trees need fungi -- 8/25/21
Today's selection -- from Finding the Mother Tree by Suzanne Simard. Tree seedlings won’t grow in sterile soil:
"I won a research grant to test whether conifer seedlings needed to connect with the mycorrhizal fungi in soil to survive. I added the twist of exploring whether native plants helped them make those connections, which I proposed to do by comparing seedlings that were planted in diverse communities to those planted alone, in bare earth. My ideas for this project, and my success at winning the grant, were owed in large part to what was happening in forestry south of the border. At the time, the United States Forest Service was transforming their practices, driven by public concerns over forest fragmentation and threats to species like the spotted owl, and scientists were recognizing that biodiversity, including conservation of fungi, trees, and wildlife, was important to forest productivity.
"Could a single species thrive on its own?
"If planted seedlings were mixed with other species, would that make for a healthier forest? Would planting the trees in clusters with other plants improve their growth, or should they be spaced far apart, in checkerboard grids?
"These tests, too, might help me get at exactly why the old subalpine firs up high, and stately Douglas firs down low, grew in clumps. They could help me understand whether native plants growing next to conifers improved connections to the soil. Whether the conifers had more colorful fungi on their root tips when growing next to broadleaf trees and shrubs.
"I picked paper birch as my test species, because I knew from childhood that it made rich humus that should be as helpful to conifers as it had been delicious in my dirt-eating days. I was also intrigued that it seemed to keep root pathogens at bay. But birch was only a weed to the timber companies. To everyone else, it was a gleaming provider of sturdy waterproof white bark, shady leaves, and refreshing sap.
|Nutrient exchanges and communication between a mycorrhizal fungus and plants.|
"The experiment should have been straightforward. Holy cats, was I in for a surprise. I planned to test how three lucrative tree species -- larch, cedar, fir -- fared in different mixtures of birch. I picked these as my other test species because they are the natives in the primary unlogged forests. I loved the cedar for its long, braided leaves, the Douglas fir for its silken bottlebrush laterals, and the larch for its starlike needles that turned golden before sprinkling to the forest floor in the fall. By now, the logging industry viewed birch as one of the most vicious of the competitors because it was thought to shade the coveted conifers, stunting their growth. But if birch saplings were helpful to the conifers, which mixtures would produce the healthiest forests? The three species of conifers differed in how much birch shade they could grow under, from very little for the star-needled larch, to lots for braided cedar, and somewhere in between for bottlebrush fir. This alone suggested that the best mixtures would vary with each species.
"I settled on a design that paired paper birch first with Douglas firs in one patch, then birch with western red cedar in another, and western larch teamed with birch in yet another section of what was, at the time, a failed plantation in a clear-cut where not even lodgepole pines had managed to claim a home. I planned the same experiment in two other clear-cuts to see how the trees would respond in slightly different terrains.
"In each species pairing, I planned for a wide variety of mixtures so I could compare the conifer species when they grew alone with when they grew with birch in different densities and proportions, and I could test my hunch that the mixtures would grow better in certain configurations, perhaps with low numbers of birch relative to larch, and higher numbers for cedar. I suspected that the paper birch enriched the soil with nutrients and provided a source of mycorrhizal fungi for the conifers. My earlier experiments also suggested that birch somehow protected the conifers against early death from Armillaria root disease.
"This amounted to a total of fifty-one different mixtures, each on its own parcel. On three clear-cut sites. After hundreds of days in plantations and in my weeding experiments observing how plants and seedlings grow together, I sensed that trees and plants could somehow perceive how close their neighbors were -- and even who their neighbors were. Pine seedlings between sprawling, nitrogen-fixing alders could spread their branches farther than if they were hunkered under a thick cover of fireweed. Spruce germinants grew beautifully nestled right up to the wintergreens and plantains but kept a wide berth around the cow parsnips. Firs and cedars loved a moderate cover of birch but shrank when a dense cover of thimbleberry also grew overhead. Larch, on the other hand, needed a sparse neighborhood of paper birches for the best growth and the least mortality from root disease. I didn't know exactly how the plants perceived these conditions, but my experiences told me to plant the test mixtures with precision. Distances between trees had to be exact, and the clear-cuts had to be on flat ground for maximum accuracy. Given that British Columbia is a province of mountains, finding three flat sites would be no small feat.
