Kakabeka, northwestern Ontario 48°23’45.7” N, 89°37’17.2” W
I stand on a stony bluff overlooking a valley filled with the textures and hues of northern forests: blue green shades of fir needles, wind-stoked flashes of brightness from trembling aspen and white birch leaves, spiky crowns of spruce trees, somber canopy gaps over the stunted trees of bogs, and thickets of young evergreens where wind has leveled older stands. I’m on a trail at the edge of one such thicket, a growth so dense that no person could pass without a severe exfoliation. A balsam fir tree overtops most of the crowd of young trees, eight meters tall and about thirty years old. The fir’s whole trunk is visible from the trail and its location on an elevated bluff yields breezes that, in the summertime, gave me intermittent relief from the hundreds of mosquitoes that gathered at my mammalian blood buffet.
A sound like fine metalwork rings from the top of the balsam fir. Tink tink. Zreep. Rivets tapped and rough edges filed. Birds rummage in the cones that swaddle the tree’s apex. Their hammers never cease, unifying the flock, telling where seed is most abundant.
As they work, shavings fall through the firs’ branches, cone scales barely heavier than air, ticking against fir needles as they fall.
In summer the slate blue cones were clenched shut. Copious dribbles of resin kept away birds and squirrels. Now, in October, the cones have browned and the dried resin has fallen. Scales have eased apart to reveal stacks of thin, translucent paper. A flick of wind shatters the cone with a gentle snap and hiss, then paper kites stream away, some carried high, others spinning to the ground. Each kite has a traveler clinging to its base, a balsam fir seed barely thicker than the paper that carries it. Although the seeds are tiny, they are dense with energy. Drawn by these stores of food, birds join the wind, sweeping their beaks through the cones. The sunlight sequestered inside each cone is thus divided into hundreds of parts. A mossy bank receives the energy latent in a fir embryo, a pine siskin fattens its flanks, and nuthatches tap winter stores under bark crevices.
Of the birds that work the balsam fir, none are as vocal as the black-capped chickadees. The forest here is dense with fir, spruce, and pine. I can see no farther than a meter or two. But the chickadees’ palaver advertises their location from tens of meters away. Like the restless movement of their bodies, chickadee sounds swing and hop, flickering through pitch and rhythm. They punch the air with guttural deer deer, then ascend an octave and give a quivering two-noted squeak like the vigorous rubbing of glass. High jabs intersperse slurs, then the voice drops to a throaty chik-a dew dew, the call that we humans used to give the species its name.
In every season that I visit the balsam fir tree, the chickadees flock me. Whether they are assessing, greeting, or just passing through by chance I do not know. Their inspection is thorough. One arrives and jumps its calls to the higher registers, then half a dozen more birds gather around me. I freeze. They perch on bouncing fir twigs, centimeters from my face, tilting and ducking their heads as they pass their impenetrably dark eyes over me. Their voices rasp as they wing from one side of my face to the other. I see them as they must see one another, not as distant shapes in a treetop but as beings of great visual intricacy: a tracery of gray plumes over their shoulders, blade-edged flight feathers, combed felt on their cheeks. Sometimes other birds are drawn to the gathering, perhaps responding to a change in the character of the chickadees’ acoustic news ticker. A northern parula warbler comes, then a magnolia warbler and a red- breasted nuthatch. These others glance, then drop out of sight. The chickadees are more curious and linger for minutes, then return to gleaning insects from fir needles or poking at cones. These are brief visits, unexceptional for them, I expect, but these chickadees are bolder and more inquisitive by far than any others I have encountered. Most striking are the fine variations of timbre and inflection that emerge from my close hearing of their chatter. At this intimate distance, a seemingly single type of call – deer – resolves into many sonic variants.
From twenty-six simple geometric shapes we’ve constructed a written language; in a few minutes of attention to their flock, I hear perhaps as many acoustic graphemes. We have only a weak grasp of how these sounds texture the birds’ experience of the world. Some calls predominate during breeding or are given near the nest. Other sounds transmit information about danger, using slight acoustic variations to encode information about the threat posed by different predators. Nuthatches eavesdrop on these variants, gleaning knowledge from their chickadee neighbors about which species of predatory owl are present in the forest. The chickadees use many other sounds as the birds interact with one another, seeming to convey the subtleties of affinities and disputes. No doubt our language and their communications diverge in many ways, but heard closely, the two are not so different in their acoustic richness.
My inspectors are a social species. Their intelligence resides within both individuals and societal relationships. A chickadee therefore lives in a dual world, a self and a network. These birds are just one example of the larger duality within the forest’s nature, one that permeates the biological world and may date back to the origins of life. Chickadee lives echo the stranger world of the balsam fir tree, the forest, and the creative ambiguities of biological networks.
