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May 21, 2019 10:00 am

The Underground Network of Microbes That Connects Trees Mapped For First Time

For the first time, scientists have mapped the millions of species of fungi and bacteria that swap nutrients between soil and the roots of trees, using a database of more than 28,000 tree species living in more than 70 countries. This interconnected web of organisms throughout the woods is being dubbed the "wood wide web." Science Magazine reports: Before scientists could map the forest's underground ecosystem, they needed to know something more basic: where trees live. Ecologist Thomas Crowther, now at ETH Zurich in Switzerland, gathered vast amounts of data on this starting in 2012, from government agencies and individual scientists who had identified trees and measured their sizes around the world. In 2015, he mapped trees' global distribution and reported that Earth has about 3 trillion trees. Inspired by that paper, Kabir Peay, a biologist at Stanford University in Palo Alto, California, emailed Crowther and suggested doing the same for the web of underground organisms that connects forest trees. 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. Local climate sets the stage for the wood wide web, the team reports today in Nature. In cool temperate and boreal forests, where wood and organic matter decay slowly, network-building EM fungi rule. About four in five trees in these regions associate with these fungi, the authors found, suggesting the webs found in local studies indeed permeate the soils of North America, Europe, and Asia. By contrast, in the warmer tropics where wood and organic matter decay quickly, AM fungi dominate. These fungi form smaller webs and do less intertree swapping, meaning the tropical wood wide web is likely more localized. About 90% of all tree species associate with AM fungi; the vast majority are clustered in the hyperdiverse tropics. Nitrogen fixers were most abundant in hot, dry places such as the desert of the U.S. Southwest. According to the data he gathered, Crowther suggests that about 10% of EM-associated trees could be replaced by AM-associated trees as the planet warms. "Microbes in forests dominated by AM fungi churn through carbon-containing organic matter faster, so they could liberate lots of heat-trapping carbon dioxide quickly, potentially accelerating a climate change process that is already happening at a frightening pace," the report says.

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