"To be as prepared as possible to look at the roots, to track if the conifers were connecting with the soil better when they were grown near paper birch than when they were alone, I ordered a dissecting microscope and a book on identifying the features of mycorrhizas and practiced with birch and fir roots collected on my way home. Jean would roll her eyes as I'd haul my samples into the storage-room-turned-office of our apartment, then tease me for burning the pot on the nights I'd promised to make dinner. My specialty was chili and hers was spaghetti, neither of us interested in cooking. I'd disappear into my cave-office until midnight, excising root tips, taking cross sections, and mounting them on slides. Before long, I was getting good at identifying Hartig nets, clamp connections, cystidia, and the many parts of the mycorrhizal root tip that helped distinguish one fungal species from another.
"Some of the species of fungi on the soft-needled firs seemed the same as those on paper birch. If this was true, maybe the birch mycorrhizal fungi jumped onto fir root tips, cross-pollinating them. Maybe this co-inoculation or sharing of fungi or symbiosis helped newly planted Douglas-fir seedlings avoid having naked roots, perhaps letting them escape the same death sentence that befell my early yellow seedlings in the Lillooet Mountains. If fir somehow needed birch, birch wouldn't be hurting fir, as foresters assumed.
"Quite the opposite.
"After months of searching, I found three flat clear-cuts, all on government land -- the sites of failed pine plantations, possibly because the soil biology was off-kilter. On one parcel, I ran afoul of a rancher who'd been grazing cows there illegally. He loudly protested my idea of converting the failed plantation into a testing ground, arguing that
he had a right to a clear-cut he'd been homesteading for years. He was less than thrilled with my countering that as a research forester, I was entitled to the clear-cut and he was trespassing on public property.
"Maudit tabernac! This was the last thing I needed.
"Preparing to plant the experiment took another few months. It involved painting every one of the 81,600 planting spots on the ground. First, though, we had to deal with root-disease infections in all three clear-cuts. About twenty thousand old stumps from the original cuttings had to be hauled out of the soil because Armillaria root disease was infecting the dead roots and spreading as a parasite to surviving trees. About thirty thousand infected pines were dead, dying, or in terrible shape, so they had to be removed along with infected native plants. The forest floor became collateral damage from the excavations, ending up with huge piles of stumps, dead saplings, and diseased native plants bulldozed to the timber edge. But this left a clean slate.
"I couldn't decide if the site looked like a farmer's field or a battlefield after the casualties got dragged off. My research grant didn't cover the installation of a cattle guard, so I painted a fake one across the road at the site entrance. I'd heard that cows don't cross lines on a road for fear they'll break their legs. It worked -- for the first few months. The next summer, my crew and I spent a month in the hot sun, painstakingly planting the seedlings in their precise locations.
Within a few weeks, all of the seedlings were dead.
"I was stunned. I had never seen such a complete plantation failure. I checked the rotting stems; there was no evidence of harmful sunscald or frost cankers. I dug up the roots and checked them under my home microscope. No obvious signs of pathological infection. But they reminded me of the embalmed spruce roots in Lillooet. No new root tips, just dark, unbranched sinker roots. Back at the site, lush swards of orchard grass had sprung up. I was puzzling over how this could have crowded in when the rancher drove up. 'Your trees are dead!' he laughed, squinting at the wreckage.
"'Yeah, I don't get it.'
"It turned out that he did get it. He very much got it. Furious at losing his grazing site, he'd seeded the clear-cut with dense grasses.
"My crew and I (with under-the-breath mutterings, mostly mine) cleared out the grass and replanted the site. The plantation failed again -- each of the mixtures. The white-barked paper birch died first, then the star-needled larch, then the soft-brushed fir, and finally the braided cedar. Following the order of their sensitivities to light and water shortages.
A third try the next year. Another failure. A fourth replanting.
"Again all the seedlings died. The site was a black hole where nothing would live. Nothing except luxuriant grass. The cows showed up to smirk at us, and I wanted to gather up all the cow shit and dump it on the rancher's truck. I guessed that the grasses had robbed the seedlings of water the first year, but I also had a troubling sense that the soil itself was suffering. I was quick to blame the rancher, but I secretly knew that my aggressive site preparation had displaced the forest floor and scraped the topsoil away. That couldn't have helped.
