Beneath the ground of our forests exists something that is one of the largest living organisms on the planet. They can be heavier than elephants or blue whales, and extend across hundreds of kilometers. They have no brain, heart or eyes yet serve as something like a life insurance policy for forests: we’re talking about what are called mycorrhizae. The name comes from the Ancient Greek words myco, which means fungus, and rhiza, meaning root, and describes a form of symbiotic relationship between fungi and trees. More precisely, it’s an enormous underground network formed by soil-based fungi that stretch their impossibly thin filaments from tree root to tree root.
Someone who’s been studying these organisms for many years is François Buscot, head of the Department of Soil Ecology at the Helmholz Center for Environmental Research in Leipzig and Professor for Soil Ecology at the University of Leipzig. “There are several hundred kilometers of fungal filaments beneath the surface of one square meter of the forest floor,” he says. “The network is very dense in the top 20 to 30 centimeters under the soil.” The filaments form an extension of the tree roots that looks a bit like cobwebs. Although the individual filaments are only 2 to 100 micrometers in diameter—a human hair is about 50—meaning in some cases not even visible to the naked eye, a mycorrhiza taken as a whole can reach enormous proportions: “In Canada, researchers discovered a network of mycorrhizae that weighed several hundred tons,” Buscot says.
This underground partnership benefits both sides. The fungi supply the trees with nutrients such as nitrogen and phosphorous, as well as water from the soil. The trees would otherwise be unable to get a majority of these resources through their roots alone. In return, the fungi receive products of photosynthesis, mainly in the form of glucose, meaning simple sugars, which they need to grow. In addition, the fungi filter out poisonous heavy metals such as lead, cadmium, nickel, mercury and chromium from the soil nutrients, thereby protecting the plant roots from these toxins.
The network additionally has benefits for the trees as a group: “It’s been discovered that the products of photosynthesis from older trees are passed along via the filaments to saplings that are located in shade,” Buscot explains. “That’s what we call nursing.”
The network also functions as a type of early-warning system against pests such as leaf-eating insects. “If a tree is attacked by parasites, it produces stress molecules such as abscisic acid. This molecule is then distributed to the other trees via the fungal filaments, alerting the others to prepare for a potential attack by ramping up their defense mechanisms.”
A tree lives with an average of about 20 to 30 different species of fungi—each of which has its own special strengths. Some fungi are more likely to be supplying nutrients, while others are specialized in distributing stress molecules.
A mycorrhiza can also network different species of trees with one another—provided that the trees are compatible with the same fungus. Roughly speaking, there are two main types of mycorrhizae in a forest: ectomycorrhiza and arbuscular mycorrhiza. They connect with tree roots in different ways.
Ectomycorrhizae form a dense net around a tree’s tiniest roots and then grow into the outer layer of the roots. This symbiosis with plants has been around for about 130 million years. There are between 15,000 and 20,000 fungal species that fall into this category, including such famous representatives as the porcini, truffle and fly agaric, a type of toadstool with a bright-red cap with white spots. The majority of trees in the forests of temperate regions—such as spruces, oaks, birch, willow, elms, pines and firs—form a symbiosis with this group of fungi.
Arbuscular mycorrhizae have been around on Earth for at least 460 million years. Instead of growing a dense net around the tree’s tiniest roots, they penetrate them directly, right into the outer plant cells. To continue spreading in the soil, these fungi are initially dependent on the glucose the trees produce. Only once they’ve fully connected to the root do they form their network. There are just a few hundred fungi that belong to this species, but they can form a symbiosis with about 80 percent of all terrestrial plants. This also includes a few trees such as poplars, acorns and ash trees.
What puzzles researchers to this day is why the fungi form these networks in the first place. The ectomycorrhiza don’t actually need to form a symbiosis with trees to thrive—they could survive in the soil without them. While arbuscular mycorrhizae need trees at the beginning of their life cycles, they could just tap the sugar from the roots without later distributing nutrients throughout the network. And how do the fungi “know” how resources should be distributed—meaning which trees have more nutrients than they need and which require more support? “We don’t have an answer for that yet,” Buscot says. “We just don’t know what the fungus gains from networking the trees together, we only know that it does it.”
Furthermore, the exchange on the network isn’t always balanced, meaning that there are phases when the symbiosis isn’t really even worthwhile for the mycorrhiza. This includes, for example, at night, when trees aren’t photosynthesizing. There’s also no advantage in winter, when the trees have lost their leaves. “At that point, the fungi are working without being paid, so to speak; they don’t get any glucose,” Buscot says. Despite this, there aren’t any forests that don’t have mycorrhiza—fungi and trees always find one another. “For an experiment, we planted trees in soil that had previously been used for agriculture,” Buscot says. “There weren’t any fungi in the soil. A few weeks later, when we examined the roots, we found mycorrhiza on them. The fungi probably arrived through the air.”
Buscot says that without these underground networks, these enormous organisms, forests probably never would have managed to spread across the planet. And whether forests could even survive without the mycorrhizae hasn’t yet been adequately researched. What’s certain is that these symbioses make forests stronger and more resilient against drought and soil contaminants. In view of global climate change, this is good news for humans on the planet as well, since forests provide oxygen, store carbon, mitigate erosion, regulate the water cycle and serve as habitats for diverse species. An added bonus, if you walk through a forest in autumn, is that you can literally collect the fruits of these underground networks: in the form of mushroom heads sprouting from the ground. The fungi are able to produce fruiting bodies when the trees lose their leaves and pump all the nutrients back into the ground. It’s as if the trees pay a nutrient bonus before shutting down for the winter—and the fungi keep working through to spring without receiving any wages.
