A forest is a place of peace. We go there to soak up the stillness, the quietude. But even the most Zen of gardens is in fact a frenetic trading floor, abuzz with an exchange of commodities and data—transactions chemical, mineral and hydrous.
Like our own tech networks, much of it is carried underground, not by fibre-optic cable but fungi. We’re only now beginning to understand the extent to which plants are connected to neighbours near and far—even those of other species—by fungal filaments called mycelia. A mushroom is the small shopfront of a vastly bigger underground corporation. Mushrooms are how fungi reproduce, but mycelia are how they live, and beneath any hectare of forest there are kilometres of them, bundled into networks called mycorrhiza that carry water, carbon, nitrogen, phosphorus and other merchandise between plants.
Mycorrhizal fungi entwine and penetrate the roots of plants, to the benefit of both. The plant sends food to the fungus, while the fungus’s thatch of mycelia effectively increases the plant’s root ball, so it can better soak up water and nutrients. Mycorrhizal networks also give their host plants a booster shot. When a fungus colonises a plant’s roots, that plant whips up a batch of defensive chemicals. There’s memory inherent in this system, which means the plant’s subsequent immune responses are faster.
Until recently, we assumed this was a one-for-one deal—each plant had its own individual fungus—but studies by Suzanne Simard at the University of British Columbia have shown that mycorrhizal networks connect hundreds of trees. Beneath a study plot of Douglas fir just 30 metres square, Simard found that one tree was linked to 47 others, thanks to eight individuals of one fungus species and three of another.
She also found that one individual fungus linked 19 trees, young and old.
The real significance of this study, perhaps, is that it focused on a single tree species—Douglas fir—and only two species of fungus, though the plot contained a much greater diversity of both. The true complexity of the underground network, if all the potential terminals were considered, would be mind-boggling.
It now seems certain that mycorrhizal fungi connect not just individuals of a species but plants of different species—and that turns our understanding of forest function on its head.
We’ve known for some time that plants assailed by browsing animals or disease broadcast chemical distress signals in the air—an early warning system that gives their neighbours time to ramp up their own defences.
Studies have now shown that plants, both herbaceous and woody, also communicate through fungal networks (although grasses and trees employ different types of mycorrhiza), even though they may not be members of the same species. A 2015 study planted seedlings of Douglas fir and ponderosa pine in proximity, separated by barriers made of a mesh small enough to stop them touching roots, but big enough for mycorrhizal contact. When researchers pulled all the needles off the Douglas fir seedlings, the stressed trees alerted, through their root fungi, the ponderosa pines to the onslaught, which prompted them to produce protective enzymes.
Broad beans and tomatoes have been shown to do the same thing, but then came a real eye-opener: the dying Douglas firs offloaded their reserves of food to their unrelated, and still-healthy, pot mates. Darwin would have conniptions: where, in any tenet of known evolution, was the conferred benefit of such sacrifice? Why, when a member of one species faces ruin, would it bequeath its resources to a different one?
It’s risky to presume to know the mind of a Douglas fir, but Simard thinks that mature trees may extend saplings a helping hand, donating carbon via the fungal network in a dendrological version of nurturing. A 1997 study found that shaded seedlings were unlikely to survive without this arboreal aid programme (which may have implications for our common logging practice of felling only mature trees).
“These plants are not really individuals in the sense that Darwin thought they were individuals competing for survival of the fittest,” said Simard in a 2011 documentary, Do Trees Communicate? “In fact, they’re … trying to help each other survive.” So, while it’s possible—likely, even—that the dying firs had no way of knowing which species their neighbours belonged to, their forfeiture was calculated on a gamble that, statistically, they might be kin.
Then again, it’s possible they simply uploaded their valuable resources to secure storage—the “cloud” that is their local mycorrhizal network—for safekeeping, assuming they might survive to recover them later. Then, in a common enough chemical phenomenon, those resources simply migrated from a sink of high concentration to low—an act of plain old diffusion—which, in this case, was the roots of the hungry ponderosa pines.
But maybe we’re not giving enough credit to the lowly fungi: in that subterranean trading floor, maybe the broker pulled all its stocks out of the tumbling Douglas fir market and put them into bullish ponderosa pines instead.
Just like our own web, fungal networks are also exploited by villains.
Some orchids are quite capable of photosynthesising for themselves, but choose instead to steal the hard-earned carbon of others, siphoning them through mutual mycelia.
In a kind of chemical warfare, some trees, like sycamores, acacias and some eucalypts, release airborne pathogens to stifle the germination of competitors. But studies have shown that mycorrhiza can be turned to such nefarious purposes as well. In a 2011 lab experiment, half a sample of marigolds were allowed to court mycorrhizal fungi; half were not.
Marigolds produce aggressive compounds that slow the growth of competing neighbours and kill nematode worms, and when researchers went looking for them in the pots, they found nearly 300 per cent more where the fungi were present.
Then, the team planted lettuce seedlings in both sets of soil. After 25 days, the lettuces in the fungus- and toxin-laden soil weighed 40 per cent less than those in the fungus-free soil, proving that—in a lab at least—mycorrhizal networks can transport pathogens in sufficient doses to cause real harm.
Insights such as these are changing the way we look at plant communities. Just like our own society, many of their do-good deals and dastardly deeds are done below the surface. They’re more entwined, over much greater tracts, than we knew. Which only reinforces the notion that ecology, first and foremost, is about the interconnectedness of all living things.