In 1999, a team of scientists led by Christian Körner did what thousands of people do every Christmas: they wrapped Norway spruce trees in tubes. Except this was in March, not December. And the trees were 40-metre-tall giants in the middle of a Swiss forest, not 2-metre pipsqueaks in a living room. (The team had to use a crane). And the tubes had no lights or baubles on them. Instead, they had a series of tiny holes, which pumped out carbon dioxide.

For years, the team fumigated five of these wild spruces. They wanted to see how trees will cope with the high levels of atmospheric carbon dioxide that we’re pumping into the atmosphere. But in the process, and almost by accident, they showed that trees of different species exchange huge amounts of carbon via an internet of fungi—a “wood-wide web” that secretly connects their roots.

When trees photosynthesize, they use the sun’s energy to refashion carbon dioxide and water into sugars. In this way, they assimilate the carbon from the gas into the molecules of their own leaves, trunks, and roots. To study this process, Körner’s team exposed their trees to a special blend of carbon dioxide, depleted in an isotope called carbon-13. These depleted levels acted like a label that the team could use to track the flow of carbon through the trees, from leaf to root, from tip to toe.

And beyond.

When Tamir Klein joined the team in 2012, his job was to see how much of the labeled carbon had made its way from the canopy to the roots. Sure enough, when he dug up the spruces’ roots, he found that they had low levels of carbon-13. But, to his surprise, so did the roots of surrounding trees, including other species like beech, pine, and larch. Somehow, the labelled carbon had not only moved from the canopies of the five spruces to their roots, but also across to unconnected trees.

“Christian was very reluctant to believe any of this. He said: you misidentified the roots,” says Klein. But he hadn’t. He and his colleagues dug up the soil around the trees to ensure that the labelled roots belonged to different individuals. “Sometimes, we even tasted the roots to distinguish them. We confirmed that the label really was being transferred.”

It wasn’t moving across the canopy. Klein only found the carbon label in the roots of nearby trees and not their leaves, so the exchanges are happening underground. Roots of neighbouring trees can sometimes graft together, and lab studies have shown that carbon can move along these bridges. But Klein showed that this wasn’t the case for his spruces: they weren’t wired up to their neighbours.

Roots can also release carbon directly into the soil, which can then be absorbed by other roots. But if the spruces were doing that, then Klein should have found labelled carbon in every nearby plant—and he didn’t. There wasn’t any trace of the stuff in understory herbs like dog’s mercury and blackberries. It was, however, abundant in fungi, growing on the roots of the spruces and other trees.

These fungi—the mycorrhiza—are found on the roots of almost all land plants, and provide phosphorus and nitrogen in exchange for carbon-based sugars. They can also colonize several hosts at once, creating a large fungal internet that ferries nutrients and signaling chemicals between neighboring plants (much like the trees of Pandora in James Carmeron’s Avatar).

“There’s a below-ground community of mycorrhizal fungi invisibly interconnecting an above-ground plant community,” explains Christina Kaiser from the University of Vienna. “But it’s usually regarded as a network for supplying nutrients in exchange for carbon, not for delivering carbon from one plant to the other in such large amounts.”

She’s not kidding about the large amounts. Klein’s team estimated that in a patch of forest the size of a rugby field, the trees trade around 280 kilograms of carbon every year. That’s around 40 percent of the carbon in their fine roots, and about 4 percent of what they produce in total through photosynthesis.

There were earlier hints of these underground carbon exchanges. In 1997, Suzanne Simard from the University of British Columbia used a similar labelling experiment to show that seedlings of paper birch and Douglas fir trees can exchange carbon via fungi on their roots. “But nothing much has happened since that influential paper,” says Klein. “No one took it to the forest level, to show that this carbon transfer could be relevant to big trees, at an ecological scale.” If anything, he showed that trees are transferring even larger amounts of carbon than Simard’s seedlings were.

“It’s an important advance,” says Simard, “given that the experiment was conducted among older trees in natural forests. It drives home the shift in how we view plant communities, driven not only by competition, but also cooperation.”

“The big question is whether this carbon sharing actually improves tree fitness or resilience to changing environmental conditions,” adds Franciska de Vries from the University of Manchester. During a drought, “the least affected trees might be able to continue photosynthesizing, and supply struggling trees with carbon, thereby increasing the resilience of the entire community.”

De Vries also wants to know if the carbon trades depend on the donor tree. Does it matter if the donor is an oak or a pine, rather than a spruce? Can large, old trees with access to lots of light supply carbon to smaller trees and saplings that might otherwise be outcompeted? “Forests might be more socialist than we thought,” she says.

To understand more about these carbon trades, Klein wants to create a mini-ecosystem, where he grows trees in common pots and allows them to develop mycorrhizal networks. “Under these controlled conditions, we can do smaller labelling experiments where we can precisely measure the rates of carbon transfer and it’s direction,” he says. “Is it really bidirectional? If you put a suffering tree next to a strong one, would the carbon flow from strong to less strong, or the other way? Or would it be neutral?”

“We don’t think there is any intention of a tree to help its neighbor,” he cautions. You could equally view the exchanges as smaller trees stealing carbon from larger ones, or as the entirely incidental side effect of mycorrhiza growing on multiple trees. But whatever the slant, it’s clear that  “even a very mixed forest is much more connected than we thought,” says Klein.