Suzanne Simard is a professor of forest ecology at the University of British Columbia. Her latest book is “Finding the Mother Tree” (May 2021). Simard’s life and work were the primary inspiration for a central character in Richard Powers’s 2018 Pulitzer Prize-winning novel, “The Overstory.”
MOUNT ASSINIBOINE, British Columbia — November. Snow blanketed the Rocky Mountains. I was on a solo backcountry ski trip, pausing in the pristine cordillera of the Healy Pass. Subalpine firs were bent over in casts of snow and ice, and the whitebark pines were spread-eagled like bouquets of bones, dead from mountain pine beetle and blister rust caused by the stress of climate change.
I was three months pregnant. In our year apart while my husband Don wrote his dissertation, perhaps because of our loneliness, there came a wordless dawning of the truth that I was 36 and he was 39, and the time for children had come. Skiing into Assiniboine was my celebration of this gift.
The beetles were on a rampage in the couloir. Four years earlier, in 1992, the outbreak had started northwest of here, in Spatsizi Plateau Wilderness Provincial Park; winter temperatures had increased by a few degrees and the coldest months stopped dropping below -30 Celsius, allowing the beetle larvae to thrive in the thick phloem of the aging pines.
Lodgepole pine had coevolved with the beetles in this landscape, naturally succumbing, after about a century, to create space for the next generation. As the trees declined, fuel accumulated as a matter of course, and wildfires were ignited by lightning or people. Flames released pine seeds from resinous cones and stimulated aspens to sprout from thousand-year-old root systems, their moist leaves reducing the flammability of the young forest. As fire fingered through the landscape, it petered out in these aspen-clad glades, leaving a mosaic of different-aged forests that was itself resistant to future fires.
But in the late 1800s, European settlers disrupted this balance by burning the forest montage to the ground in search of gold, creating a vast blanket of new pine stands whose uniformity was later enhanced by fire suppression and herbicide spraying that ensured the aspens wouldn’t interfere with profits. As these pine trees turned 100 and the climate warmed, the beetle populations exploded, and the landscape ran red like blood flowing through water.
The air rushed cleanly into my lungs as I glided among the dead whitebark pines, intoxicated to be following tracks and carving new turns around rockfalls and tree wells. Don was taking the afternoon to build a cradle. Contentment had enveloped us both. But in the heart of the saddle between peaks, I stopped to check some tracks in the fresh snow and felt a familiar rush of fear. The paw prints were as large as saucers, claw marks an inch deep.
Wolves. A lone skier was easy pickings.
I skied away, across the pass. Soon, though, I was lost. When I circled back to the center, I shuddered to be back to my original tracks, already frozen in the drifting snow. And covered with fresh prints.
Maybe three wolves. Hunting me?
I instinctively kept skiing down the pass. Naked alpine larch, clustered in bowls below the peaks, their golden needles already fallen, were behind me. Down here, the subalpine firs were knotted together in small groves, their numbers increasing as I descended. Telemarking with 30 pounds on my back strained my legs. My baby, no bigger than an ounce of gold, didn’t throw off my balance. I cinched up the hip buckle to stabilize myself on the icy, broken terrain and turned slowly, one link at a time.
I made a large traverse to the east to evade a ravine, avoiding a steep section before heading back. It was hard to see because the trees were tightly spaced. Younger lodgepole pines. There must have been a fire a few decades back. Soon I was off course again and checked my compass. If I didn’t stay oriented and get back to the main route, this could get dire.
My fear built on some background frustration about my research. I had growing evidence that forests have intelligence — that they are perceptive and communicative — but I didn’t feel ready to take on the establishment. The fellows would ignore me or, worse, laugh at my talk of the sentience of plants.
No, I was pregnant and needed to stay quiet to protect my child, the most precious thing in my life. A recent CBC radio interview had drawn some interest from local naturalists and environmentalists and even a few like-minded foresters, but it was met with silence from the provincial capital. Without even so much as an email from the policy guys, I wondered whether giving interviews was worth it. Or talks at conferences, for that matter. I couldn’t go public more than I already had; there was too much at stake now.
