Surprising Greenhouse Gas Source: Trees Actually Emit Methane Rather Than Store It

Methane is considered to be one of the most powerful greenhouse gasses. It's about 25 to 33 times stronger than carbon dioxide. Due to the global warming potential of methane, multiple studies have been conducted to identify sources of greenhouse gasses and things that store it.

Now, a study from the University of Delaware found a new, previously unaccounted source of methane -- tree trunks in upland forests. The study, published in the journal Ecosystems, showed that methane emissions from tree trunks could upset the positive effect caused by the ability of upland forest soils to take up and store methane.

"The tree trunks constantly have low but detectable emissions of methane. Soils are providing an environmental service of sequestering this potent greenhouse gas, but the trunks are releasing methane equivalent to 4 percent of what could be captured by CWD and soils at the ecosystem scale," said Rodrigo Vargas, assistant professor in the Department of Plant and Soil Sciences at UD's College of Agriculture and Natural Resources and lead investigator of the study, in a press release.

For the study, the researchers measured the carbon dioxide and methane fluxes of the soil and tree trunks' coarse woody debris (CWD) in a 30-acre area of upland forest at Fair Hill Natural Resources Management Area. They used Off-Axis Integrated Cavity Output Spectroscopy (OA-ICOS), a state-of-the-art greenhouse gas analyzer based on laser absorption technology, to measure the fluxes of methane and carbon dioxide.

The researchers observed that tree trunks and fresher CWD emit methane. However, as the CWD rot away over time, it starts behaving like soil and consumes methane rather than emitting it. The amount of methane and carbon dioxide being released by the tree trunks varied with the species.

The researchers note that tulip poplar trees release a lot of methane and carbon dioxide, while beech trees release the most methane within the forest but emit very little carbon dioxide.

Temperature also plays a crucial role in methane emissions. The researchers found that methane scatters more when it's warmer. When the temperature is below 17 degree Celsius, it becomes a key driver of methane flux.

It is still unknown how living trees produce methane. The researchers believe that it has something to do with internal rotting or infection inside the tree. They noted that more research is needed to further understand the mechanism behind the trees' methane emissions.