IT’S HARD TO miss the headliners at Kew Gardens. The botanical collection in London is home to towering redwoods and giant Amazonian water lilies capable of holding up a small child. Each spring, its huge greenhouses pop with the Technicolor displays of multiple orchid species.

But for the really good stuff at Kew, you have to look below the ground. Tucked underneath a laboratory at the garden’s eastern edge is the fungarium: the largest collection of fungi anywhere in the world. Nestled inside a series of green cardboard boxes are some 1.3 million specimens of fruiting bodies — the parts of the fungi that appear above ground and release spores.

“This is basically a library of fungi,” says Lee Davies, curator of the Kew fungarium. “What this allows us to do is to come up with a reference of fungal biodiversity — what fungi are out there in the world, where you can find them.” Archivists — wearing mushroom hats for some reason — float between the shelves, busily digitizing the vast archive, which includes around half of all the species known to science.

In the hierarchy of environmental causes, fungi have traditionally ranked somewhere close to the bottom, Davies says. He himself was brought to the fungarium against his will. Davies was working with tropical plants when a staffing reshuffle brought him to the temperature-controlled environs of the fungarium. “They moved me here in 2014, and it’s amazing. Best thing ever, I love it. It’s been a total conversion.”

Davies’ own epiphany echoes a wider awakening of appreciation for these overlooked organisms. In 2020, mycologist Merlin Sheldrake’s book “Entangled Life: How Fungi Make Our Worlds, Change Our Minds, and Shape Our Futures” was a surprise bestseller. In the video game and HBO series “The Last of Us,” it’s a fictional brain-eating fungus from the genus Cordyceps that sends the world into an apocalyptic spiral. (The Kew collection includes a tarantula infected with Cordyceps — fungal tendrils reach out from the soft gaps between the dead arachnid’s limbs.)

While the wider world is waking up to these fascinating organisms, scientists are getting to grips with the crucial role they play in ecosystems. In a laboratory just above the Kew fungarium, mycologist Laura Martinez-Suz studies how fungi help sequester carbon in the soil, and why some places seem much better at storing soil carbon than others.

Soil is a huge reservoir of carbon. There are around 1.5 trillion tons of organic carbon stored in soils across the world — about twice the amount of carbon in the atmosphere. Scientists used to think that most of this carbon entered the soil when dead leaves and plant matter decomposed, but it’s now becoming clear that plant roots and fungi networks are a critical part of this process. One study of forested islands in Sweden found that the majority of carbon in the forest soil actually came from root-fungi networks, not plant matter fallen from above the ground.

Martinez-Suz’s research focuses on mycorrhizal fungi — a large group of fungi that coexist with plant root systems. The mycorrhizal fungi form networks that can go around and sometimes inside plant roots, transferring nutrients and water to the plants in exchange for carbon. Around 90 percent of plant species are known to make these symbiotic trade networks with different species of fungi. “These plants are covered by these fungi. It’s incredible. They are small but they are everywhere,” says Martinez-Suz.

This has serious implications for tree-planting schemes. Planting new forests is a major hope for carbon sequestration, but there is increasing evidence that the mycorrhizal networks might be crucial to the success of these attempts. One replanting study found that a forest of birch and pine trees planted onto heath moorland in northern Scotland did not increase soil carbon stocks even after nearly 40 years in the ground. The researchers who carried out the study think that it might be because the influx of new trees upset the delicate moorland mycorrhizal networks already present.

“Replacing the complete set of fungi with other fungi has implications for long-term carbon sequestration in soil and biodiversity,” says Martinez-Suz. Her current project involves comparing samples from forests in low-pollution sites like northern Finland with those in heavily polluted regions like Belgium and the Netherlands. The fungi in polluted regions are less diverse, she says, and this might have a knock-on effect on how well those forests store carbon.

The major culprit here is nitrogen pollution, which enters soils through burning fossil fuels for electricity and transport, and through agriculture. An excess of nitrogen changes the composition of soil fungi, so that the fungi that are the best at retaining nutrients and pumping carbon into the soil decrease.

But there is some hope that forests can turn things around. One study in the Netherlands found that when nitrogen pollution reduced, beneficial fungi species started to return to the forests. The danger, Martinez-Suz says, is that if ecosystems are pushed too far then there might not be any fungal spores remaining to boost populations.

If we’re to better understand how these fungi influence critical ecosystems, then we need to get to grips with all of these species. Mycologists think that nearly 90 percent of the world’s fungi species are still to be discovered, and the archivists at Kew are only halfway through the long process of digitizing their collection so that researchers can easily know where and when a species was found.

Around 5,000 extra specimens enter the fungarium each year, and the shelves are crammed with samples waiting to be dehydrated and stored. Many of them, Davies says, are sent by amateur mycologists who are fascinated by the world of fungi. “People in academic institutions like this will send them stuff to work on and do identifications, because they are world experts even though they have no formal training. They’re just really obsessive. It’s so cool.”