Forested areas absorb billions of tons of carbon dioxide each year—and new research suggests trees could also be sequestering another important greenhouse gas: methane.
Microbes that live on trees take up between 25 million and 50 million metric tons of methane each year, according to a study published in Nature. The findings suggest that certain forests could serve as net methane sinks and that reforestation could have greater benefits than expected.
“It just shows us there’s so very little we know about the role of trees within the global methane cycle.”
“I think it’s very exciting research,” said Luke Jeffrey, an aquatic biogeochemist at Southern Cross University in Lismore, Australia, who was not involved in the new study. “It just shows us there’s so very little we know about the role of trees within the global methane cycle.”
Methane is the second-most abundant greenhouse gas emitted by human activity, after carbon dioxide (CO2). It enters the atmosphere through sources such as agriculture, mining, and the decomposition of waste in landfills. Though methane is some 200 times less prevalent than CO2 in the atmosphere, it traps heat more effectively.
Methane also finds its way into the air through natural sources, such as trees. The phenomenon is particularly prevalent in areas where the water table is high, such as the Amazon basin.
Previous research has indicated that naturally occurring microbes can curb these emissions. A 2021 study, for instance, found methane-eating microbes living in the bark of the Australian paperbark tree (Melaleuca quinquenervia). These microbes offset methane emissions of swamp-dwelling paperbarks by about a third.
In the new study, researchers led by Vincent Gauci, an environmental scientist at the University of Birmingham in the United Kingdom, measured methane uptake and emissions in trees around the world.
Top, Down, and All Around
In addition to studying trees in different types of forests—tropical, temperate, and hemiboreal—Gauci and his colleagues documented the methane uptake and output at different heights on the trees.
In line with previous results, the researchers found that trees in tropical zones emit methane during wet periods. But in the dry season, tropical trees behave more like their temperate and hemiboreal counterparts: They continue to emit methane at the ground-level base of their trunks, but just a few tens of centimeters above the ground, the emission rate slows so much that other processes outcompete and trees actually behave as methane sinks.
“It’s quite interesting, because you don’t expect to find a [methane] sink,” Gauci said.
Gauci explained the results by describing the role of trees in the methane cycle. Tree roots take up methane from the soil around them. That methane diffuses back out to the atmosphere through bark. But the effect diminishes farther up from the soil, especially during dry periods when the water table is meters below the soil surface. At the same time, microbes in the bark metabolize methane from the air all along the trunk. Eventually, the rate at which these microbes absorb methane from the atmosphere outpaces the rate that the gas diffuses out from the tree.
Terrestrial Laser Scanning
As part of their research, the team estimated the area of Earth’s surface covered by trees and woody plants by using a technique called terrestrial laser scanning. They set up a laser scanning instrument to bounce light off surfaces and constructed a 3D image of forested areas in different biomes. They then extrapolated the findings to global woody surface area. The technique is “like the sorts of laser scans that you find in front of these new cars that prevent crashes,” Gauci said.
The surface area of trees around the world covers 143 million square kilometers—about the size of all the land area on Earth. Few prior estimates of global bark surface area exist, Jeffrey said, so “this is a great advancement in the literature, to have this new global surface area that we can work with.”
“We know we need to reduce our emissions. But this also tells us that if deforestation is diminishing the sink, that’s not going to help either.”
On the basis of their surface area estimate and methane measurements, the researchers determined that microbes on trees around the world could capture between 25 million and 50 million metric tons of methane each year. Such methane sequestration could have as much climate impact as sequestering between 197 million and 399 million metric tons of CO2, according to the researchers.
The newly discovered methane sink means that planting more trees could offer an additional 10% benefit to the climate on top of what’s already predicted from reforestation efforts, according to the team’s calculations.
“We know we need to reduce our emissions,” Gauci said. “But this also tells us that if deforestation is diminishing the sink, that’s not going to help either.”
—Skyler Ware (@skylerdware), Science Writer
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