Family Trees
The Walrus|June 2021
How I tapped into the vast social network beneath the forest floor
Suzanne Simard

Suzanne Simard is a professor of forest ecology at the University of British Columbia. Her research into how trees cooperate, share resources, and communicate through underground fungal —  or mycorrhizal —  networks has reached global influence, from Peter Wohlleben’s The Hidden Life of Trees to the film Avatar.

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 belowground 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 ahem. 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.

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.

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