To save a pine tree, researchers fight fungus with fungus
January 5, 2016
In a cluttered Bozeman laboratory, Cathy Cripps lays a whitebark pine seedling across the plate of a microscope, adjusts a few knobs, and peers into the eyepiece.
The young tree is just a three-inch-tall spray of green needles, bursting from a carrot-shaped sleeve of black dirt. A constellation of white freckles, each speck no larger than a period on this page, dots the soil like dandruff. Under the microscope, however, the spots blossom into bizarre and spectacular structures, with elegant coralline branches and cotton candy-like threads that coat the tree's labyrinthine roots. These otherworldly bodies are mycorrhizal fungi, symbiotic organisms that trade favors with plants.
"It always amazes me how tiny they are, how delicate," says Cripps, a Montana State University mycologist, who wears wire-frame glasses and a gray ponytail. "And yet, they do all this absolutely essential stuff." Indeed, this particular fungus, Suillus sibiricus, may help save the whitebark pine — with a little help from her.
Fungal partnerships made the West: Gaze upon a stand of lodgepole pine, a sagebrush steppe, or windblown tallgrass, and you're seeing the fruits of microscopic mutualism. Mycorrhizal fungi draw sugars from plant roots and, in turn, pipe essential nutrients, like nitrogen, back to their hosts through underground filaments. Though most fungi feed on decomposing material, many familiar species, including chanterelles and boletes, lead mycorrhizal lives.
Cripps especially adores the alpine fungi of Montana's Beartooth Plateau, which colonize stunted shrubs in inhospitable climes. A decade ago, however, she noticed a disturbing phenomenon while traversing her field sites: vast red swaths of dying whitebark pine. The trees, whose seeds nourish grizzly bears and Clark's nutcrackers, were falling victim to beetles and a disease called blister rust, twin scourges of the Northern Rockies. To combat the crisis, foresters have tried raising rust-resistant whitebark seedlings in nurseries. But cultivating whitebarks is arduous and complicated — the seeds require elaborate temperature cycles, for example — and rival trees, like spruce and fir, often outcompete introduced seedlings. One study found that just 42 percent of planted whitebarks survive.
As she wandered through surviving whitebark groves, Cripps, like any good mycologist, scanned the forest floor. The soil, she saw, teemed with S. sibiricus, whose yolk-colored mushrooms are called slippery jack. Whitebark and slippery jack grew in close companionship. Maybe the failed nursery seedlings just needed their fungal partner.
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She wasn't the first to consider the value of mycorrhizae; Austrian scientists have been inoculating European stone pine seedlings with fungal spores for 50 years. Today, you can't walk into a gardening store without knocking over bottles of fungal additive. But while most store-bought products use generic recipes, researchers are coming to realize that local fungi help native plants more than commercial mixes. In a 2015 study, Mia Maltz, a Ph.D. candidate at the University of California, Irvine, who's used fungal inoculation to grow sage in soil contaminated by borax mines, analyzed 28 fungus-based restoration projects. She found that adding mycorrhizae nearly always improved plant growth; that native fungi outperformed store-bought products; and that the benefits endured years after planting. Native mycorrhizae may help blue grama grass withstand drought, sagebrush ward off invading cheatgrass, and ponderosa pine recolonize disturbed forests.
In 2010, Cripps and a graduate student named Erin Lonergan foraged for slippery jack in the mountains of Montana and Alberta. Back at Montana State, they used a coffee grinder to mill the mushrooms' spore-laden undersides into slurry, then employed a cattle vaccination gun to inject the concoction — dubbed "Cathy's magic powder" by some foresters — into the soil around whitebark seedlings in Glacier National Park's nursery. Finally, that September, volunteers planted a thousand inoculated seedlings around Canada's Waterton Lakes National Park.
Four years later, Cripps and Lonergan reported that the fungal injection had improved pine survival in the wild by 11 percent compared to control plots — a substantial gain, considering the challenges of keeping planted whitebarks alive. They believe it will generate even greater benefits in places that have lost their mycorrhizae altogether. "We'd expect this to work best where there might not be any Suillus left, like intensely burned areas," Lonergan says. "We want to keep these fungi in the system."
But that won't be easy: As whitebarks perish in the Northern Rockies, Cripps fears their mycorrhizal partners will also vanish, making restoration even more difficult. One morning, Cripps drives up rutted dirt roads into the Gravelly Mountains, three hours southwest of Bozeman, to check on some rust-resistant seedlings she'd planted in a patch of fire-scorched earth. On the way, she passes through endless groves of rust-killed pines, their gray, skeletal limbs twisted against the sky.
"Where goes the pine, so goes the Suillus," she murmurs as we flash past a ghostly copse. Cripps sighs heavily, mourning a die-off only she can see.