Thursday, July 22, 2021

Rebecca Giggs / A Better Way to Look at Trees

A Better Way to Look at Trees

What pioneering new research has revealed about the forest

This article was published online on June 17, 2021.

Above all else in the plant kingdom, trees make good trellises for our self-regarding thoughts. Robert Frost knew this when he wrote “Two roads diverged in a yellow wood.” A woodland is the right spot to yield to reflection. Though the life of a tree has little in common with the life of a person, we are accustomed to approaching trees on personal, even introspective, terms. As trunk is a synonym for torso, as branch can be interchangeable with limb, trees of great variety (especially the old ones) give body to human concerns.

Consider the coastal eucalyptus, forced by sea winds to grow prostrate along the ground—how the maxim “Better bend than break” takes shape in its supplicating posture. Or meditate on Sakura, the cherry blossom, and its instructive transience. We look to trees for their symbolism, and to have our own comparatively stunted existence put into perspective. High up in the Sierra Nevada mountains, bristlecone pines preside—seemingly more stone than wood, partly fossilized. Some rise from saplings at a tempo so slow that they endure through generations, even whole civilizations—thousands of years—living off the ephemeral sustenance that all trees rely on: light, water, a smattering of nutrients drawn from the soil. These ancient pines have been called sages and sentinels, as though it were their edict to stand watch over cycles of human progress and folly.

Yet have we ever really understood trees in the plural? Since the turn of the millennium, a remarkable recasting of our attention—away from the gravitas of individual trees and toward the question of what trees do together, as a collective—has been under way. What passes between trees, the nuance of their exchanges, and the seemingly delicate mechanism of their connections—that mystery has inspired a rich new realm of research, and along with it, a subgenre of literature dedicated to spreading a revised conception of the powers and processes that allow arboreal plants to thrive. The title of the German forester Peter Wohlleben’s hugely popular 2015 book, The Hidden Life of Trees: What They Feel, How They Communicate—Discoveries From a Secret World, sums up the paradigm shift and captures the tone of awed revelation shared by researchers and readers alike. What a tree is—tree botany in its essentials—feels utterly changed. Will our self-centered thoughts, as we stand in the never-silent forest, change too, and how?

Meg lowman and suzanne simard are two pathfinders who have worked for decades in this field (that is, the forest), and they have now written books not just to instruct, but to reorient and inspire. Lowman—who goes by “Canopy Meg” in educational settings—is an ecologist and a conservationist on a mission to correct trunk bias, our myopic attachment to the tree’s upright midsection. For a plant to be considered a tree—as distinct from a shrub or a vine—it must have a woody stem of cellulose made rigid by an organic polymer, lignin. Reasons for fixating on tree trunks are not hard to come by. The commercial worth of a tree (aside from the fruit-bearing and oil-producing types) principally depends on its timber. Trunk appeal surely also lies in the eye of the beholder. Being ground-dwelling mammals, we live closer to tree trunks than to boughs or roots—and the mind readily personifies their surface, seeing eyes in knots, dimples and dewlaps in folds of bark. The result, Lowman argues, is a failure to engage with the expansive wilderness above: the floating world of the forest canopy, a mantle of enormous significance, as the subtitle of Lowman’s new book, The Arbornaut: A Life Discovering the Eighth Continent in the Trees Above Us, conveys.

Lowman’s focus zooms in on the foliage. Having grown up in a cottage built around the girth of an elm tree—a fairy-tale prologue to the lifelong pursuit in store—she devoted her early scientific career to a deceptively simple ambition: She aimed to study leaves in the wild, from budburst to drop. As Lowman describes the venture, she improvised with slingshots, weights, and caving tackle to rig a tree’s branches for a low-impact ascent, reverse spelunking (as cat-footed as is humanly possible), up into the greenery. There she discovered the fascinations of the “phylloplane,” the surface of a leaf, and its little occupants—weevils and walking sticks, moths, fly larvae, bees, caterpillars. How eerie to think that more than half the planet’s terrestrial animals live up there, overlooked—underlooked?—by most of us.

