says, referencing the erroneous New Age notion that plants enjoy music, “but they are sensitive to acoustics.”
“I have a lot of respect for them as—I don’t know if conscious is the right word, but as very aware beings. Just working with plants and seeing how responsive they are. That’s new for me in the last 10 years or so. The facts are not new to me, but the change in world view is.” Why only in the last decade, I wonder, after spending 30 years in the field? “I’m someone who changes their mind slowly,” he says.
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