Trees may be even bigger climate heroes than we thought

Trees are important for the health of our planet. For one, they pull carbon dioxide, a planet-warming gas, out of the air. But they also take up methane gas, scientists have now learned. Methane is even more potent as a greenhouse gas than carbon dioxide. This means that to slow climate change, protecting forests is more important than ever.

Much of the methane in air comes from human activities. These include agriculture, storing wastes in landfills and burning fossil fuels. But methane has natural sources too. Microbes in wetland soils emit this gas. Trees, too, were known to release methane, especially those growing in wet soils. These trees take up the gas from soil and emit it through their trunks. 

Vincent Gauci studies methane emission from trees. He’s an environmental scientist at the University of Birmingham in England. Gauci knew trees in wet places, such as the tropics, give off methane from their trunks. Next, he wanted to study its release from trees in drier soils. He expected these upland trees would give off methane, though less than those at wetter sites.

But that’s not what he found.

“We were surprised to see the exact opposite,” says Gauci. The trees were actually taking in methane. Think of how many trees there are on the planet, he notes. That could add up to a lot of methane being removed from the air.

An accidental discovery

When trees take in or release gases — such as carbon dioxide and oxygen — it’s called gas exchange. Researchers typically measure methane exchange low on the trunk, near the ground. That’s because methane comes into the tree from the roots and is quickly released from the trunk. But Gauci’s team took their measurements higher up on the tree. The reason? They thought this would better estimate how much methane the whole tree might emit.

Two students working with Gauci measured methane release from trees at two sites. One was in the United Kingdom. The other was in Panama. Later, the team looked at the results from both sites together.

They expected methane emissions would drop as they measured higher up the trunk. At some point, they thought, methane release should drop to zero. But they didn’t know at what height that might be.

Their hypothesis at first seemed to hold. Higher up the trunk, methane release dipped. But at around chest height, the numbers dropped below zero — here, trees were absorbing methane.

“When we saw those patterns, it was like ‘whoa, what’s going on here?’” Gauci recalls.

That’s when his team went back to past data from the Amazon rainforest. They saw the same pattern in those data. Around one meter (3.3 feet) off the ground, the trees switched from emitting methane to absorbing it.

The team looked at trees in Sweden to further check their findings. In the end, they analyzed trees from four regions: 100 trees in Brazil, 18 in Sweden and 24 in each of Panama and the United Kingdom. On average, the result was the same at all sites: Overall, the trees took up more methane than they released.

Measuring gas exchange

To measure how much methane a tree soaked up, the researchers built an air chamber around a tree’s trunk. It was enclosed by a plastic sheet with foam edges to seal off a volume of air. A laser analyzer measured how much methane was in the air within this chamber. That let the team analyze how methane levels varied over time.

The rate of gas exchange varied at different sites. Methane uptake was slower in colder places. In the United Kingdom and Sweden, it took 20 minutes or more for methane levels in the air sample to even out. In the warm tropics, that methane exchange happened in just a few minutes.

In the Amazon, the switch from emitting to absorbing methane also happened closer to the ground. Overall, trees there took up more methane compared to trees at cooler sites.

Using data from the four sites, the team estimated how much methane all the world’s forests may be absorbing. It might be as much as 50 million tons, they calculated. “It’s a sizable chunk,” says Gauci. That’s about as much methane as wafts from all the world’s landfills.

Gauci’s team shared its findings July 24 in Nature.

So much more to learn

The scientists believe methane is taken in by microbes that live in tree bark. These microbes consume methane and convert it to carbon dioxide. Microbes metabolize faster in warm temperatures. That’s why they take in methane faster in the tropics.

Very little is known about these microbes, says Gauci. In fact, little is known about gas exchange in tree bark and branch surfaces. And those surfaces add up to a huge area. “If we were to unwrap all the trees and roll them flat, they would basically cover the entire Earth’s land surface,” he says.

This research is important and eye-opening, says Kazuhiko Terazawa, a forest ecologist. He studies trees and methane at the Hokkaido Research Organization in Japan. The study’s estimates for how much methane trees might take up surprised him. If correct, he says, this means the land absorbs nearly twice as much methane as people had thought it did.

But, adds Terazawa, without more data it’s hard to know how accurate these new estimates are. Many things affect how much methane a tree may take up. For instance, its location, species and size all play a role. So does the time of year. Overall, he says, this study shows that this issue needs more research.

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Weifeng Wang agrees. He’s an ecologist at Nanjing Forestry University in China. He also studies how trees interact with methane. After reading the new study, Wang wished he’d taken measurements higher up the trunk in his own work. But he’s skeptical of the new global estimates. He doesn’t see enough evidence to support the team’s estimates for how much methane all the world’s forests could take in. In particular, he’d like to see data from more tree species at more sites.

Both Wang and Terazawa would like to see this approach extended to the whole tree. For instance, how do the leaves and the tree canopy interact with methane? It also would be helpful to know more about the bacteria in bark that may consume methane.

One final note, Terazawa says: People release far more methane than what nature can remove. So we still have to reduce these emissions and cut back on fossil fuels. 

This study is just the beginning for Gauci. He thinks there is much more to learn. But it’s certain that “trees are even more important for climate than we thought,” he says. “We have to protect our forests.”