What Satellite Images of Trees Can Reveal About Underground Fungus

Fungi are a lot like glaciers: Nearly all the action takes place below the surface. Beneath the soil, fungi tangle with trees, forming mutually beneficial relationships. Scientists recently used these close relationships and mapped tree health with the help of fungus data. They published their findings in the journal Global Change Biology.

"Every tree species has a distinct spectral signal, a kind of measurable aura," ecologist Sean McMahon said in a press statement. "Now we can tell who their underground friends are, an indicator of their nutrient status, from the sky." McMahon is temperate program coordinator of the Smithsonian’s Forest Global Earth Observatory (ForestGEO) network, which monitors the growth and survival of millions of trees on five continents.

Nearly all of those trees are involved in close relationships with fungi. The unseen world underground is a vast mesh of tree roots and threadlike fungal hyphae. It’s a good system; the fungi help trees get the water and nutrients they need, then reap the sugary benefits of the trees’ photosynthesis. Two types of fungus interact this way: ectomycorrhizal fungi (ECM), which hang out near and on tree roots, and arbuscular mycorrhizal fungi (AM), which actually dig into the cells of tree roots.

AM fungus in the roots of a flax plant. Image credit: MSturmel via Wikimedia Commons // Public Domain

A team of researchers from across the United States wondered what the fungi could reveal about their tree buddies. They compared maps from Landsat satellites with records on the fungal partners of 130,000 trees at research sites in Virginia, Indiana, Wisconsin, and Missouri.

"We discovered that groups of tree species associating with one type of fungi were spectrally distinct from groups of species associating with other types of fungi," said the paper’s first author, California Institute of Technology postdoctoral researcher Joshua Fisher. In other words, the trees’ under-the-covers, fungal hanky panky affected the way they appeared on the maps. Trees with AM pals have more phosphorus in their leaves. They make leaves sooner, and those leaves decay faster, which means faster nutrient recycling for everyone involved. ECM-associated trees, on the other hand, take longer to process nutrients.

Fisher and his colleagues realized that this could mean a new way of measuring and tracking forest health. Previous studies had shown that certain tree species preferred one type of fungus over another, but the sheer amount of labor involved in checking each tree/fungus pair in a forest is prohibitive.

The researchers tested their theory by trying to determine based on satellite maps which tree-fungus pairings grew in their forest research sites. They were able to predict the correct pairings 77 percent of the time, which is pretty impressive considering they were looking at these underground relationships from space.

Encouraged, the team will continue testing their concept in another 63 ForestGEO sites.