Rather than storing it, tree trunks in upland forests actually emit methane, a powerful greenhouse gas, say a study.
Upland forest soils usually take up and store methane, but this effect can be counteracted by methane emissions from tree trunks, the research team found.
Because of methane's global warming potential, identifying the sources and "sinks" or storehouses of this greenhouse gas is critical for measuring and understanding its implications across ecosystems.
The new findings, published in the journal Ecosystems, represent a new, previously unaccounted source of this powerful greenhouse gas.
"We believe our work can help fill in some gaps in methane budgets and environmental processes in global ecosystem models," said the study's leader Rodrigo Vargas, Assistant Professor at University of Delaware in Newark, US.
In a 30-acre area of upland forest in the US, the researchers tested a cluster of trees, soil and coarse woody debris (CWD) -- dead wood lying on the forest floor in various stages of decomposition -- to measure fluxes of methane and carbon dioxide.
"What we've found in this study is that some coarse woody debris acts kind of like the soil and consumes methane while other pieces of coarse woody debris emit small amounts of methane, which is also what we saw with living tree trunks," Daniel Warner from University of Delaware said.
While tree trunks have been known to release carbon dioxide, this research showed that they were also releasing 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 four per cent of what could be captured by CWD and soils at the ecosystem scale," Vargas said.
Overall, the tree trunks acted as a source of carbon dioxide and as a small source of methane, but the magnitude of gases emitted varied with the species.
The researchers said their next step would be to identify the mechanisms of methane production and transport in tree trunks.
"At this moment, the mechanisms of methane production in upland forests are not clear. 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," Vargas said.
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