The array of leaves is staggering, too. In the tropical canopies Lowman surveys, the shape of a leaf is typically governed not only by a tree’s DNA, she reveals, but by that leaf’s position in the forest. Leaves in the understory are blackish-green platters, often dusty with pollen, and thinner than those above. Leaves cresting into the sky are liable to be yellower, smaller, and leathery. The middle strata are a mixed salad: Leaves that catch sun flecks are distinct from their dimly lit neighbors, though they may emanate from the same bough. If insects roost in nearby air plants, a tree’s leaves may be more prone to getting skeletonized. If a tree sustains nests of ants, the ants may prey on leaf-eating grubs, resulting in more intact leaves. Elevation and wind can vary a leaf; moisture can increase its likelihood of being burdened with moss and lichen. In turn, trees together can engineer the weather they grow in; Amazonian canopies induce rain by releasing enough water droplets through transpiration to create low-level clouds. Showers from these clouds change the air temperature, triggering winds that draw additional moist air inland from the oceans, watering the trees with further rain.

Though we often talk of trees as though they were nature’s metronomes, observing the steady tick of time in their corrugated rings, Lowman’s research makes clear that a single tree is not all one age. In non-deciduous forests—those that don’t undergo a seasonal fall—the leaves on an individual tree have staggered life spans. The lifetime of a leaf offers clues to its function, and to the tree’s overall strategies for survival. On the coachwood, darker, larger leaves live longer; more nutrients go into their production, so retaining them makes sense. The foliage of other trees turns over quickly—perhaps because the tree has evolved to keep pace with high levels of insect defoliation. Leaves on the giant stinging tree of eastern Australia (a nettle capable of growing to 40 meters) last only four to six months; nearly half of the tree’s leaf-area disappears into the maw of the single beetle species that is impervious to its sting. Trees possibly gain secondary benefits from herbivory. Leaves may, in effect, be sacrificed so as to bring “frass” (insect excrement) to enrich the ground around a tree’s base. Each leaf has its biography, its society, and—with the aid of Lowman’s pen—an obituary. If a tree was once understood as a mostly static living object, here we see it rippling with change, configured by its surroundings.

Fashioned by a host of extrinsic factors, a tree also exerts its influence in previously invisible ways. Leaves collect light, of course, and thereby beget the energy a tree needs for fresh growth, regeneration, and reproduction. But leaves, including their stems and buds, also emit airborne biochemicals. Some plant matter, having caught fire, releases smoke that signals to certain seeds that conditions are conducive to germination. Leaves assailed by grazers might effuse what some scientists call “wound hormones”—in certain trees, this response can convey more than the fact of injury. A beech leaf torn by the mouth of a munching deer and a beech leaf snipped mechanically, for example, release different concoctions of chemicals; deer saliva is the trigger in the first case. Studies done on other plants exposed to vapors from damaged leaves have shown that unharmed neighbors begin to ramp up production of defensive toxins, targeted to deter specific herbivores. On Lowman’s continent high above, she gathers evidence to show that, besides being a habitat for tree-living creatures, a canopy is the lively and fluctuating expression of tree interaction and strategy.

Suzanne simard, a preeminent forest ecologist who teaches at the University of British Columbia, goes underground to uncover camaraderie in tree plantations in Finding the Mother Tree: Discovering the Wisdom of the Forest. Like Lowman’s, her imagination was kindled in childhood, during an emergency that she recounts early in the book. The family dog had fallen into a lakeside outhouse, and frantic digging ensued to extract the pet from the pit. Entranced, Simard watched as leaf litter—shed by birches, hemlocks, cedars, and firs—was raked back to expose a swath of fungal tendrils glistening like spun sugar. Pickaxes cut through humus (a fermenting paste of dead plant life), the wickerwork of tree roots, a narrow band of white mineral sand, and yet more fungi tangled below. It is to this surprisingly vital world that Simard has returned, again and again, throughout the course of her professional life.

Simard’s transformative contribution to arboreal science has been to explain the function of mycorrhizal networks—a webbing of thready fungi, reticulated through and expanding beyond tree roots, fastening trees to one another in the soil. Picture a mirror canopy beneath the forest floor. This subsurface layer is composed not of leaves, but of more filamentous stuff: a cross-hatching of fungal fibers, milk-pale, inky, or translucent. To the trees’ advantage, these organic structures act as conduits for shuttling water, carbon, nitrogen, and biochemical information between trees that are related (progenitor and seedling), between trees of the same species (say, beech to beech), and even between trees of different species (alder to pine). The fungi—there are thousands of varieties—benefit from absorbing sugars in the exchange, which their cells could not otherwise obtain. By linking multiple trees, each fungus diversifies its source of nutriment and hedges against the demise of a single tree or species. The trees leverage the fungi, the fungi exploit the trees: a relationship of co-cultivation.

