Passage IV

NATURAL SCIENCE: This passage is adapted from the book Mycophilia: Revelations from the Weird World of Mushrooms by Eugenia Bone (@2011 by Eugenia Bone).

There are a number of fungi that live in mutualist relationships in which a balance of interests occurs between two organisms. Lichen has a mutualistic relationship with photosynthesizing algae and bacteria. And there are also commensal relationships, where the fungus may not be doing the host any good or any harm, either—the raison d'être of some yeasts in our body, for example, is unknown and may be commensal. But mycorrhizal fungi are the princes of mutualism. "Fungi can’t make their own food," said Gary Lincoff. "So they made a strategic choice to team up with plants."

Ninety percent of natural land plants are thought to have mycorrhizal fungi partners. It's a masterpiece of evolution: Mycorrhizal fungi break down nutrients like phosphorus, carbon, water, and nitrogen into a readily assimilative form and deliver them to the plant in return for sugar produced by the plant via photosynthesis. The fungus needs sugar for energy and to launch its spores, and the tree needs nutrients because (despite what I learned in school) tree roots don't do the job adequately. Tree roots primarily anchor the tree in the soil. While tree roots will absorb moisture if watered and nutrients if fertilized, it is the mycorrhizal fungus growing on and in the tree roots that provides the tree with the lion's share of its nutrition and water. Mycorrhizal fungi significantly expand the reach of plant roots, and by extending the root system, increase the tree's nutrient and water uptake.

In the wild, mycorrhizal fungi are key to not just the health of single trees but to healthy forest ecosystems. A single fungal genotype or clone can colonize the roots and maintain the nutritional requirements of many trees at once. And multiple fungi can colonize the roots of all or most of the trees in a forest. The hyphae, those threadlike strings of cells that are the fungus, function as pathways for shuttling nutrients, water, and organic compounds around the forest. The mycologist Paul Stamets believes that mycorrhizal fungi function as a giant communications network between multiple trees in a forest—he calls it "nature’s Internet." Others have described this linkage as the "architecture of the wood-wide web."

Weaker plants are able to tap into this network, too, like hitchhikers on a nutritional superhighway. Young seedlings struggling to grow in the shadow of established trees tap into the larger, older tree's fungal network to improve their nutritional uptake. This network exists to benefit not only established trees and seedlings of the same species but also trees from different species, and at different stages of development. So one multitasking fungus, its hyphae attached to the roots of multiple trees in the forest, can simultaneously provide a different nutritional load as needed to different trees. It's a couture service.

The old trees in a forest function as hubs for these mycelial networks. "Like spokes of a wheel," said Suzanne Simard, a professor of forestry at the University of British Columbia who studies mycorrhizae. Rhizomorphs (ropes of hyphae) connect the foundation tree with other trees—like an express stop on a subway system where lots of local trains come through—and the bigger the tree, the larger the hub. That's because the largest trees have the greatest root system, and the more roots there are, the more real estate there is for the fungus to colonize. "In one forest, we found 47 trees linked by two species of fungi composed of 12 individuals," said Simard. (By individuals, she means two genetically distinct fungal entities.) "Talk about two degrees of separation!" Even nonphotosynthesizing plants take advantage of "the hub." Parasites like the Indian pipe depend totally on mycorrhizal fungi for its nutritive needs. It taps into the nutrients and water provided by the mycorrhizae and connects via the mycorrhizae to a photosynthesizing plant for sugar.

Despite the fact that fungi are microscopic organisms, the functions they perform are often on an ecosystem or landscape scale. If you could take an x-ray look at the soil, you’d see that underneath the forest duff there is a layer of mycorrhizal mycelium running between, on, and in the roots of plants. It's like a stratum of life between the duff and the soil that holds water and nutrients in the ground. And when that stratum is disrupted, or not present, plants suffer. In fact, ecosystems with inadequate mycorrhizal fungi can experience catastrophic losses of plant biomass.