In the fight to cut greenhouse gas emissions, we tend to think of trees as our allies, but new research suggests they might not be as "green" as we give them credit for. Researchers at the University of Delaware have found that some types of trees actually emit methane through their trunks, partly countering their role as a greenhouse gas sink.

Though it comprises a smaller percentage of greenhouse gases than carbon dioxide, methane makes up for it in potency, with its effects on the environment estimated to be 25 times stronger than CO2. Climate scientists have been trying to catalog sources and sinks of methane and CO2 to take stock of the Earth's greenhouse gas budget and find ways to improve it.

Oceans and forests are generally considered sinks, meaning they capture and store greenhouse gas emissions, but they aren't completely offsetting the gases burping out of the Arctic, reservoirs and livestock.

To study how much methane and carbon dioxide forests may be producing or storing, the University of Delaware team took to the woods in Cecil County, Maryland. Over a growing season spanning April to December, they used a tool called an Off-Axis Integrated Cavity Output Spectroscope (OA-ICOS) to analyze whether soil, tree trunks and coarse woody debris (CWD), or the rotting wood on the ground, are sources or sinks.

The biggest surprise the team found was that trees give off methane through their trunks. The amount depends on the species, but generally those emissions don't seem to be as high as their carbon dioxide output.

"The tree trunks constantly have low but detectable emissions of methane," says Rodrigo Vargas, lead researcher on the study. "Soils are providing an environmental service of sequestering this potent greenhouse gas, but the trunks are releasing methane equivalent to four percent of what could be captured by CWD and soils at the ecosystem scale."

Where trees are pulling that methane from remains unknown, but the researchers believe the clues point to one of two sources: the trees might be absorbing it from the soil through their roots, or the wood has already begun to rot from the inside.

"At this moment, the mechanisms of methane production in upland forests are not clear," says Vargas. "Methane can be either transported from the soils upward inside the stem and diffused to the atmosphere or produced inside the stem by fungi or archaea — single-celled microorganisms."

CWD was found to swing wildly between being a methane source and sink, and it seems to depend on its age. Since these fallen bits of wood are essentially the middleman between living trees emitting methane, and the soil absorbing it, the older a piece was, the more likely it was to be absorbing the gases.

"When a tree falls over, it's still functionally the same in terms of methane emissions," says Daniel Warner, lead author on the study. "Over time, as it decays, my theory is that it gets colonized by soil bacteria that consume methane and it shifts to behave more like the soil, resulting in a methane sink."

Since these findings are based on one section of forest, the researchers plan to investigate whether they carry across to other forests and types of trees, and whether that emission footprint is large enough to factor into global methane models.

"When people develop ecosystem to global scale methane budgets, there's always a chunk in which it is uncertain from where that methane is coming," says Warner. "Methane emissions by vegetation and tree trunks are seen as a newly-considered source that might bring that budget closer in to our estimates. It's good to keep chipping away at that."

The research was published in the journal Ecosystems, and Warner demonstrates the work in the video below.

View gallery - 2 images