Listen to an audio version of this story
For more audio from The Walrus, subscribe to AMI-audio podcasts on iTunes.
The trees by the creek were dense and plump, and the ones at the top of the slope looked sparser and smaller. The soil would be drier there, water shedding off the granite knoll like a toboggan sweeping downslope. By comparing the architecture of the network of the dry upper stand with this moist lower forest, I could see if the linkages up there, where water was more precious, were denser, more plentiful, more crucial to the establishment of a seedling.
At the first old tree, twenty metres in as I headed up the hill toward the crest, saplings skirting its crown like a hula hoop, I pulled out my T-shaped increment corer to check its age, thankful the handle was orange because the leaves of the thimbleberry shrubs were as big as dinner plates and could swallow anything that dropped. I fit the bit shoulder-high into a furrow of the tree’s chunky bark and cored the tree to the pith, drawing out a small cross-section of its striped insides.
Examining the core, my pen dotting each decade, I slowly counted her years: 282. I cored another dozen trees around my first one, all different heights and girths, and they ranged in age from five years to the same couple of centuries as the first. These forests, in the interior of British Columbia, experienced fire every few decades or so, when the summers were dry and there was plenty of fine fuel—when twigs and needles from old trees gathered on the forest floor, blades from deep grasses senesced and dried, and thickets of new firs started to choke out the watery aspens and birches. With a single spark, patches of the forest would burn, the old trees usually surviving, the understory swept clean. If the fire scorched the floor in tandem with a good cone year, a new bunch of seeds germinated.
I stuffed the tree cores inside colourful straws, sealed the ends with masking tape, and labelled each one so I could double-check the ages and measure the annual radial growth increments under a microscope at my University of British Columbia lab. There, I could compare each year’s growth with the corresponding annual rain and temperature records. I ran my thumb across the tip of my trowel to make sure it was sharp, followed a thick root running from the base of my first old tree to where it tapered to the width of a finger, and sliced open the forest floor in search of brown truffles, the scabby below-ground mushrooms of Rhizopogon. The trowel cut through the litter and fermentation layers and slit open the humus to reveal the dense grains of underlying minerals.
After half an hour, mosquitoes biting my forehead, my knees sore on twigs, I hit a truffle the size of a patisserie chocolate. It was resting smack between the humus layer and the mineral horizon, and I scraped away the organic crumbs and found a beard of black fungal strands running from one end of the truffle to the old tree’s roots. I followed another pulpy skein in the other direction, and it led me to a cluster of root tips that looked like white translucent pussytoes. One root tip was especially welcoming, and I gently tugged it, like pulling a stray thread in a hem. A seedling a hand’s length away shuddered slightly. I pulled again, harder, and the seedling leaned back in resistance. I looked at my old tree, then at the little seedling in the shadows. The fungus was linking them.
I tracked another root from the elder and found another truffle, and another. I raised each to my nose and breathed in its musty, earthy smell of spores and mushroom and birth. I traced the black pulpy whiskers from each truffle to the riggings of roots of seedlings of all ages, and saplings too. With each unearthing, the framework unfolded: this old tree was connected to every one of the younger trees around it. Later, one of my graduate students would return to this patch and sequence the DNA of almost every Rhizopogon truffle and tree—and find that most of the trees were linked together by the Rhizopogon mycelium, the network of fine underground filaments of a fungus, and that the biggest, oldest trees were connected to almost all of the younger ones in their neighbourhood. One tree was linked to forty-seven others, some of them twenty metres away. We figured the whole forest was connected by Rhizopogon alone.
Listen to an audio version of this story
For more audio from The Walrus, subscribe to AMI-audio podcasts on iTunes.
We would publish these findings three years later, in 2010, followed by further details in two more papers. If we’d been able to map how the other sixty fungal species connected the firs, we surely would have found the weave much thicker, the layers deeper, the stitching even more intricate. Not to mention the arbuscular mycorrhizal fungi adding interstitial components to such a map as they possibly joined the grasses and herbs and shrubs in an independent web. And the ericoid mycorrhizal fungi linking the huckleberries in their own network, and the orchid mycorrhizas with their own too.
