Forests absorb billions of tons of carbon dioxide every year – and new research suggests that trees may also bind another important greenhouse gas: methane.

Microbes that live on trees absorb between 25 and 50 million tons of methane each year, according to a study published in NatureThe results suggest that certain forests could serve as net methane sinks and that reforestation could bring greater benefits than expected.

“I think this is 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 that we know so little about the role of trees in the global methane cycle.”

After carbon dioxide (CO), methane is the second most commonly emitted greenhouse gas by humans.₂). It enters the atmosphere through sources such as agriculture, mining and the decomposition of waste in landfills. Although methane is about 200 times less abundant than CO₂ in the atmosphere it stores heat more effectively.

Methane also enters the air through natural sources such as trees. The phenomenon is particularly common in areas with high groundwater levels, such as the Amazon basin.

Previous research has shown that naturally occurring microbes can curb these emissions. For example, a 2021 study found methane-eating microbes living in the bark of the Australian paperbark tree (Melaleuca quinquenerviaThese microbes offset the methane emissions of the paperbarks living in the swamp by about a third.

In the new study, researchers led by Vincent Gauci, an environmental scientist at the University of Birmingham in the UK, measured the uptake and emission of methane in trees around the world.

Above, below and all around

Gauci and his colleagues not only examined trees in different forest types – tropical, temperate and hemiboreal – but also documented the uptake and release of methane at different heights of the trees.

Consistent with previous results, the researchers found that trees in tropical zones emit methane during the rainy season. However, in the dry season, tropical trees behave more like their counterparts in temperate and hemiboreal zones: they continue to emit methane at the soil base of their trunk, but just a few dozen centimeters above the ground, the emission rate slows down so much that other processes are crowded out and the 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 looking at the role of trees in the methane cycle. Tree roots absorb methane from the soil around them. This methane diffuses back into the atmosphere through the bark. However, higher up from the ground, the effect diminishes, particularly during dry periods when the water table is several meters below the soil surface. At the same time, microbes in the bark metabolize methane from the air along the entire trunk. Eventually, the rate at which these microbes absorb methane from the atmosphere exceeds the rate at which the gas diffuses out of the tree.

Terrestrial laser scanning

As part of their research, the team estimated the area of ​​the Earth’s surface covered by trees and woody plants using a technique called terrestrial laser scanning. They set up a laser scanning instrument that reflected light off surfaces and created a 3D image of forest areas in different biomes. They then extrapolated the results to the global forest area. The technique is “like the kind of laser scans you find in front of these new cars that prevent accidents,” Gauci said.

The surface area of ​​trees around the world covers 143 million square kilometers – about the entire land area of ​​the Earth. There are few previous estimates of global bark surface area, Jeffrey said, so “it’s a big advance in the literature to have this new global surface area to work with.”

Based on their surface area estimate and methane measurements, the researchers determined that microbes on trees around the world could sequester between 25 and 50 million tons of methane annually. Such methane sequestration could have as large an impact on the climate as the sequestration of 197 to 399 million tons of CO₂say the researchers.

The newly discovered methane sink means that planting additional trees could provide a 10 percent benefit to the climate on top of the reforestation efforts already expected, the team calculates.

“We know we need to reduce our emissions,” Gauci said. “But this also shows us that deforestation is of no use to us if it reduces the sink.”

—Skyler Ware (@skylerdware), science writer

Quote: Ware, S. (2024), Microbes in tree bark absorb millions of tons of methane every year, Eoss, 105, https://doi.org/10.1029/2024EO240376. Published on 23 August 2024.

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