"Douglas fir and western larch form symbioses only with ectomycorrhizal fungi, the ones that wrap the outside of the root tips, whereas the grasses formed relationships only with arbuscular mycorrhizal fungi that penetrate the cortical cells of their roots. The seedlings starved to death because the kind of mycorrhizal fungi they needed had been replaced by the kind only the damned grasses liked. It dawned on me that the rancher had helped me get at my deepest question: Is connection to the right kind of soil fungi crucial for the health of trees?
"I replanted a fifth year, but this time I collected live soil from beneath old birch and fir trees in the adjacent forest. I placed a cup of it in each of one-third of the planting holes. I planned to compare these seedlings to another batch planted straight in another third of the razed ground without any transferred soil. For good measure, I placed old-growth soil that had been radiated in the lab to kill its fungi in the final third of the planting holes. This would help me figure out if the living fungi or the soil chemistry alone accounted for any seedling improvements with the soil transfers. After five tries, I felt on the cusp of a discovery.
"I returned to the site the following year. The seedlings planted in the old-growth soil were thriving. As predicted, the seedlings without transferred soil, or with the dead, radiated transferred soil, were dead. They had met the usual morbid fate that had been plaguing them -- and us -- for years. I dug up samples of the seedlings and took them home to my microscope. As I expected, the dead seedlings had no new root tips. But when I looked at the seedlings grown in the old-growth soil, I jumped out of my chair.
"Merde! The root tips were covered with a dazzling array of different fungi. Yellow, white, pink, purple, beige, black, gray, cream, you name it.
"It was about the soil.
"Jean had become an expert on Douglas-fir forests and the widespread poor seedling growth in dry, cold country, and I grabbed her to come look. She took off her glasses, peered into the microscope, and shouted, 'Bingo!'
"I was overjoyed. But I also knew I was only scratching the surface. Enormous clear-cuts had recently emerged on Simard Mountain, obliterating the old-growth forests. I had driven the new logging road along the shoreline where we used to moor Grampa's houseboat. Where the Jiggs outhouse used to be. And Grampa Henry's waterwheel, and his flume. Now one clear-cut morphed into the next. The cutting and monoculture planting and spraying had transformed my childhood forest. While elated with my revelation, I was heartbroken by the relentless harvesting, and it was my responsibility to stand up. To act against the government policies that I felt weakened the tree-soil links. The land. Our connection to the forest.
"I also knew the religious fervor behind the policies and practices -- a fervor backed by money. On the day I left my experiment, I stopped to absorb the forest's wisdom. I walked up to an elder birch along the Eagle River where I had collected the soil for transferring to the planting holes. Running my hands across the papery bark stretched across its wide, sturdy girth, I whispered the tree thanks for showing me some of its secrets. For saving my experiment.
"Then I made it a promise.
"A promise to learn how trees sense and signal other plants, insects, and fungi. To get the word out. The death of fungi in the soil, and the breakdown of the mycorrhizal symbiosis, held answers about why the little yellow spruce in my first plantations had been dying. I'd figured out that accidently killing the mycorrhizal fungi also killed trees. Turning to the native plants for their humus, and putting the fungi in the humus back into the plantation's soil, helped the trees.
"In the distance, helicopters were spraying the valleys with chemicals to kill the aspens, alders, and birches in order to grow cash crops of spruces, pines, and firs. I hated this sound. I had to stop it.
"I was especially puzzled by the war on alder, because Frankia -- the symbiotic bacteria inside its roots -- had the unique ability to convert atmospheric nitrogen into a form the small shrub could use to make leaves. When the alders shed their leaves in the fall and decayed, the nitrogen was released into the soil and became available for the pines to take up with their roots. The pines relied on this transformation of nitrogen because these forests burned every hundred years, sending much of the nitrogen back into the atmosphere.
"But I would need much more proof about soil conditions and how trees might connect with, and signal to, other plants if I hoped to move the needle on forest practices. Alan had encouraged me to return to the university to get a graduate degree to keep improving my skills. I was twenty-six, starting my master's at Oregon State University in Corvallis in a few months, and I decided to conduct an experiment to test whether alder was a real pine killer, as believed by the policies, or whether alder improved the soil with nitrogen and gave pine a boost.
"My bets were on the latter.
"My hunch would prove to be far more prescient than I could have imagined. I knew my conviction to dig into free to grow could rankle the policymakers. I just didn't have any idea how much."