I skied back about 300 feet and found old tracks from earlier skiers. The wolf prints crossed them three times. It looked like at least five animals.
I skied farther. The lodgepole pine trees grew sparser, and their puffy crowns reached closer to the ground. There should be a special word for the type of mourning you know is to come. In a decade, almost 45 million acres of mature pine forest would be dead, representing about a third of the forested area of British Columbia. The beetles would continue to chew their way through whitebark, western white and ponderosa pines, through the U.S. from Oregon to Yellowstone, and would start infesting the jack-pine hybrids across the boreal forest of Canada, producing a total epidemic across North America in an area roughly the size of California. It would surpass any insect outbreak in recorded history and provide fuel for devastating wildfires down the road.
I passed a grove of bare aspens. The prints were melting in steaming urine. Dark orange-yellow. I kept to the main route out of the narrow valley, adrenaline making my pack lighter. The wolves stayed in front, just out of sight, leaving only traces.
Their tracks headed straight for the main northbound trail, and I suddenly calmed. The wolves were not pursuing me; they were leading me out of the valley. As the vista widened, my trail converged with one from the south. I turned onto it, while the wolf tracks veered abruptly north. A gust of wind blew over them as they disappeared into the trees.
It was as though the wolves had said goodbye.
I lit a candle in the snow for my brother and for his spirit in those wolves. The lodgepole pines were tall and strong, and their lofty crowns shadowed me while they steadfastly watched over some sub-alpine firs. I needed to linger here where canyon rock and crystallized tree crowns and packs of wolves had come together. The sun climbed over the granite peaks, and I tilted my face toward it. I pulled out a sandwich, ready to stay forever. I felt welcomed, whole. Pure and clean and untroubled.
As I ate, I wondered why trees — these aspens and pines — would support a mycorrhizal fungus that provides carbon (or nitrogen) to a neighboring tree. Sharing with individuals of its own species, especially its own genetic family, seemed like an obvious benefit. Trees disperse most of their seeds — by gravity or wind or the odd bird or squirrel — in their small local area, meaning that many individuals in an immediate neighborhood are related.
The pines clustered at the edge of this meadow were probably relatives of the same family, their genes diversified by pollen drifting in from distant fathers. These parent trees shared some of the genes of the trees around them, and passing along carbon to each other to increase the survival of their seedlings, their own offspring, would help ensure those genes got passed to future generations. A later study would show that the roots of at least half the pines in a stand are grafted together, and the larger trees subsidize the smaller ones with carbon. Blood runs thicker than water. This makes perfect sense from an individual-selection perspective. It’s Darwinian.
But my work was showing that some carbon also moved to unrelated individuals, ones of an entirely different species. From birch to fir and back again. I looked at the white aspen, its bark basking in the sun, and wondered if it shuffled carbon to the firs under its crown. The other way around too: firs to aspens.
Mycorrhizal fungi are generalists — they colonize plant root tissue, sometimes even intracellularly. They might invest in many tree species to hedge their bets for survival, and the off chance that some carbon would move to a stranger was simply part of the cost of moving it to relatives.
But this was not what my trees were showing. They were offering me evidence that the pattern of carbon movement was not just by chance, an unfortunate consequence of the moveable feast. No, my trees were demonstrating that they had a lot of skin in the game. Over and over, the experiments showed that carbon moved from a source tree to a sink tree — from a rich to a poor one — and that the trees had some control over where and how much carbon moved.
A squirrel chattered in the branch of a knotty juniper, waiting for me to toss it scraps of my sandwich. It kept an eye on the Clark’s nutcracker at the top of a pine, probably with a whitebark pine seed in its beak. A raven — a species that also coveted those energy-rich seeds — gurgled a song.
Whitebark pine depends on all these species and more, including grizzly bears, to scatter its heavy seeds. Why would the old pines trust their reproductive success to these birds and animals, whose interest in the seeds was only as food? A few seeds needed to be left to germinate and grow into offspring to ensure the successful reproduction of the elders; why trust that enough would remain? If one of these seed dispersers disappeared, perhaps in a fire or during a particularly harsh winter, then others might deliver the goods. In the same vein, why would a tree pass carbon to a generalist networking fungus — a Suillus or Cortinarius — that could then pass the carbon to an unrelated tree? From the pine to the understory subalpine fir?