Through the corset of branches, I saw a hawk circle overhead. Solitude is rare in the forest, and I felt slightly uneasy. But the breeze lulled me, and I continued my work, using the finest tip of my Swiss Army knife to excavate a germinant no bigger than a daddy-long-legs. I pulled on the collar of the exposed stem, and a radical—one of the tiny primordial roots—slid out of the old-blood humus. This courageous root was vulnerable, and it survived by emitting biochemical signals to the fungal network hidden in the earth’s mineral grains, its long threads joined to the talons of the giant trees. The mycelium of the old tree branched and signalled in response, coaxing the young roots to soften and grow in a herringbone and prepare for the ultimate union with it.
Squatting, I peered at the radical through my hand lens and fumbled to split open the fragile root with my dirt-caked fingernails, to steal a glimpse of the fungal mycelium that might have succeeded in encasing the cortical cells, finishing the courtship. My nails were so blunt! I twisted around to let the sun pour on my hands, and I scoured the ragged root for signs of tallow between the cells. On invasion, the fungus envelops the root cells, forming a latticework—a Hartig net. The fungus delivers nutrients, supplied by the vast mycelium of the old trees, to the seedling through this Hartig net. The seedling in return provides the fungus with its tiny but essential sum of photosynthetic carbon.
The roots of these little seedlings had been laid down well before I’d plucked them from their foundation. The old trees, rich in living, had shipped the germinants waterborne parcels of carbon and nitrogen, subsidizing the emerging radicals and cotyledons—primordial leaves—with energy and nitrogen and water. The cost of supplying the germinants was imperceptible to the elders because of their wealth—they had plenty. The trees spoke of patience, of the slow but continuous way old and young share and endure and keep on.
Once the Hartig net was firmly embedded in the radical of the new sprouts and the old trees were dispatching sustenance, making up for the paltry rates of photosynthesis by the cotyledons, the fungus could then grow new hyphal threads to explore the soil for water and nutrients. As the miniature crowns of the seedlings spawned new needles, they would feed the mycelium with their own photosynthetic sugars so the fungus could travel to even more distant pores. Once on solid footing, life running smoothly, the growing root could then support a fungal mantle—a coating—as though donning a jacket of mycelium from which even more fledgling hyphae could grow into the soil. The thicker the mantle and the greater the number of fungal threads the root could feed, the more extensively the mycelium could laminate the soil minerals and the more nutrients it could acquire from the grains and transport back to the root in trade. Root begets fungus begets root begets fungus, the partners keeping a positive feedback loop until a tree is made and a cubic foot of soil is packed with a hundred miles of mycelium. A web of life like our own cardiovascular system of arteries, veins, and capillaries.
I continued through the trees, ducking under the crowns of thick-barked elders, striding through grassy gaps sprinkled with seedlings, swimming through thickets of spindly saplings, the data of my graduate students churning through my mind as if in a calculator. These young trees got their start in the shadow of the old by linking into their vast mycelium and receiving subsidies until they could build enough needles and roots to make it on their own. The Douglas fir seeds that another of my graduate students had sown around mature trees had a 26 percent increased survival rate where he’d allowed them to link into old-tree fungal networks compared with where he’d isolated them in bags with pores allowing only molecules of water to filter through. The seedlings in this forest were regenerating in the network of the old trees.
Resting on a stump, I took a long drink of water and noticed a cluster of seedlings no bigger than roofing nails. A below-ground network could explain why seedlings could survive for years, even decades, in the shadows. These old-growth forests were able to self-regenerate because the parents helped the young get on their own two feet. Eventually, the young ones would take over the tree line and reach out to others requiring a boost.
With the sun straight overhead, I checked my email on my BlackBerry. I marvelled at this little machine, how the internet made me feel so connected to the world. This forest was like the internet, but instead of computers linked by wires or radio waves, these trees were connected by mycorrhizal fungi. The forest seemed like a system of centres and satellites: the old trees were the biggest communication hubs and the smaller ones the less-busy nodes, with messages transmitting back and forth through the fungal links. Back in 1997, the journal Nature had published an article of mine and had called this network the “wood-wide web.” That was turning out to be much more prescient than I’d imagined. All I had known back then was that birch and fir transmitted carbon back and forth through a simple weave of mycorrhizas. This forest, though, was showing me a fuller story. The old and young trees were hubs and nodes, interconnected by mycorrhizal fungi in a complex pattern that fuelled the regeneration of the entire forest.
