Tree bark in the world’s forests absorbs methane, study finds

My colleagues and I have shown, for the first time at a global scale, that tree bark in the world’s forests removes the greenhouse gas methane. The finding could have significant consequences for efforts to confront climate change.

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

Our extensive research now confirms that trees can also take greenhouse gases out of the air in another way. Forests may therefore deliver greater climate benefits than was previously recognised.

Methane has been responsible for around one-third of the warming observed since the pre-industrial era. Its concentration in the atmosphere has increased quickly for nearly two decades.

This poses a serious threat to Earth’s climate, as a given quantity of methane retains far more heat in the atmosphere than the same quantity of CO₂. However, whereas CO₂ may persist in the air for centuries, methane remains there for about ten years.

Because methane has such a short atmospheric lifespan, changes in either the sources that release it or the processes that remove it - known as methane sinks - can produce swift effects. Increasing methane removal could therefore offer a rapid climate benefit and help limit worsening climate change.

For this reason, scientists are keen to establish both how methane enters the atmosphere and how it is removed by different processes.

Tree bark and methane exchange

My team of ecologists and climate scientists has been examining methane exchange between the atmosphere and tree bark, a surface whose contribution to the climate had previously received little attention.

Wetlands are already recognised as the principal natural methane source. Trees in swamps and floodplains can release methane through the lower parts of their trunks.

Until now, however, methane exchange in trees growing in well-drained, non-flooded soils - which includes the majority of the world’s forests - had received limited study.

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

Our equipment consisted of a straightforward plastic chamber fitted around each tree trunk and linked to a laser-based methane analyser.

Initially, we searched for methane being released by trees. Some trees do emit a small quantity near the base of their trunks.

The unexpected result emerged when we took measurements further up. Trees were absorbing methane from the atmosphere, and this removal became stronger at greater heights on the trunk. Overall, methane uptake from the air outweighed methane emissions.

Global scale of tree bark methane uptake

We then explored whether this process mattered globally. That required an estimate of the worldwide surface area of tree bark. Using terrestrial laser scanning, we mapped the woody surfaces of trees, including their smallest twigs.

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

An untapped methane sink in forests

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

That is comparable with soils, the only other land-based methane sink. It means temperate and tropical trees are 7%-12% more beneficial for the climate than they currently receive credit for.

Unlike soil, whose area does not change, forests shrink and grow through deforestation and reforestation. Those shifts can affect atmospheric methane levels. Reforesting and planting trees in suitable locations could draw more methane out of the atmosphere.

Decarbonising the global economy and energy system is plainly the central means of tackling climate change. Yet tree bark’s ability to absorb methane presents an additional nature-based climate solution.

Plantation forestry may offer new opportunities to increase methane uptake, either by choosing trees that are especially effective at removing atmospheric methane or by 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 take methane into account.

This evidence further underlines the importance of trees and forests to the climate system, while showing how much remains to be understood 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.