I threw my crust toward the squirrel, and the raven and nutcracker swooped to jockey for the prize. Tail twitching, the squirrel launched off its stump. Just as the old whitebark pines were glad to feed their seeds to birds and squirrels, depending on more than one for dispersal, there must be a similar evolutionary advantage to a tree hosting many mycorrhizal fungal species making up the linking network, letting it benefit from a diverse suite as insurance in case one element got lost.
Maybe even more important was the fungi’s ability to reproduce rapidly. Their short life cycle would enable them to adapt to the rapidly changing environment — fire and wind and climate — much faster than the steadfast, long-lived trees could manage. The oldest Rocky Mountain juniper is about 1,500 years old and the oldest whitebark pine around 1,300, in Utah and Idaho, respectively. Meanwhile, the trees here would take decades to produce their first cones and seeds and then do so only sporadically thereafter, but their fungal network could spawn mushrooms and spores each time it rained, potentially enabling its genes to recombine several times a year.
Maybe the fast-cycling fungi could provide a way for the trees to adjust swiftly to cope with change and uncertainty. Instead of waiting for the next generation of trees to reproduce with more adaptive ways of coping with the soils warming and drying as the climate changes, the mycorrhizal fungi with which the trees are in symbiosis could evolve much faster to acquire increasingly tightly bound resources. Perhaps the Suillus and Boletus and Cortinarius fungi could respond more immediately to the warming winters that had spawned the mountain pine beetle outbreak and help the trees still gather nutrients and water to maintain a level of resistance.
The raven won the battle for my sandwich crust and spiraled past the Clark’s nutcracker in a cloud of feathers and squawks. Not only was the squirrel too slow, it had no hope of wresting anything from the beak of a bird. It would have to unearth whitebark seeds instead, after the birds buried them. Or it could feast on a mushroom left drying in the branches of a pine. It wouldn’t live long with neighbors like the raven and nutcracker if it had to rely only on whitebark pine seeds they’d overlooked. Likewise, the fungus could hedge its bets, hitching its spores on legs or feathers, or catching an updraft to colonize new hosts.
If the fungus acquires more carbon from one tree than it needs for its own growth and survival, then it could supply the excess to the other networked trees in need and, in so doing, diversify its carbon portfolio — insurance in acquiring essential resources. The fungus could shuttle carbon produced by a rich aspen to a poor pine in the middle of the summer to ensure it had two different healthy hosts — sources of photosynthetic carbon — in case there was a calamity and one died. Like investing in stocks along with bonds in case the market crashed.
The fungus might not care what species the hosts were, as long as at least one of its carbon sources remained viable. Investing in diverse plant communities is a lower-risk strategy than investing in just one species. The more stressful the environment, the more successful those fungi are at associating with multiple tree species.
Though my puzzling elated me, something still didn’t quite fit together. As I shouldered my pack and turned to the fork that led south along Bryant Creek, I thought about the greater group of interacting species: the whole community of plants, animals, fungi and bacteria. Individual selection might explain how the fluorescent Pseudomonads interacted with the mycorrhizal fungi of birch to reduce Armillaria root disease in Douglas fir. Could selection also operate at the group level?
Do cooperative guilds of species — like guilds of people in societies — exist? Where multiple tree species are linked by a network for mutual aid, in the way it takes a village to raise a child, despite a risk that there might be cheaters in such guilds? This sharing would work if our behavior was ruled by steadfast tit for tat, like the two-way transfer between birch and fir and their principle of reciprocity, changing the direction of net transfer over the course of the summer. Quid pro quo.
But what about longer-term shifts in trade? Such as when firs eventually grow taller than birches. Would the quid pro quo rule of engagement change, and how might this compare to our human lives when they become more complex and our relationships transform with age? If someone helps me with childcare, how do I repay her if she moves far away? I wondered why two tree species would continue to trade carbon over the long term, given the uncertainty of the future.