As Simard frames it, the trees she and her team study are engaged in a kind of mutual aid. Resources are rerouted from trees in the sunlight to those that grow in their shade, from trees that have surplus water to those that are dehydrated. Signals are telegraphed from bug-infested trees to adjacent, healthy trees. Saplings detached from the network fail to thrive. As an aged tree reaches its terminus, it might use mycorrhizal linkages to entrust sizable carbon stores to its young; these, Simard names “Mother Trees” (mothering here being tantamount to self-sacrifice). Rather than being competitive organisms, each tree invests in the well-being of the forest as a whole, via mycorrhizae.

Simard’s and lowman’s explorations have ushered in a new kind of tree, or a new vision of tree life, different from the tree life that poets have romanticized: the solitary, singular tree, a heavy anchor flung into the past, emblematic of fortitude or witness. This newfound tree is networked, sensitive, companionate, and communicative; it matters as part of a conjoined whole, the canopy or a mycorrhizal woodlot. It displays caretaking toward offspring and, far from being siloed in its own world, it engages in a dynamic exchange. Such findings make trees seem capable of so much more than we once imagined. The notion that plants “do” anything, outside of surging toward the light and siphoning water, would imply threshold competencies that have long been regarded as mental, or at the very least sensory. Biologists have traditionally held that the faculties required for communication belong to life-forms with brains, eyes, ears, nostrils, and tongues (at a minimum, skin), not to plant life. Can something made mostly of wood demonstrate an awareness of other organisms nearby? Can it be strategically responsive, and exhibit kinship, or a sense of self? Is a tree intelligent? In stories, trees that interact are declared anthropomorphic, because fellow feeling is considered a human trait. To speak of trees as social beings remains, in some quarters, heretical.

No wonder, though, that this account of a forest has also struck many as beguiling. The portrayal of resource-sharing in the woods sounds so benevolent, so wise, in a world where inequality continues to increase. While strife and delusion travel with terrifying speed in our networked, online existence, the spectacle of intricate, protective arboreal cooperation beckons as blissful, utopian. The discovery of a covert unity and nurturance among separate trees acquires a special resonance against the backdrop of the coronavirus pandemic. What looks lone and immobile is, in fact, linked and supportive. Squint, and qualities once deemed anthropomorphic begin to seem, well, vegetalmorphic.

Yet perhaps we haven’t truly let go of trunk bias and the narcissism of seeing ourselves in trees. Maybe we have only shifted to looking for messages of community resilience over spiritual salvage. We are discrete beings and know no other way of life so intimately as we know our own. As social mammals, we make a virtue of parental ministration where other life-forms appear to have no need for it. By choice, we seek dialogues; we enter into collective arrangements that many hold to be a common good; we tend to our communities. Trees do not make this choice; almost certainly they do not consider themselves selves; they know no ideology of mutual aid.

Indeed, some trees are, biologically speaking, monastic—secluded in small groves, they profit from dispersing their seeds into rivers to be carried far away by ocean currents. Others, such as the strangler fig, are innately parasitic. Tree flourishing doesn’t necessarily entail solidarity. Lowman makes the point that tropical trees in high-diversity rain forests may not benefit from germinating near their “conspecifics” (their parents), because then a population of devouring insects, adapted to feed off one plant species, could more readily hop between adult and sapling. So mycorrhizal fostering of young trees would not be advantageous in a biodiverse environment: It would bond new trees to old in a proximity that increases the chance of defoliation, and also the spread of species-specific pathogens. What looks, to us, like ruthlessness and self-interest might best serve a tree’s genetic inheritance in the long run.

Returning from a hike recently, I glimpsed red hemorrhaging from the base of a tree set back from the trail, and an instinct released a bleat of adrenaline within me so swiftly—pain, there’s pain—that I stumbled on the path. I drew the brush aside and saw that the bleeding thing was a bloodwood tree, its vivid “blood” only sap. A tree has no nervous system, no pain receptors, no neurons, and very likely the bloodwood was only extruding a borer insect by inundating it with fluid. What any tree “feels,” what it “wills” or “wants,” is so far removed from our reality that even to use scare quotes is misleading. Plant intelligence remains staunchly nonhuman. And yet, in that moment, I could not stop sympathy from welling up, a response that felt more animal than cerebral. For a second, I touched the gleaming sap, glossy but solidifying in the air. It gave off no warmth. I thought then of fungi, a flickering presence in this landscape, appearing spasmodically as puffballs, conks, and earthstars, only to melt away back underground: hidden organisms, dainty, deathly. That a tree’s durability might rest on such a fragile life raft seemed the most important message to hear.