Climate change has not been good for the world’s trees. Scientists have linked human-caused climate change to die-offs of eucalyptus trees in Australia, beeches in Germany, and pines in the American West. Many of them believe these die-offs occur because climates are changing faster than trees can adapt.

Climate—specifically temperature and precipitation—largely determines where trees and other plants can grow. The problem is that plant-growing ranges are expected to move with the changing climate.

In the Northern Hemisphere, tree-growing ranges are predicted to shift northward in latitude and upward in elevation. As this happens, trees are expected to be out of sync with their environments. As the cooler, wetter conditions they initially grew in, adapted to, and evolved in shift, trees could become climate refugees stranded in a hotter, drier world. In fact, these range shifts are likely already happening, which, according to some, can be seen in the current wave of tree die-offs. But what if climate wasn’t the whole story? 

This question is now being asked by a growing number of researchers who have begun challenging the long-held assumption that climate alone will guide plant migrations. As proof, they argue, one need only look in the soil.

According to one of these contrarian studies, published last summer in the journal Ecological Monographs, one group of soil-dwelling organisms known to help trees could make or break their successful migration: beneficial mycorrhizal fungi.

Mycorrhizal fungi are symbiotic organisms that use trees and other plants as hosts. While some mycorrhizal fungi are parasitic, many are mutualists that benefit their hosts. This includes providing their host trees with nitrogen, phosphorus, and other nutrients. Mycorrhizal fungi can also protect their tree partners from heavy metals in the soil and disease-causing agents, including other fungi. Some scientists believe that these fungi and the vast underground networks they create help trees exchange nutrients with one another, an underground community known as the “wood wide web.” In exchange for these services, trees provide their fungal collaborators with food in the form of sugars produced via photosynthesis, something the fungi—being more akin to animals than plants—can’t do.

Given the importance of mycorrhizal fungi to trees, the study asked the question: Can trees migrate without their mycorrhizal partners? The answer, the researchers found, is probably no. The study focused on two northeastern North American tree species—American beech (Fagus grandifolia) and sugar maple (Acer saccharum)—and the mycorrhizal fungi they partner with, ectomycorrhiza for beech and arbuscular mycorrhiza for maples.

Both tree species are common at lower elevations throughout the eastern United States. But their ranges are expected to migrate upward in elevation to sites now dominated by conifers, which are expected to become climatically out of sync with these sites in the decades ahead. What’s more, many of these sites already have climates conducive to beeches and maples. But, according to the study, only the American beech is likely to be a successful migrant because the soil at these higher-elevation sites contains sufficient amounts of the ectomycorrhiza, which the trees use. The outlook for sugar maples, however, is less optimistic due to lower levels of their companion fungi, arbuscular mycorrhiza, in that same soil.

“Climate is definitely important for plant growth … and soil chemistry as well,” Jordon Tourville, the study’s lead author, said. “But the fact that mycorrhizal fungi had an effect independent of those things and a significant effect at that was quite striking."

Tourville, who is a staff scientist at the Appalachian Mountain Club, based in Boston, conducted the study over a three-year period while completing his PhD at the State University of New York. To isolate the effects of mycorrhizal fungi from the effects of climate and soil chemistry, he and his colleagues grew beech and maple seedlings under varying conditions designed to simulate the migration of the two tree species upslope in elevation. This included growing seedlings in climate-controlled greenhouses in varying soil chemistries, some of which contained mycorrhizal fungi and some of which did not. Greenhouse seedlings were also grown in soil collected from high-elevation locations where the trees are expected to migrate. And seedlings were grown on-site at the same high-elevation sites where the soil was collected, sometimes in the local soil and sometimes not.

The experiments revealed that seedlings grown with the trees’ companion mycorrhizal fungi had higher survival rates than seedlings grown without them. The researchers observed this trend in both greenhouse-raised seedlings and seedlings grown at upslope migration sites.

What all this means, the study concludes, is that although new areas might become climatically suitable for trees as the climate warms, seedlings are unlikely to establish themselves if their suitable fungal partners aren’t present. This finding, Tourville said, complicates efforts to predict when and where trees will likely migrate as the climate warms because most predictions are based on scientific models that use only climate.

“If a suitable climate opens up further upslope of a species’ range, [these models] assume that it would be fine for that species to grow there. What these models don’t really take into account are things like soils and biotic interactions. I think fungi represent one aspect of migration that hasn’t been considered but really needs to be.”

Tourville and his colleagues’ study assumes the migration of beeches and maples is natural and unaided by people. Nonetheless, some see implications for human-assisted migration, where people help species move, in the study’s findings. 

Martín Nuñez, an associate professor in the Department of Biology and Biochemistry at the University of Houston, peer-reviewed the study for Ecological Monographs and said the study has “huge implications” for the assisted migration of trees. “What this paper is showing is that some species really need their specific partners,” Nuñez said. “There’s this highly specialized association that perhaps humans need to help a bit. Perhaps we can move the fungi into new areas to facilitate the movement of these [tree] species with climate change.”

Nuñez, whose previous research examined how fungi helped nonnative pines establish themselves in South America, said moving trees with their fungal partners is likely to create a whole series of technical problems. Study coauthor Tom Horton, a professor emeritus at the State University of New York, agrees, saying one of the biggest obstacles is likely to be the fungi themselves. “These fungi are really fickle, and they don’t like to be grown in the lab for the most part, or a nursery,” Horton said. Tourville is also skeptical that lab-grown mycorrhizal fungi would thrive or even survive over the long term at a migrated site.  

Nancy Johnson, an ecologist and a professor at Northern Arizona University who was not involved in the study, shares Horton and Tourville’s concerns about lab-grown fungi, calling the widespread use of lab-grown mycorrhizal fungi “a super bad idea.” Johnson, who considers assisted migration “controversial,” said another likely obstacle for both natural and assisted migration of trees is that mycorrhizal fungi are not the only important and often-overlooked group of organisms in the soil that affect plant health and survival.

“There’s no question that plants have a microbiome,” said Johnson. “Just like animals have a microbiome in the gut, plants have a microbiome in their roots and the surrounding soil.… Mycorrhizal fungi are just one important player in this microbiome that all plants are just bathed in.”

Johnson’s own 2020 study, which used a similar methodology to Tourville and colleagues', found that ponderosa pines grown within their native soil grew better on warmer, drier sites than pines grown without their soil.

According to Johnson, the plant microbiome includes not only fungi but also bacteria, protozoa, nematodes, and potentially even viruses. Mycorrhizal fungi are important to the microbiome because they provide the “scaffolding upon which all these other organisms can set up shop and do their thing,” she added.

Moving this larger soil microbiome, however, comes with its own risks, including the unintended introduction of invasive species and pathogens. For Johnson, the technical, ethical, and ecological questions involved in assisted migration raise the question of whether it should even be attempted. While the implications of Tourville and his colleagues’ study for assisted migration remain unknown, Johnson said the study makes one thing clear: It’s not enough to look at climate alone when trying to predict plant migrations under climate change.

“I think the implication of this study is mycorrhizal fungi are super important,” Johnson said. “There’s a lot more to [plant] migration than just temperature and moisture.”