Tree bark in the world’s forests absorbs methane, offering a climate change opportunity

My colleagues and I have shown, for the first time at a global scale, that tree bark in the world’s forests takes in the greenhouse gas methane - a finding with potentially major consequences for efforts to combat climate change.

When trees photosynthesise, their leaves absorb carbon dioxide (CO₂), which is stored as biomass in trunks and branches, creating a long-term carbon reserve.

Our extensive research now confirms that trees also take up greenhouse gases in another way, meaning forests may deliver greater climate benefits than had previously been recognised.

Methane has accounted for roughly one-third of the climate warming observed since preindustrial times. Atmospheric methane levels have climbed quickly for nearly two decades.

This presents a serious challenge for Earth’s climate, as methane retains far more heat in the atmosphere than the same quantity of CO₂. Unlike CO₂, which can remain in the atmosphere for centuries, methane persists for approximately ten years.

Because methane remains in the atmosphere for such a comparatively short period, changes either to its sources or to the processes that remove it - known as methane sinks - can produce rapid effects. Increasing its removal could therefore offer a swift climate gain and help limit worsening climate change.

For this reason, researchers are keen to understand both how methane enters the atmosphere and the various mechanisms through which it is removed.

How tree bark exchanges methane with the atmosphere

That is why my team of ecologists and climate scientists has examined methane exchange between the atmosphere and tree bark, a surface whose contribution to the climate had previously been overlooked.

Wetlands are recognised as the main natural source of methane, and trees in swamps and floodplains may release methane through the lower parts of their trunks.

However, methane exchange in trees growing on well-drained, non-flooding soils - including most forests worldwide - had received little research attention until now.

We assessed methane exchange from hundreds of tree stems in forests across a climate gradient, from the Amazon and Panama to Sweden and woodland near Oxford in the UK.

Our method used a straightforward plastic chamber fitted around a tree trunk and linked to a laser-based methane analyser.

Initially, we searched for methane released by trees. Some trees do emit small quantities from the base of their trunks.

The unexpected result came when we took measurements further up the trunks. Trees were drawing methane out of the atmosphere, and this removal became more powerful with height. Overall, atmospheric methane uptake was the dominant form of methane exchange.

We then explored whether this mechanism matters at a global level. This required us to estimate the total worldwide area of tree bark. We used terrestrial laser scanning to map woody tree surfaces, reaching even the smallest twigs.

We found that flattening the bark from every tree on Earth would produce enough surface area to cover all of the planet’s land. This could provide an enormous interface for gas exchange between the atmosphere and tree bark, although the mechanism remains poorly understood.

Tree bark as an untapped methane sink

Our conservative initial calculation suggests that trees absorb around 25 to 50 million tonnes of atmospheric methane annually, with tropical forests accounting for most of this uptake.

That figure is comparable with soils, the only other land-based methane sink. It means that temperate and tropical trees are 7%-12% more beneficial for the climate than current assessments acknowledge.

Yet, whereas soil area is not changing, forest cover shrinks and grows through deforestation and reforestation. Such shifts can affect atmospheric methane levels. Reforesting and planting trees in suitable locations could remove more methane from the atmosphere.

Decarbonising the global economy and energy system is plainly the central means of tackling climate change. However, tree bark’s ability to absorb methane provides another route for a nature-based climate solution.

In plantation forestry, it may be possible to develop new approaches for increasing methane uptake, including choosing trees especially effective at removing atmospheric methane or altering microbial communities in tree bark.

Countries could receive stronger incentives to protect existing natural forests and prevent additional deforestation. Costly reforestation schemes could also become more financially feasible through credible carbon offset programmes that account for methane.

This evidence further underlines how important trees and forests are to the climate system, while showing that much remains to be discovered about these valuable ecosystems.

Vincent Gauci, Professorial Fellow, School of Geography, Earth and Environmental Science, University of Birmingham

This article is republished from The Conversation under a Creative Commons licence. Read the original article.