Trees can act as giant methane chimneys

A new study has suggested that trees can act like chimneys, moving methane gas produced by soil microbes up through roots, stems and leaves before releasing it into the atmosphere.

Researchers have long known that methane comes from anaerobic processes in waterlogged soils such as swamps, wetlands and rice fields, as well as in the guts of termites and ruminant animals such as cows and sheep.

But in 2006, a team proposed the surprising idea that plants, too, produce methane - as much as 10-30% of the world's total methane emissions.

If true, that would require a major overhaul of global carbon budgets.

Now, according to a report in Nature News, a team led by atmospheric scientist Andrew Rice of Portland State University in Oregon, have determined that tree trunks can act as giant methane chimneys.

They can move methane gas produced by soil microbes up through roots, stems and leaves before releasing it into the atmosphere.

This effect could account for as much as 10% of methane emissions globally. It could also help to explain why methane fluxes are higher than expected in wet tropical regions.

The team responsible for the latest work measured methane flow in three tree species, which were flooded to create conditions ripe for anaerobic microbes to start churning out methane.

According to Rice, the work does not rule out the possibility that plants themselves can produce methane aerobically.

For instance, light at a certain intensity and wavelength could create a photolytic reaction that produces methane, as the 2006 work suggested.

"The question is the magnitude of that source," said Rice.

The latest study also found that the isotopic composition of the microbial methane transported through the trees was almost identical to that of the methane emissions observed in the 2006 study.

This means that it could be tough to distinguish in the field between methane produced anaerobically and that produced aerobically.

"It's getting clearer that living vegetation is maybe playing a more active role in emitting methane to the atmosphere than we previously thought," said Frank Keppler, a geochemist at the Max Planck Institute for Chemistry in Mainz, Germany.