A new study from the University of Delaware is one of the first in the world to show that tree trunks in upland forests actually emit methane rather than store it, representing a previously unaccounted new source of this powerful greenhouse gas.

Methane is about 25 times stronger than carbon dioxide as a greenhouse gas, with some estimates as high as 33 times stronger due to its effects when it is in the atmosphere. 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.

Upland forest soils usually take up and store methane, but this effect can be counteracted by methane emissions from tree trunks, the research team from the University of Delaware's College of Agriculture & Natural Resources found. Their work was published in the scientific journal Ecosystems.

"We believe our work can help fill in some gaps in methane budgets and environmental processes in global ecosystem models," said study leader Rodrigo Vargas, an assistant professor in the department of plant and soil sciences.

Shreeram Inamdar, professor of watershed hydrology and biogeochemistry, is co-investigator on the project with Vargas, and doctoral student Daniel Warner is the lead author of the paper. The research was funded by the U.S. Department of Agriculture, with additional support from Delaware's Federal Research & Development Matching Grant Program.

Maryland study site. In a 30-acre area of upland forest at the Fair Hill Natural Resources Management Area in Cecil County, Md., 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.

Warner visited the site over the course of one growing season, April to December, and measured the carbon dioxide and methane fluxes of the soil, tree trunks and CWD to determine whether those three components were sources or sinks of these greenhouse gases.

In terms of carbon dioxide, research has been done for decades on the fluxes of tree trunks — known as stem respiration — and soil — known as soil respiration — but research to determine the importance of carbon fluxes with regard to CWD still lags.

It's a different story for methane, however. While studies have been done on methane fluxes in connection to soils, which usually consume the methane and are considered methane sinks, very few studies have dealt with CWD and tree trunks in upland soils.

"What research has been done is generally lab incubations of wood, where they measure how much methane is released over time. 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," Warner said.

To understand the differences between the actions of the CWD pools, Warner and his colleagues found that fresher CWD has a positive methane flux, which is similar to how a living tree behaves. The researchers also found that CWD had a high rate of variability when it came to methane emissions.

Tree trunks and methane fluxes

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 4% 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 by species.

Tulip poplar is one species that released a lot of methane and carbon dioxide, whereas beech trees released the most methane within the forest but emitted very little carbon dioxide. "It might be some species-specific trait that's controlling the flux," Warner noted.

Temperature also played a key role in regulating the magnitude of the fluxes.

"Methane in soils seem to follow a temperature gradient where higher temperatures are related to higher uptake of methane, but that's not necessarily the case for CWD or for tree trunks," Vargas said.

Warner said it's hard to develop a temperature relationship with methane, because there are two processes that oppose each other. They found that beyond a threshold of 17°C for soil temperature, the variability of methane consumption expands dramatically.

As for where the methane originated, Warner said it's still a scientific frontier, but this study provides enough clues to give the researchers some theories.

The first one is that methane is produced in hot spots in the soil.

"By hot spot, we mean a place where conditions are conducive to methane production, and then that methane is sucked up by the tree roots, transported through its vascular system and released out of its trunk," Warner said. "We know that happens in wetlands, but in uplands, maybe it happens in one specific spot and nowhere else."

The other mechanism that could be causing methane fluxes from trunks is internal rotting or infection inside the tree that produces an environment where methanogenic bacteria can survive, and then methane diffuses out of the tree.

"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.

Both Warner and Vargas agreed that the next steps should be to test the generality of these observations across different forests and identify the mechanisms of methane production and transport in tree trunks. Finally, they suggest that global and ecosystem models should take into account methane produced from tree trunks as a new source of methane to the atmosphere.