The big old trees on this knoll were spaced farther apart, and the saplings were fewer and farther between, limited by drought. The thimbleberries and huckleberries had disappeared, replaced by the long bunched leaves of pinegrass, the bonnets of silky lupines, and the occasional red-dotted buffaloberry shrub. The lupine and buffaloberry were nitrogen fixers, adding nitrogen to this slow-growing stand. Though the south-facing slope was dry, the plant community was intact, with no invasive weeds like the ones creeping along the roadside where I’d parked.
This forest was at the northern edge of the arid Great Basin, but to the south, it was mainly too dry for trees, and bunchgrasses grew instead in native prairies. These native grasslands were under pressure from exotic weed invasions, and in this case, the mycorrhizal networks were sapping them of life. Knapweeds, spread by animals including cattle, tapped into the mycorrhizas of the grass tillers and stole phosphorus right out of their roots. Instead of the fungi of knapweed helping the grasses thrive, as they had with birch and fir, they were accelerating the decline that had begun with humans herding cattle. They did this possibly by sending the native grasses some poisons or an infection to finish the murder. Or by starving them, taking over their energy, and degrading the native prairie.
With my increment borer, I cored a handful of ancients on the knoll. The youngest was 227, the oldest 302—elders of the forest. Their thick bark was scarred by flames, more pronounced than the trees in the wetter area below because it was hotter and drier here, a magnet for lightning-caused fires. This explained the wide range of ages.
I scraped the soil with my trowel. Just like the old trees near the creek, those on this crest were decorated with truffles and tubercules—clusters of mycorrhizal roots covered in a fungal rind—and golden fungal strands that ran from them like shooting stars. Here, too, the trees and fungi were in an intimate web. Compared with the trees down below, there were even more connections where the soil was drier and the trees more stressed. This made sense! Here on the crest, the trees invested more in mycorrhizal fungi because they needed more from them in return.
I leaned against the oldest tree, at least twenty-five metres tall with branches like the ribs of a whale. Seedlings were germinating in a crescent along the northern dripline of the tree, their needles stretched like spider legs, and I excavated one with my knife. Fungal threads streamed off the end of its roots, and I felt intoxicated. I pressed the seedling and its woolen mycorrhizas between the pages of my notebook so I could look more closely at home. But I already knew that these little seedlings were linked into the network of the old trees, receiving enough water to get them through the driest days of summer. My students and I had already learned that the deep-rooted trees brought water up to the soil surface at night, by hydraulic lift, and shared it with shallow-rooted plants, helping the archipelago stay whole during prolonged drought.
Without such attachment, the deaths of seedlings on hot August days can be nearly immediate, their needles turning red and the collars of their stems wounded with burns, leaving not a trace by snowfall. For these young recruits, small resource gains in moments of vulnerability make the difference between life and death. But, once their roots and mycorrhizas reach the labyrinth of russet pores, where water clings in films to soil particles, they ratchet up their game and grow a foundation. A root system like that, unfettered in its opportunity, was far more resilient than the chunky pistons grown in Styrofoam tubes in the nursery, where the seedlings intended for plantations were so stuffed with water and nutrients they couldn’t—didn’t need to—sprout adequate roots to partner with fungi to connect with the soil. Their thick needles needed streams of water under the hot August sun, but their roots continued to grow as though imprisoned, unable to reach the old trees for help when the soil cracked in the dry clear-cuts.
I walked from the northern crescent of seedlings back to the old tree, the ground directly underneath its canopy bare even of grass. Not a seedling grew here. Its crown was so dense that it intercepted most of the precipitation and sun, and its roots were so thick that they took up most of the nutrients and water. We later found that there was a sweet zone, a donut around the dripline, the fringe of the crown, where the water dripped off the outermost needles and some seedlings flourished. Not too close to be starved by the needs of the old tree and not too far away for the grasses in the intervening meadows to rob them of what they required.
I ducked under the opposite edge of the old tree’s crown—facing south, where the sun beat down—and gazed down the slope rolling into scree. It was so hot and dry on this side that not even a network could save a seedling from burning up. In the extremes—such as a desert—even the fungus could fail to bring life to a tree. An old log lay on the angle of repose, poised to roll over broken stones, and chunks of heartwood were newly exposed, beetles and ants flowing in lines with white fungi in their clutches. Claw marks. Bear, I thought, from at least a few days before. Douglas fir seedlings cascaded off the north side of the log, where there was a sliver of shade along its length, and spilled onto the forest floor. The scrap of advantage from the shade meant a little less water lost, a slightly thicker film coating the soil pores, the difference between survival and not. I wondered if the white fans of mycelium were linked to the old tree and helped keep the wood moist. These seedlings were alive, I figured, only because the fungi were importing water from somewhere.
