For the first time ever, scientists have made a complete map of the "wood wide web," the underground network of bacteria and fungi that connects trees and passes nutrients from the soil to their roots, as Science Magazine explained.
The paper, published in Nature Thursday, draws on a database of more than 1.1 million forest inventory plots including more than 28,000 species of trees in more than 70 countries.
"It's the first time that we've been able to understand the world beneath our feet, but at a global scale," report co-author Prof. Thomas Crowther of ETH Zurich told BBC News.
Crowther had previously completed a map of all the world's trees and concluded that there were about three trillion, Science Magazine reported. Stanford University biologist Kabir Peay then reached out to Crowther to see if they could collaborate in mapping the fungal and bacterial networks below the trees as well. Science Magazine explained how the researchers were able to accomplish this goal:
Each tree in Crowther's database is closely associated with certain types of microbes. For example, oak and pine tree roots are surrounded by ectomycorrhizal (EM) fungi that can build vast underground networks in their search for nutrients. Maple and cedar trees, by contrast, prefer arbuscular mycorrhizae (AM), which burrow directly into trees' root cells but form smaller soil webs. Still other trees, mainly in the legume family (related to crop plants such as soybeans and peanuts), associate with bacteria that turn nitrogen from the atmosphere into usable plant food, a process known as "fixing" nitrogen.
The researchers wrote a computer algorithm to search for correlations between the EM-, AM-, and nitrogen-fixer–associated trees in Crowther's database and local environmental factors such as temperature, precipitation, soil chemistry, and topography. They then used the correlations found by the algorithm to fill in the global map and predict what kinds of fungi would live in places where they didn't have data, which included much of Africa and Asia.
"I haven't seen anybody do anything like that before," University of California, Irvine ecologist Kathleen Treseder told Science. "I wish I had thought of it."
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The map also has important implications for climate change, BBC News explained. That's because EM fungi, more common in temperate forests, store more carbon in the soil. AM fungi, more prevalent in the tropics, release carbon back into the atmosphere more quickly. The database found that 60 percent of trees are connected to EM fungi, but these fungi are also more vulnerable to climate change. As temperatures warm, they will be replaced with species favoring AM fungi, which will release more carbon. If greenhouse gas emissions aren't lowered by 2100, EM fungi could decline by 10 percent, the paper's authors concluded.
"The types of fungi that support huge carbon stores in the soil are being lost and are being replaced by the ones that spew out carbon in to the atmosphere," Crowther told BBC News.
Treseder told Science she thought the paper's conclusions about the relationship between carbon and fungi were "a little bit more tenuous" because, she said, there is still more to be learned about how soil fungi interact with carbon. But she also didn't dismiss the paper's conclusions.
"I'm willing to be convinced," she said.