More than one-third of the world’s tree species, from tropical magnolias to mountainous pines, are at risk of extinction. The International Union for the Conservation of Nature issued this stark update to its Red List of threatened species in October. Trees now account for more than a quarter of all species on the Red List and are at risk of extinction in nearly every country.

Fungi — or a lack thereof — could partly explain why trees are failing to adapt to climate change. A majority of tree species depend on underground symbiotic fungi, called ectomycorrhizal fungi, for the nutrients and water they need to survive (SN: 7/13/09). Like other organisms, ectomycorrhizal fungi may be struggling to adapt to climate change, especially heat and drought. But there’s still a lot that scientists don’t know about how these critical fungi are responding to climate change, or how that affects trees’ ability to survive in different regions.

“These are absolutely critical interactions for both the aboveground and belowground world,” says Michael van Nuland, a soil microbe scientist who focuses on tree-fungi relationships. “But we’re still struggling with understanding how these relationships are going to change with climate change.”

Van Nuland, working at the Society for the Protection of Underground Networks, and colleagues published a study earlier this year in PNAS examining where climate change is shrinking the overlap between trees and underground fungi, consequently limiting where trees can move.

The researchers used North American distribution data for 50 tree species and for 402 species of soil fungi and their DNA to map “suitable habitat,” where tree species and soil fungi overlapped. The team also used climate data to see what conditions are in the modern suitable habitat for tree-fungi relationships. Then, they modeled future climate conditions and how both trees and fungi would respond.

The final maps revealed that, as expected, suitable habitat for both trees and fungi tends to shift northward, into cooler and wetter conditions. But 35 percent of all tree-fungi pairings face shrinking areas where both trees and fungi will be able to survive. Without the right fungi in tow, trees won’t be able to move north along with their climate.

“If we really want to conserve trees and their diversity, we need to understand mycorrhizal plant interactions,” those between roots and fungi, says Aimée Classen, a soil ecologist at the University of Michigan who was not involved in the study. “I think we’re really moving in the right direction.”

That only about a third of tree-fungi pairs faced shrinking habitat surprised van Nuland. “It feels a little low, which probably means it’s a conservative estimate,” he says.

It’s a different kind of habitat loss than extinction risk assessments, including those by the IUCN, typically consider, van Nuland says. It’s not only about habitat size; it’s about ecological function, too. “It’s species interaction loss,” he says. “You’re missing a critical element you need to survive, just as you might be lacking the right climate.”

Deforestation is a problem for fungi, too. “When you clear-cut a forest, you’re clear-cutting the belowground fungal network,” van Nuland says. “It’s just that it happens out of sight.”

Suitable habitat shrinkage was driven largely by a lack of biodiversity in soil fungi at the edges of the overlap, the data show. Trees that were able to migrate in response to climate change had more options for soil fungi at the edges of their suitable habitat, so the odds of finding a good partner were better. Tree species that were falling behind in migration were in places with lower soil fungi diversity.

“It really showed us that fungi are playing a role in helping trees move across the landscape in response to climate change,” van Nuland says. “Fungi unlock the potential for trees to escape.”