Remote sensing of tree canopies reveals the identity of belowground fungi

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Scientists can sniff out subterranean fungi based on how a tree's leaves reflect light. (Unsplash/Alex Zarubi)

A plant's leaves reflect light according to the type of fungal symbiont associated with its roots, according to a new study that suggests aerial imaging of treetops could give insight into distributions of mycorrhizal fungi, which play key roles in global carbon and nutrient cycling.

Researchers, who published their findings May 19 in Geophysical Research Letters, used remote sensing from airplanes to measure light reflectance in canopies of more than 100,000 individual trees in U.S. forests. The team identified spectral signatures that were strongly associated with mycorrhizae type.

Mycorrhizae are symbiotic connections between fungi and plant roots, upon which the vast majority of plants rely for essential nutrients.

"[Mycorrhizae] are big players in terms of climate change and carbon cycles and nutrient cycles," said Josh Fisher, a co-author of the study and a scientist at NASA's Jet Propulsion Laboratory. "As we ramp up CO2 in the atmosphere, our number-one way to get that CO2 out is through plants. And if the plants don't have enough nutrients, they won't be able to do it." 

There are two main types of fungi involved in these symbioses: arbuscular mycorrhizal fungi, or AM, and ectomycorrhizal fungi, or ECM. In exchange for nutrients, plants pay these microbes with sugar, Fisher explained.

"Just like different merchants, they charge different amounts and will give back different amounts of nutrients," he said. "So if we know the distribution of mycorrhizal types across the landscape, we can have a better sense of how much carbon those plants might be able to draw down because of their available nutrients from the fungi."

Each plant species usually forms associations with either AM or ECM, rarely both, which means that detailed maps of tree species in a forest are easily converted into maps of mycorrhizae. But because this information isn't known for most places on Earth, mycorrhizae distribution across the planet is not well-documented.

Remote sensing from airplanes or satellites is a way to capture a complete picture of conditions on Earth, and this approach has been used to measure deforestation and changing coastlines, among other things. 

Optical remote sensing instruments detect light that scatters off objects on Earth, including trees. Objects with different chemical properties reflect different amounts of light at different wavelengths. As such, an object's unique pattern of reflectance is called a spectral signature or fingerprint.

In previous work, Fisher and his colleagues found that ECM and AM fungi were associated with different canopy spectral signatures, but they used a remote sensing tool called multispectral imaging that only detects reflectance in certain bands of the electromagnetic spectrum, giving only coarse features.

In the new study, the researchers used hyperspectral imaging, which detects light reflectance across the electromagnetic spectrum, providing more detailed spectral fingerprints.

The researchers gathered on-the-ground records for 112,975 individual trees at six U.S. sites and assigned them as ECM or AM depending on the tree species. The team aligned the geolocations of these trees with hyperspectral images taken by a NASA imaging instrument onboard an aircraft.

The researchers found that the light reflectance signatures of tree canopies closely matched the type of mycorrhizae — ECM or AM — regardless of the tree species. 

"Our technology can see below the ground; these vast networks of fungi that we know are there but we can't see, suddenly, we can see, because [mycorrhizae] act like fingers on a puppet, where the tree is the puppet, and they can control how the trees move — not physically, but spectrally," Fisher told The Academic Times. 

The researchers saw differences between trees with ECM or AM across the electromagnetic spectrum, although the differences were bigger or smaller at different wavelengths. According to Fisher, these differences correspond to the abundance of compounds such as nitrogen, phosphorus, cellulose and water in the canopy.

"Those [differences] are related to what the fungi are getting to the tree through the roots, but they go right on up to the leaves," Fisher said. "It's kind of neat to be able to see the fingers under the puppet."

The findings were consistent across different sites and forest types in the study. But because the sites were all U.S. forests, the researchers say that additional verification with on-the-ground data in other countries would be needed to confirm that these mycorrhizal spectral signatures are consistent in different areas. 

Fisher said that the findings lay the groundwork for using remote sensing to map global distributions of mycorrhizae. Such maps could improve calculations of carbon and nutrient cycling, processes that are important for regulating Earth's climate, and provide important baseline data for understanding potential changes in forests over time or in response to climate change.

The hyperspectral images in the study were taken from planes, so this data isn't yet available for the whole planet. But according to Fisher, the researchers are preparing for a future NASA mission called Surface Biology and Geology that will take hyperspectral and thermal images of Earth from space.  

The researchers note that they didn't consider other types of mycorrhizal fungi besides AM and ECM in this study. A potential avenue for future research is investigation of forests dominated by ericoid mycorrhizae, for example.

The study, "Tree canopies reflect mycorrhizal composition," published May 19 in Geophysical Research Letters, was authored by Daniel Sousa, Joshua B. Fisher and Ryan P. Pavlick, California Institute of Technology; Fernando Romero Galvan, Cornell University; Susan Cordell, Christian P. Giardina and Faith Imran-Narahari, USDA Forest Service; Thomas W. Giambelluca and Creighton M. Litton, University of Hawai'i at Mānoa; Gregory S. Gilbert, University of California, Santa Cruz; James A. Lutz, Utah State University; Malcolm P. North, U.S. Forest Service; David A. Orwig, Harvard University; Rebecca Ostertag, University of Hawai'i at Hilo; Lawren Sack, University of California, Los Angeles; and Richard P. Phillips, Indiana University. 

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