Inside their skulls, the sophistication of the neural capacity of black-capped chickadees increases in autumn. The part of the brain that stores spatial information gets larger and more complex, allowing the birds to remember the locations of the seeds and insects that they cache under bark and in clusters of lichen. The superior memory of the birds that I hear in the tips of the fir tree is a neuronal preparation for the hungry days of late autumn and winter. The seat of spatial memory in the brains of chickadees that live in these northern forests is particularly voluminous and densely wired. Natural selection has worked winter into the birds’ heads, molding the brains so that chickadees can survive even when food is scarce.
Chickadee memories also live within societal relationships. The birds are keen observers of their flockmates. If one bird should happen on a novel way of finding or processing food, others will learn from what they see. Once acquired, this memory no longer depends on the life of any individual. The memory passes through the generations, living in the social network. If black-capped chickadees are like their European relatives, regional traditions color this cultural knowledge. Birds in one part of the forest may favor a particular style of opening cones or capturing insects, a style transmitted to them from the happenstance learning of their ancestors. Generations ago, a bird in the western part of the forest may have discovered a faster way of extracting a fir seed. The bird’s eastern counterpart also invented a new cone-breaking technique, slightly different from the western approach. Now the innovators are dead, but west and east still differ, even though the two methods are equally effective. These traditions trump individuality. Birds conform to their group’s preferred habits even when they have successfully tried the other way of feeding.
Bird behavior is of great importance to the balsam fir tree. Although the majority of the tree’s seeds are wind dispersed, bird beaks are often the blow that sends cone scales flying. The birds’ hunger has two opposing effects on the trees’ future. The balsam fir’s reproductive efforts are trimmed by the thorough work of foraging birds, a loss for the trees. In eating the seeds, birds divert energy that could have nourished young seedlings. The trees’ stores are rerouted into bird stomachs and keep wild gray flames on the wing. This robbery is a heavy burden; it takes fir trees two years to build the energy needed to produce a full seed crop. But in hiding the stolen loot through the forest, chickadees and other birds lodge fir seeds in rotting logs and other prime seed beds. In winter many of these seeds are recovered and eaten, but some are forgotten. Bird memories are therefore a tree’s dream of the future. Despite its neurological abstraction in the mind and culture of another species, chickadee memories are as important for the balsam fir as are soil, rain, and sunlight.
The chickadees’ two ways of holding knowledge “in mind,” in the individual and in the network, mirror the principles of the balsam fir tree’s own intelligence and behavior. Even though it lacks a nervous system, the tree’s cells are awash in hormones, proteins, and signaling molecules whose coordination allows the plant to sense and respond to its surroundings.
Some plant responses are long term, such as the growth of branches into light or roots into fertile soil. Plant architecture is not a haphazard affair but is the result of constant assessment and adjustment as conditions change. Twigs sense the luminosity of their particular location on the tree and grow accordingly. Flat fans of needles grow in the shade, to maximize exposure to meager sunlight, but in strong sunlight needles take on an upswept form to both gather sun and minimize shading of needles below. Branches offset themselves vertically from those around them, avoiding shading and wiggling themselves into sunlight.
Other responses last just a few minutes. The upper surface of a fir needle is a waxed floor, an unbroken green sheen. Underneath, two silver lines run lengthwise down the needle. Seen through a magnifying lens, the blur of silver resolves into a dozen rows. The rows are wheat-crop straight, hundreds of bright white dots on a green background. These dots are breathing pores, each one made from the gap between two curved cells. The cells integrate information about the state of the needle’s internal environment, then open or close pores to admit gases or release water vapor. Every cell inside the needle is making similar assessments and decisions, sending and receiving signals, modulating its behavior as it learns about and responds to its environment.
When such processes run through animal nerves, we call them “behavior” and “thought.” If we broaden our definition and let drop the arbitrary requirement of the possession of nerves, then the balsam fir tree is a behaving and thinking creature. Indeed, the proteins that we vertebrate animals use to create the electrical gradients that enliven our nerves are closely related to the proteins in plant cells that cause similar electrical excitation. The signals in galvanized plant cells are languid – they take a minute or more to travel the length of a leaf, twenty thousand times slower than nerve impulses in a human limb – but they perform a similar function as animal nerves, using pulses of electrical charge to communicate from one part of a plant to another. Plants have no brain to coordinate these signals, so plant thinking is diffuse, located in the connections among every cell.
