Why are dead trees emitting methane?

A new study published in the journal Frontiers in Environmental Science has investigated the source of methane emissions in coastal forests that are destroyed by rising sea levels – the so-called “ghost forests.” The scientists have found that communities of microbes in the soils under dead trees (also called “snags”) are producing methane, while the snags act like filtering straws as the gas rises through the wood.

“We’re tracing where the methane is originally coming from, and what we’re finding is that it’s coming from the soils, and as it moves through the tree, it’s changing as well,” said study co-author Marcelo Ardón, an associate professor of Forestry and Environmental Resources at North Carolina State University. “The methane is being processed as it moves through those snags.”

In order to localize the source of methane emissions, Professor Ardón and his colleagues analyzed methane gas samples from standing dead trees in ghost forests from five sites on the Albemarle-Pamlico Peninsula on the North Carolina coast. 

“It’s important to be able to understand how methane is moving through the ecosystem,” study lead author Melinda Martinez, a postdoctoral scholar at the U.S. Geological Survey. “These ghost forest regions are temporary ecosystems; they will become marshes. But even during the transient state, we want to know how ghost forests produce and move this greenhouse gas.”

By measuring concentrations of the gas in the soil, in the soil water, and inside the snags, the scientists found that methane concentrations generally decreased the higher they got from the ground in the dead trees. Afterwards, they also took samples from the trees and placed them in vials to see if they were producing any methane when isolated in oxygen-free environments.

“Only five out of 100 vial samples produced any significant results, which provides further evidence that the methane being emitted from standing dead trees originated in the soil,” said Dr. Martinez.

Since increasingly more scientists agree that methane has a higher warming potential than carbon dioxide, mapping methane fluxes in ghost forests is essential for better understanding the environmental impact of forest-to-ghost forest transition and building strategies for mitigating climate change. 

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By Andrei Ionescu, Earth.com Staff Writer