Maybe birch and fir, and Armillaria ostoyae fungus and fluorescent Pseudomonads, are in a prisoner’s dilemma where, in the long run, the benefits of group cooperation outweigh the costs of individual prerogatives. Studies show time and again that cooperation is commonly chosen in groups, even when betrayal of others could lead to a better individual reward. Fir can’t survive without birch due to a high risk of infection from Armillaria, and birch can’t survive in the long run without fir because too much nitrogen would accumulate in the soil, causing the soil to acidify and birch to decline.
In this scenario, the little fluorescent Pseudomonad bacteria serve two functions: They produce compounds that inhibit the spread of Armillaria root disease among the trees, ensuring there is still a source of carbon energy for the community, and they transform nitrogen using the carbon exuded by the mycorrhizal network. Was this still in line with selection at the individual species level, or was it at the level of the group?
Distracted, I practically skied straight into a couple of biologists tracking the radio-collared wolves that had been guiding me earlier. They knew the pack well; the leader was an old mother wolf.
I asked why they were tracking the wolves. As the shadows of the peaks grew longer, the lead tracker, a lean, windburned woman with a dark ponytail, told me about the pressure to cull wolves in the park to alleviate the decline of caribou. She pushed her sunglasses back on her head as she spoke, radiating a fierce intelligence.
“It’s the clear-cutting,” I responded, meeting her gaze. The sprouting willows and alders were attractive browse for moose, so their populations were increasing and attracting wolves. The problem was that the wolves also hunted mountain caribou, which were in precipitous decline due to habitat loss and interactions with people.
She nodded in agreement as she shifted on her skis and checked that her avalanche beacon was on.
“Yes, the snow gets so deep in the clear-cuts, the caribou can’t outrun the wolves,” she said, looking toward the trail where the mother wolf had gone. And there were more and more clear-cuts as the beetle-killed pines were being salvaged.
“Gotta go or we’ll lose them,” her assistant said, squinting at a tracking device and tightening the chest strap on his pack. The researcher’s eyes narrowed toward the pass ahead.
“See ya,” she said, and I said goodbye too, appreciating her unwavering pursuit. They evaporated into the pines as seamlessly as they had appeared, reminding me that a person could easily disappear out here without a trace. It was past noon. I had to keep moving or I’d be skiing the last miles in the dark.
The trail along Bryant Creek was fast and gently downhill, and as I raced past swaying pines, the sun at my back and the avalanche tracks falling behind me, I was grateful the wolf biologists had packed down the trail with their skis. I reached my car as the sky’s pink and purple stripes faded to black across the tilted sheets of sedimentary crust.
Ecosystems are similar to human societies — they’re built on relationships. The stronger those are, the more resilient the system. And since our world’s systems are composed of individual organisms, they have the capacity to change. We creatures adapt, our genes evolve, we can learn from experience. A system is ever-changing because its parts — the trees and fungi and people — are constantly responding to one another and to the environment. Our success in coevolution — our success as a productive society — is only as good as the strength of the bonds with other individuals and species. Out of the resulting adaptation and evolution emerge behaviors that help us survive, grow and thrive.
We can think of an ecosystem of wolves, caribou, trees and fungi creating biodiversity just as an orchestra of woodwind, brass, percussion and string musicians assemble into a symphony. Or how our brains, composed of neurons, axons and neurotransmitters, produce thought and compassion. Or the way brothers and sisters join to overcome a trauma like illness or death — the whole greater than the sum of the parts.
The cohesion of biodiversity in a forest, the musicians in an orchestra, the members of a family growing through conversation and feedback, through memories and learning from the past, even if chaotic and unpredictable, leveraging scarce resources to thrive. Through this cohesion, our systems develop into something whole and resilient. They are complex. Self-organizing. They have the hallmarks of intelligence. Recognizing that forest ecosystems, like societies, have these elements of intelligence helps us leave behind old notions that they are inert, simple, linear and predictable — notions that have helped fuel the justification for rapid exploitation that has risked the future existence of creatures in the forest systems, like us.
Excerpted from the author’s just-out book, “Finding the Mother Tree” (May 2021, Alfred A. Knopf)