As a child, Suzanne Simard often roamed Canada’s old-growth forests with her siblings, building forts from fallen branches, foraging mushrooms and huckleberries and occasionally eating handfuls of dirt (she liked the taste). Her grandfather and uncles, meanwhile, worked nearby as horse loggers, using low-impact methods to selectively harvest cedar, Douglas fir and white pine. They took so few trees that Simard never noticed much of a difference. The forest seemed ageless and infinite, pillared with conifers, jeweled with raindrops and brimming with ferns and fairy bells. She experienced it as “nature in the raw” — a mythic realm, perfect as it was. When she began attending the University of British Columbia, she was elated to discover forestry: an entire field of science devoted to her beloved domain. It seemed like the natural choice.

By the time she was in grad school at Oregon State University, however, Simard understood that commercial clearcutting had largely superseded the sustainable logging practices of the past. Loggers were replacing diverse forests with homogeneous plantations, evenly spaced in upturned soil stripped of most underbrush. Without any competitors, the thinking went, the newly planted trees would thrive. Instead, they were frequently more vulnerable to disease and climatic stress than trees in old-growth forests. In particular, Simard noticed that up to 10 percent of newly planted Douglas fir were likely to get sick and die whenever nearby aspen, paper birch and cottonwood were removed. The reasons were unclear. The planted saplings had plenty of space, and they received more light and water than trees in old, dense forests. So why were they so frail?

Simard suspected that the answer was buried in the soil. Underground, trees and fungi form partnerships known as mycorrhizas: Threadlike fungi envelop and fuse with tree roots, helping them extract water and nutrients like phosphorus and nitrogen in exchange for some of the carbon-rich sugars the trees make through photosynthesis. Research had demonstrated that mycorrhizas also connected plants to one another and that these associations might be ecologically important, but most scientists had studied them in greenhouses and laboratories, not in the wild. For her doctoral thesis, Simard decided to investigate fungal links between Douglas fir and paper birch in the forests of British Columbia. Apart from her supervisor, she didn’t receive much encouragement from her mostly male peers. “The old foresters were like, Why don’t you just study growth and yield?” Simard told me. “I was more interested in how these plants interact. They thought it was all very girlie.”

Now a professor of forest ecology at the University of British Columbia, Simard, who is 60, has studied webs of root and fungi in the Arctic, temperate and coastal forests of North America for nearly three decades. Her initial inklings about the importance of mycorrhizal networks were prescient, inspiring whole new lines of research that ultimately overturned longstanding misconceptions about forest ecosystems. By analyzing the DNA in root tips and tracing the movement of molecules through underground conduits, Simard has discovered that fungal threads link nearly every tree in a forest — even trees of different species. Carbon, water, nutrients, alarm signals and hormones can pass from tree to tree through these subterranean circuits. Resources tend to flow from the oldest and biggest trees to the youngest and smallest. Chemical alarm signals generated by one tree prepare nearby trees for danger. Seedlings severed from the forest’s underground lifelines are much more likely to die than their networked counterparts. And if a tree is on the brink of death, it sometimes bequeaths a substantial share of its carbon to its neighbors.

Although Simard’s peers were skeptical and sometimes even disparaging of her early work, they now generally regard her as one of the most rigorous and innovative scientists studying plant communication and behavior. David Janos, co-editor of the scientific journal Mycorrhiza, characterized her published research as “sophisticated, imaginative, cutting-edge.” Jason Hoeksema, a University of Mississippi biology professor who has studied mycorrhizal networks, agreed: “I think she has really pushed the field forward.” Some of Simard’s studies now feature in textbooks and are widely taught in graduate-level classes on forestry and ecology. She was also a key inspiration for a central character in Richard Powers’s 2019 Pulitzer Prize-winning novel, “The Overstory”: the visionary botanist Patricia Westerford. In May, Knopf will publish Simard’s own book, “Finding the Mother Tree,” a vivid and compelling memoir of her lifelong quest to prove that “the forest was more than just a collection of trees.”

Since Darwin, biologists have emphasized the perspective of the individual. They have stressed the perpetual contest among discrete species, the struggle of each organism to survive and reproduce within a given population and, underlying it all, the single-minded ambitions of selfish genes. Now and then, however, some scientists have advocated, sometimes controversially, for a greater focus on cooperation over self-interest and on the emergent properties of living systems rather than their units.

Suzanne Simard in Nelson, British Columbia, holding a Douglas fir seedling, right. She studies the way trees exchange carbon, water and nutrients through underground networks of fungus.

Before Simard and other ecologists revealed the extent and significance of mycorrhizal networks, foresters typically regarded trees as solitary individuals that competed for space and resources and were otherwise indifferent to one another. Simard and her peers have demonstrated that this framework is far too simplistic. An old-growth forest is neither an assemblage of stoic organisms tolerating one another’s presence nor a merciless battle royale: It’s a vast, ancient and intricate society. There is conflict in a forest, but there is also negotiation, reciprocity and perhaps even selflessness. The trees, understory plants, fungi and microbes in a forest are so thoroughly connected, communicative and codependent that some scientists have described them as superorganisms. Recent research suggests that mycorrhizal networks also perfuse prairies, grasslands, chaparral and Arctic tundra — essentially everywhere there is life on land. Together, these symbiotic partners knit Earth’s soils into nearly contiguous living networks of unfathomable scale and complexity. “I was taught that you have a tree, and it’s out there to find its own way,” Simard told me. “It’s not how a forest works, though.”

In the summer of 2019, I met Simard in Nelson, a small mountain town not far from where she grew up in southern British Columbia. One morning we drove up a winding road to an old-growth forest and began to hike. The first thing I noticed was the aroma. The air was piquant and subtly sweet, like orange peel and cloves. Above our heads, great green plumes filtered the sunlight, which splashed generously onto the forest floor in some places and merely speckled it in others. Gnarled roots laced the trail beneath our feet, diving in and out of the soil like sea serpents. I was so preoccupied with my own experience of the forest that it did not even occur to me to consider how the forest might be experiencing us — until Simard brought it up.

“I think these trees are very perceptive,” she said. “Very perceptive of who’s growing around them. I’m really interested in whether they perceive us.” I asked her to clarify what she meant. Simard explained that trees sense nearby plants and animals and alter their behavior accordingly: The gnashing mandibles of an insect might prompt the production of chemical defenses, for example. Some studies have even suggested that plant roots grow toward the sound of running water and that certain flowering plants sweeten their nectar when they detect a bee’s wing beats. “Trees perceive lots of things,” Simard said. “So why not us, too?”

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