Recent research is showing that trees, especially in tropical wetlands, are a major source of the second most important greenhouse gas in the atmosphere, methane. The knowledge that certain woodlands are high methane emitters should help guide reforestation projects in many parts of the world.

There are many mysteries in the Amazon. Until recently, one of the most troubling was the vast methane emissions emerging from the rainforest that were observed by satellites but that nobody could find on the ground. Around 20 million tons was simply unaccounted for. Then Sunitha Pangala, a British post-doc researcher, spent two months traveling the Amazon’s waterways strapping gas-measuring equipment to thousands of trees. She found that trees, especially in the extensive flooded forests, were stimulating methane production in the waterlogged soils and mainlining it into the atmosphere. Her 2014 expedition plugged a gaping hole in the planet’s methane budget. And she had discovered a hitherto ignored major source of the second most important greenhouse gas in the atmosphere. It now seems that most of the world’s estimated 3 trillion trees emit methane at least some of the time. Nobody is arguing that trees are therefore bad for climate and should be cut down. Indeed, in most cases, their carbon storage capability easily outweighs their methane emissions. But in a world where corporations plant trees to offset their carbon emissions, we badly need to know if their numbers add up, or if they are undermined by the complex chemistry of trees and methane.

“In the seasonally flooded part of the Amazon, the trees become a massive chimney for pumping out methane,” says one researcher.

Forest scientists have long amused their students by cutting holes in tree bark and setting fire to gases hissing from the trunk. The first recorded measurements were made in 1907, when Francis Bushong of the University of Kansas cut a campus cottonwood and found the gas coming off was 60 percent methane. Yet “it was only about a decade ago that scientists thought to measure whether methane was actually emitting from trees growing in forests,” says Patrick Megonigal of the Smithsonian Environmental Research Center in Maryland, a pioneer in the work. For a while, few forest researchers wanted to know. They were not keen to hear that trees might not be quite as good for the climate as they hoped. Perhaps they feared a rerun of the furor in 1981, when Ronald Reagan used research on the discovery of volatile organic compounds from trees to falsely claim that they “cause more pollution than automobiles.” Similarly, climate scientists saw forests as absorbing methane, rather than releasing it. It only slowly dawned on anyone that trees might do both. Among the first was Vincent Gauci, then at the UK’s Open University and now at Birmingham University. “When I was first working on this, it was poo-pooed,” he says. When Pangala, then also at The Open University, made her first measurements of trees emitting methane in the swamps of Borneo, she had the same experience. Despite finding that the trees increased standard estimates of emissions from the swamps sevenfold, “it took 18 months to get it published,” she says. “We were rejected by several journals. They just weren’t interested.” But the results kept coming. In 2017, Pangala published the findings from her Amazon expedition, during which she had travelled its tributaries and flooded forests, taking measurements of methane from surface water, floating aquatic plants, soils, and the stems and leaves of almost 2,400 trees in 13 floodplain locations.

Sunitha Pangala installs a device that measures a tree's methane emissions, in the Amazon. Courtesy of Sunitha Pangala

“We found a consistent story that the trees all emit a lot of methane,” she says. “In the seasonally flooded part of the Amazon, the trees become a massive chimney for pumping out methane.” Emissions from individual trees were more than 200 times higher than any previously measured anywhere. This was not trivial. Every hectare of flooded forests was emitting several kilograms of methane each day. The on-the-ground findings doubled previous estimates of Amazon methane emissions to around 40 million tons a year. The trees were emitting as much methane as all the tundra ecosystems of the Arctic, whose permafrost contains huge amounts of the gas — a store that is expected to be released in ever-greater quantities as the region warms and its soils thaw. Pangala’s “bottom-up” findings were confirmed by “top-down” measurements from aircraft flown by the U.S. National Oceanic and Atmospheric Administration (NOAA) and others across the same areas. It was a game-changer. “She blew the story open,” says Kristofer Covey of Skidmore College in New York. “The work was very thorough and considered. She provided a full ecosystem picture, and showed the missing methane was coming from trees. It was very difficult to argue with.” Not least because it explained a long-standing data gap, first identified by hydrologist Christian Frankenberg, now at the NASA Jet Propulsion Laboratory in California. He had pointed out in 2005 that remote sensing data from satellites suggested the Amazon was emitting twice as much methane as researchers on the ground could account for. Now the world knew why. “She closed the Amazon methane budget,” says Covey.

After water vapor and carbon dioxide, methane is the most important greenhouse gas. In fact, molecule for molecule, it is a much more potent planet-warmer than CO2. Human sources – most prominently rotting landfills, coal mines, rice paddies, cattle, and leaks from natural gas pipelines – have raised atmospheric concentrations by around 250 percent. They are reckoned to be responsible for around a fifth of global warming. But there are many natural sources, too, including microbial activity in wetlands, termites, the guts of ruminants, and, it now turns out, most of the world’s trees.

The most intense tree emissions are almost certainly from forested areas of tropical wetlands, such as the Amazon.