The old trees were the mothers of the forest. The hubs were mother trees. Well, mother and father trees, since each Douglas fir tree has male pollen cones and female seed cones. But it felt like mothering to me, with the elders tending to the young. Yes, that’s it. Mother trees. Mother trees connect the forest.
This mother tree was the central hub that the saplings and seedlings nested around, with threads of different fungal species, of different colours and weights, linking them, layer upon layer, in a strong, complex web. I pulled out a pencil and notebook. I made a map: mother trees, saplings, seedlings. Lines sketched between them. Emerging from my drawing was a pattern like a neural network, like the neurons in our brains, with some nodes more highly linked than others.
My little trail joined another, like a frayed thread joining a rope. I knew the networks were complex, with thick cords like freeways amid a gauze of fine hyphae that behaved like secondary routes. The thick cords themselves consisted of many simple hyphae that had twined together, forming an outer rind around a space. Information chemicals could travel through these cords like water through a pipe. The main trail widened, and after a few more curves, the small road would lie ahead. The thick pipes of fungal species like Rhizopogon were designed for long-distance communication, and the fine mycelial fans of fungal species like Wilcoxina must be adept at rapid response, able to transmit chemicals swiftly to trigger fast growth and change. When my grandmother Winnie was diagnosed with Alzheimer’s, I’d read about what makes our brains either plastic or rigid. Maybe the long-distance Rhizopogon were analogous to the strong links in our brains arising from repetition, pruning, and regression, giving us long-term memory. Maybe the finer Wilcoxina hyphae, which grew faster and more abundantly, helped the mycorrhizal networks adapt to new opportunities, not unlike our own rapid, flexible responses to new situations, which Grannie was losing.
Grannie Winnie still had long-term memory. She knew she had to put on clothes; she just couldn’t remember how many shirts to wear when it got hot or whether to clip her bra in front or behind. Just as Rhizopogon strands deal with long-distance transport of solutions, Grannie’s memory about wearing clothes came from lifelong brain pathways. But her ability to adjust quickly and her short-term memory were dwindling with the loss of new synapses, as if she were losing connections analogous to the ones created by the mycelial Wilcoxina fans for trees.
The thick complex strands running out from the mother trees must be capable of efficient, high-volume transfer to the regenerating seedlings. The finer spreading mycelia must help the new germinants modify to accommodate pressing, rapid needs, such as how to find a new pool of water on a particularly hot day. Pulsing, active, adaptive in providing for the growing plants—like fluid intelligence.
The complex mycorrhizal network unravelled into chaos with clear-cutting. With the mother trees gone, a forest would lose its gravitas. But, within a few years, as seedlings grew into saplings, the new forest would slowly try to reorganize into another network. Without the pull of the mother trees, though, the new forest network might never be the same. Especially with widespread clear-cutting and climate change. The carbon in the trees, and the other half in the soil and mycelium and roots, might vaporize into thin air. Compounding climate change. Then what? Wasn’t this the most important question of our lives?
I reached a colossal tree, a rampart, her branches thick right to the ground and as big as trees themselves. Her large size and old age were magnificent compared to her neighbours. She looked like the mother of all mother trees. What foresters call a “wolf tree”—far older, bigger, and with a much wider crown than the others, a lone survivor of previous calamities. She had lived through centuries of ground fires that others had—at one time or another—succumbed to. I waded through seedlings to get to the fringe of her crown and picked up a cone perhaps clipped by a squirrel, its bracts dusted in white spores. Her life had started when the Secwépemc people cared for this land, long before the Europeans came, when Indigenous people regularly lit fires to create habitat for game or to stimulate the growth of valuable native plants or to clear routes for trading with neighbouring nations, keeping the fuels low so the flames were never intense enough to have burned off her thick bark completely. I was sure that, if I cored her, her rings would be calloused with char every twenty years or so, like the stripes of a zebra. I was struck by her endurance, her rhythm that spanned centuries. It was a matter of survival, not a choice, not an indulgence. Light glanced off her bark, incandescent, the sun dropping.
Excerpted from Finding the Mother Tree: Discovering the Wisdom of the Forest by Suzanne Simard, published by Allen Lane Canada. Copyright © 2021