The climate protection effect of trees apparently goes well beyond the binding of carbon dioxide: Microbes living in their bark bind large amounts of the powerful greenhouse gas methane from the atmosphere, a study has revealed. According to calculations, the newly discovered methane absorption increases the climate benefit of trees in temperate and tropical latitudes by around ten percent. This therefore increases the importance of protecting forests in the fight against climate change, say the researchers.
After carbon dioxide, it is the second most important greenhouse gas in the context of climate change: methane occurs in lower concentrations in the atmosphere, but its warming potential is much greater than that of CO₂. According to estimates, methane concentrations in the air have almost tripled since the beginning of industrialization, contributing about a third to climate change. A large proportion of man-made methane emissions are caused by the combustion of fossil fuels and by agriculture. What is particularly problematic is that global warming is leading to an increased release of the gas from natural sources such as thawing permafrost.
However, methane is also constantly being removed from the air: in addition to decomposition through chemical reactions in the atmosphere, microbes in the soil were previously considered the most important sinks for methane. These are certain types of bacteria that can use the gas as an energy source for their metabolism. The role of trees in the global methane balance, however, has been unclear until now. Measurements have shown that trees can release the gas through certain production processes, at least in the area close to the ground. On the other hand, there have also been indications that decomposition processes can occur on the surface of tree trunks.
From a certain trunk height, the methane disappears
In order to shed more light on the role of trees in the global methane system, researchers led by Vincent Gauci from the University of Birmingham have now systematically recorded the net exchange of methane between tree trunks and the air. Their focus was on forest trees in the tropical Amazon region, deciduous trees in Great Britain and conifers in Sweden. As part of the study, the methane concentrations were recorded and evaluated using chambers that were attached to the trunks at different heights – up to a maximum of two meters.
As the researchers report, they found the same vertical pattern of methane exchange at all locations: The values in the area of the tree base confirmed previous indications that trees can release small amounts of methane. Net emissions occur up to a trunk height of 130 centimeters. But in the higher trunk areas, degradation significantly exceeds the release, the researchers found: There is a significant net absorption that significantly exceeds the tree’s production and thus also binds methane from the atmosphere. Further research results showed that this is due to the activity of methane-degrading bacteria that live in and on the bark in the higher trunk areas. Methane absorption was strongest in the tropical forests, which is probably due to the fact that the methane-eating microbes thrive particularly well in the warm, humid conditions, say the researchers.
Considerable importance is emerging
But how significant could the effect be in the context of the earth’s methane system? To shed light on this question, the team used a laser scanning method to record average tree bark areas. This data was then combined with estimates of the earth’s forest areas based on satellite data. The calculation results highlighted the potentially enormous importance of these plant surfaces. If the bark of all the trees in the world were laid out flat, it could cover the entire surface of the earth. In conjunction with their previously obtained data, the researchers then came to a rough estimate of the importance of the material for the earth’s methane system: tree bark could remove around 25 to 50 million tons of methane from the atmosphere every year. This estimate is in a similar range to that assumed for the role of soil microbes in methane degradation, the team emphasizes.
“We have so far considered the contribution of trees to the environment mainly through the absorption of carbon dioxide through photosynthesis and the storage of carbon. However, these results reveal a new aspect of how trees make an important contribution to the climate,” says Gauci. “The Global Methane Commitment, launched at the COP26 climate summit in 2021, aims to reduce methane emissions by 30 percent by the end of the decade. Our results suggest that planting more trees and reducing deforestation should certainly be important components of any approach to achieving this goal,” says the scientist.
However, the team says further research is now needed to better understand the newly discovered factor in the global methane balance. In particular, they now want to learn more about the methane-eating bark microbes and their distribution.
Source: University of Birmingham, specialist article: Nature, doi: 10.1038/s41586-024-07592-w