Because methane only lasts in the atmosphere for around a decade, removing major sources could have a quicker effect on global temperatures than removing CO2, which lasts for centuries. That does not mean that cutting down the world’s trees would cool the planet, however. Far from it. In most places at most times, trees’ ability to absorb and store carbon dioxide trumps any contribution their methane emissions make to the atmosphere. But equally, it can’t be ignored, says Pangala. The numbers are too high. “We find a total footprint of 50-65 million tons of methane annually from wetland trees,” she says. “That is a third of the total from natural wetlands. A third we didn’t know about at all until recently.” The most intense tree emissions are almost certainly from forested areas of tropical wetlands, such as the Amazon. But the role of trees outside wetlands cannot be discounted. “We know emissions from [non-wetland] trees are lower, but there is a far larger area of upland forests in the world to emit,” says Megonigal. Likewise, trees outside the tropics do not generally emit on the scale of those in the tropics. Temperatures are too low. But even so, some forests in the mid-latitudes may at times emit enough methane to negate the methane-absorbing capacity of their soils, turning their ecosystems from net methane sinks to net sources, says Megonigal. So how and why do trees give off methane? Wetlands are known natural sources of methane, which used to be called “marsh gas.” So, in retrospect, it should not be too surprising that their trees play a role. But what role? Some researchers see wetland tree trunks merely as passive conduits for methane generated by micro-organisms in the waterlogged soils. Tree trunks may look solid, but they contain spaces and channels through which gases travel up and down. “A large proportion of the volume of a tree stem is gas,” says Covey — anywhere between a quarter and half.

Tropical wetlands, such as this mangrove forest in Bali, give off the most intense tree-based emissions of methane. Alamy

But it seems that wetland trees are much more than conduits. They also create the conditions, and provide the raw materials, for methane generation by micro-organisms. “In wetland systems, trees send a lot of carbon into their roots,” says Pangala. This delivery, known as rhizodeposition, provides the essential raw materials for methane-generating micro-organisms that congregate among the trees’ roots. “Trees are bioreactors”, says Gauci. “Without them, methanogenesis, even in wetlands, might be much less.” Many trees, especially outside wetlands, also actively generate methane. Some methane comes from photochemical reactions in their foliage. More may be from microbes living in the trunks that themselves generate methane, says Gauci. Some researchers have termed trees as crypto-wetlands or vertical wetlands. The scale of these processes remains unclear. But what we are learning, says Covey, is that the chemical interactions between trees and the atmosphere are extremely dynamic. “Until recently, in climate terms, we have seen forests mostly as carbon sinks,” he says. “ The reality is very different, there is much more going on.” Not all of that action is bad news. For as well as emitting methane, trees also absorb the gas. Indeed, the same tree may be a net source or sink depending on the season, its age, or even which bit of the tree you are talking about. Many emit methane close to their base while absorbing it further aloft.

There is an urgent need to include tree emissions in greenhouse gas emissions inventories.

The bottom line, says Pangala, is that almost all trees can both emit and absorb methane. But finding out the net balance is very hard because it changes so much. And that methane is, of course, only part of a much bigger picture of the role of trees in climate. “In the wider world of climate change, their benefits are almost always much greater,” says Pangala. “Even for an individual tree, the methane element usually turns out to be quite small compared to carbon storage.” And besides storing carbon, they recycle moisture, create shade, stimulate cloud formation, protect biodiversity, and cleanse the air. But even if trees are rarely “bad” for the climate, clearly some can be better than others, says Pangala. So if the world is to embark on a sustained program of reforesting the planet as a means to fight climate change, then “let’s choose trees with a small methane footprint.” Her former supervisor at The Open University is on the case. Gauci currently is working on the Indonesian island of Sumatra with the owners of huge plantations of acacia trees growing on drained peat bogs. Dried peat emits carbon dioxide, and to prevent that, the Indonesia government is requiring peatland concession holders to plug drains and raise the water table. But the risk, says Gauci, is that rising waters will trigger a burst of methane emissions from the waterlogged trees. He hopes to find a perfect combination of trees and water levels — a “sweet spot that will minimize carbon emissions but avoid a methane bomb.” Methane, lit on fire, escaping an Eastern cottonwood tree. Credit: Oak Ridge National Laboratory

The need to quantify such complex interactions and trade-offs – and to find a planetary “sweet spot” – is growing fast. There is an “urgent need to include tree emissions in greenhouse gas emissions inventories,” wrote a team of scientists in a 2016 paper in the journal Scientific Reports. The need is all the greater, notes Covey, when governments and corporations are planting trees with the promise that they will thereby offset their industrial emissions by adding trees that soak up CO2, thus meeting their international obligations for cutting greenhouse gas emissions. The climate benefits could be inflated if methane emissions from the trees are ignored, he says: “The danger is that we end up trading real emissions on the carbon markets for perceived offsets.” “Ultimately we want to get to the situation where if you… know the type of trees and soil and temperature and water table, we will be able to calculate how much methane gets into the atmosphere,” says Pangala. But that still requires a lot more science, and a lot more data.