The balsam fir tree also remembers. If caterpillars or moose browse its needles, the nibbling assault lodges itself in the chemical makeup of the tree, in a manner analogous to the changes in a chickadee’s nerve cells after a near miss with a predator. The tree’s subsequent growth is more heavily defended by unpalatable resins, like a bird turned jumpy by its bad experience with a hawk. The fir also remembers air temperatures dating back nearly a year, a memory that helps the tree to know when to winterize its cells. Plant memories can cross generations, as the offspring of stressed parents inherit an enhanced capacity to generate genetic diversity when they breed, even if this next generation experiences benign conditions. We only partly understand how plants hold these memories. It seems from experiments with cress plants that changes in the proteins that wrap DNA may be partly responsible. By looping DNA either tightly or loosely, plants can store information about which genes will be most useful in the future. Plant memory is thus captured in biochemical architecture.
Roots and twigs have memories of light, gravity, heat, and minerals. Darwin discovered some of these abilities by rotating young bean roots and showing that they remembered their previous orientation for many hours. He compared the roots’ behavior to that of a headless animal, with memory suffused through the body. Whether the balsam fir has exactly the same abilities as the beans and cress plants is unknown, but the tree possesses the same internal chemical and cellular networks as these laboratory grown species.
Part of a plant’s intelligence exists not inside the body but in relationship with other species. Root tips, in particular, converse with species from across the community of life, especially with bacteria and fungi. These chemical exchanges locate decision making in the ecological community, not in any one species. Bacteria produce small molecules that serve as signals, allowing the cells to make collective decisions. These same molecules soak into root cells, where they combine with plant chemicals to promote growth and regulate the architecture of roots. Roots also signal to bacteria, providing them with sugars that both nourish the bacteria and switch on their genes. This halo of food and encouraging chemical signals cause the bacteria to cluster in gel-like layers around the root. Once established, the bacterial layer defends the root from attack, buffers it from changes in salt concentrations, and stimulates growth.
Roots converse with fungi, sending chemical messages through the soil. On receiving the message, symbiotic fungi grow toward the root and reply with their own chemical ooze. Root and fungus then change the surfaces of their cell membranes to allow more intimate contact. If the chemical signals and cell growth occur in the right sequence, root and fungus entangle and begin an exchange of sugars and minerals. In addition to food, the root chimera moves information from one plant to another in the form of chemical signals that travel through the fungus. These molecules carry messages about attacking insects and drying soil, the stressors of plant life. The soil is therefore like a street market. Roots gather to exchange food and in doing so they also hear the neighborhood news.
Nearly 90 percent of all plant species form belowground unions with fungi. Our eyes therefore tell half truths when we gaze on a forest, a prairie, or a leafy urban park. The verdure of plants that we see is only part of the network that brings the community into being. For many trees, especially those like balsam fir that grow in cold, acidic soils, the fungus/ root relationship is particularly well developed, comprising a sheath of fungal tissue around every root. Working together, both fungus and plant can thrive in the challenging physical environment of boreal forest soils.
This network of communication also includes leaves. There plant cells not only sniff the air to detect the health of neighbors but also use airborne odors to attract helpful caterpillar-eating insects. Sound plays a role in this communication. When a leaf senses the vibrations of a caterpillar’s moving jaws, these chewing sounds cause the leaf to mount a chemical defense against the insect. Leaf cells therefore integrate chemical and acoustic cues as they sense and respond to their surroundings.
A leaf is not just composed of plant cells, though. The waxy surfaces of leaves are peppered with fungal cells, and leaf interiors are host to dozens of fungal species. Like fungi in the roots, leaf fungi have smaller cells than plants and lack photosynthetic pigments. Fungi are closer kin to animals than they are to plants, gaining their food not from sunlight but by absorbing food into their bodies. This suggests a reason for the ubiquity and success of intimate plant/fungus relationships. The two partners are sufficiently different that each can bring a talent that the other lacks. The union melds two different parts of life’s family tree, yielding creatures whose physiology is nimble and many-skilled, both in leaves and in the soil. A leaf populated with fungi is able to deter herbivores, kill pathogenic fungi, and withstand temperature extremes much better than one composed only of plant cells. There may be one million species of leaf-dwelling (“endophytic”) fungi on the planet, making them one of the world’s most diverse groups of living creatures.
Virginia Woolf wrote that “real life” was the common life, not the “little separate lives which we live as individuals.” Her sketch of this reality included trees and the sky, alongside human sisters and brothers. What we now know of the nature of trees affirms her idea, not as metaphor but as incarnate reality. In the forest, Woolf’s common life is the only life.
David George Haskell is a professor at the University of the South in Sewanee, Tennessee, and will give the keynote address at this year’s Northern Woodlands Conference. This piece was adapted with permission from his book The Songs of Trees: Stories from Nature’s Great Connectors, published on April 4, 2017, by Viking, an imprint of Penguin Publishing Group, a division of Penguin Random House, LLC. Copyright © 2017 by David George Haskell.