Last summer, the popular podcast series RadioLab aired an episode that caused listeners' jaws to drop. Titled "Tree to Shining Tree," the half-hour program explored the incredible relationship between trees and the below-ground organisms they depend on to survive.

While we won't spoil some of the more incredible revelations regarding this hidden symbiosis, the takeaway is rather startling: Beneath our feet, an intelligent, multi-layered network of fungi, bacteria and other micro-organisms, collectively known as the soil microbiome, is actively influencing the leafy life we see above.

In a new study published in the journal Nature Ecology and Evolution, researchers at the University of Tennessee say these soil organisms play a critical role in influencing a naturally occurring phenomenon known as "tree migration." While many of us likely instantly picture trees sprouting legs, pulling up roots and running away, the concept actually involves the movement of tree populations in geographical space over time.

Largely, these migrations are influenced by environmental changes. With climate change heating up regions all around the world, some species of trees are traveling north to escape the heat at average rates of 62 miles a century.

In the United States, the migration is already well underway. A 2010 U.S. Forest Service study found that 70 percent of tree species are already showing tree range migration, with maple, beech and birch potentially gone entirely in the Northeast by 2100. 

As this map shows, heat-sensitive species such as maple, beech and birch are expected to shift north within the next century in response to a warming climate. (Photo: U.S. Global Change Research Program)

"One general expectation is that tree ranges will gradually move toward higher elevations as mountain habitats get hotter," lead researcher Michael Van Nuland told ScienceDaily. "It is easy to see the evidence with photographs that compare current and historical tree lines on mountainsides around the world. Most document that tree lines have ascended in the past century."

During their research, Van Nuland and his team discovered that the relationship between trees and soil organisms includes a migration contingency plan. To ensure that their above-ground partners can migrate successfully, these invisible biotic communities create "soil highways" to guide young trees in their move towards cooler conditions.

To prove their theory, the team collected soil from beneath a common cottonwood species at both the lower elevation it currently sits and the higher elevation it's expected to migrate to in the future due to climate change. They then planted a number of cottonwood saplings in the soil samples and monitored their growth. As expected, trees placed in the soil near the bottom of the mountain thrived, while those in soil from the higher altitude did not. The opposite occurred for trees found at higher elevations.

"This indicates that we need to work with the trees near the bottom of the mountain, because they are the ones that will feel the most stress from warming temperatures," Van Nuland said. "So we have to figure out a way to coax them to move up."

The team concludes that the research could help scientists one day create bacteria or fungi designed to help certain species migrate at a faster rate relative to climate change.

"These results suggest that variable plant–soil biotic interactions may influence the migration and fragmentation of tree species, and that models incorporating soil parameters will more accurately predict future species distributions," they added.