Trees reveal climate surprise – bark removes methane from the atmosphere

According to the international team of researchers led by the University of Birmingham, microbes living in bark or in the wood itself are removing atmospheric methane on a scale equal to or above that of soil. They calculate that this newly discovered process makes trees 10% more beneficial for climate overall than previously thought.

Methane is responsible for around 30% of global warming since pre-industrial times. Although most methane is removed by processes in the atmosphere, soils are full of bacteria that absorb the gas and break it down for use as energy. Soil had been thought of as the only terrestrial sink for methane, but the scientists say that their research suggests that trees may be as important, perhaps more so.

Lead researcher, Professor Vincent Gauci of the University of Birmingham, says: ‘The main ways in which we consider the contribution of trees to the environment is through absorbing CO2 through photosynthesis and storing it as carbon. These results, however, show a remarkable new way in which trees provide a vital climate service.’

He continues: ‘The Global Methane Pledge, launched in 2021 at the COP26 climate change summit aims to cut methane emissions by 30% by the end of the decade. Our results suggest that planting more trees, and reducing deforestation surely must be important parts of any approach towards this goal.’

In the study, published in the journal Nature, the researchers investigated upland tropical, temperate and boreal forest trees. Specifically, they took measurements spanning tropical forests in the Amazon and Panama; temperate broadleaf trees in Wytham Woods, in Oxfordshire, UK; and boreal coniferous forest in Sweden. The methane absorption was strongest in the tropical forests, probably because microbes thrive in the warm wet conditions found there. On average the newly discovered methane absorption adds around 10% to the climate benefit that temperate and tropical trees provide, reports the research team.

By studying methane exchange between the atmosphere and the tree bark at multiple heights, the researchers were able to show that while at soil level the trees were likely to emit a small amount of methane, from a couple of metres up the direction of exchange switches and methane from the atmosphere is consumed.

In addition, the team used laser scanning methods to quantify the overall global forest tree bark surface area, with preliminary calculations indicating that the total global contribution of trees is between 24.6–49.9mn tonnes of methane. This fills a big gap in understanding the global sources and sinks of methane, say the researchers.

The tree shape analysis also shows that if all the bark of all the trees of the world were laid out flat, the area would be equal to the Earth’s land surface. ‘Tree woody surfaces add a third dimension to the way life on Earth interacts with the atmosphere, and this third dimension is teeming with life, and with surprises,’ comments co-author Yadvinder Malhi of the University of Oxford.

Professor Gauci and colleagues at Birmingham are now planning a new research programme to find out if deforestation has led to increased atmospheric methane concentrations.