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New Research Reveals Upland Forest Trees Emit Methane

A recent study from the University of Delaware reveals that tree trunks in upland forests actually emit methane rather than store it. This represents a new, previously unaccounted source of this powerful greenhouse gas.

Methane is approximately 25 times stronger than carbon dioxide, with some estimates as high as 33 times stronger because of its effects when it is in the atmosphere.

Due to methane’s global warming potential, detecting the sources and storehouses or “sinks” of this greenhouse gas is vital for measuring and understanding its repercussions across ecosystems.

Upland forest soils typically absorb and store methane, but this effect can be counteracted by methane emitted from tree trunks, the research team from UD’s College of Agriculture and Natural Resources discovered. Their research findings have been 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.

Rodrigo Vargas, Assistant Professor, University of Delaware

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

Maryland study site

In a 30-acre area of upland forest at Fair Hill Natural Resources Management Area in nearby Cecil County, Maryland, the team analyzed soil, a cluster of trees, and coarse woody debris (CWD) - dead wood that can be found on the forest floor in different stages of decomposition - to measure fluxes of carbon dioxide and methane.

The researchers used a sophisticated greenhouse gas analyzer based on laser absorption technology, called Off-Axis Integrated Cavity Output Spectroscopy (OA-ICOS), which closely resembles a proton pack from the movie “Ghostbusters.”

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

Soils and CWD fluxes

With regards to carbon dioxide, research on the fluxes of soil, known as soil respiration, and tree trunks, known as stem respiration, has been conducted for decades, but research to establish the importance of carbon fluxes pertaining to CWD is still far behind.

With regards to methane, studies have been conducted on methane fluxes in connection to soils, which typically consume the methane and are perceived as methane sinks, however, there are very few that analyze tree trunks and CWD 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.

Daniel Warner, Doctoral Student, University of Delaware

To comprehend the differences between the actions of the CWD, Warner and colleagues discovered that fresher CWD has a positive methane flux, which is a lot like how a living tree behaves.

“When a tree falls over, it’s still functionally the same in terms of methane emissions. 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,” said Warner.

The research team also discovered that CWD had a high rate of inconsistencies with regards to  methane emissions.

As it decays it becomes a lot more variable. Some of the super-decayed wood was still releasing methane but a lot of it was consuming methane. If you have a CWD pool with less diversity regarding the degree of decomposition, you can expect it to play a more uniform role in terms of methane emissions or sinks.

Daniel Warner, Doctoral Student, University of Delaware

Tree trunks and methane fluxes

While tree trunks have been known to emit carbon dioxide, this research revealed that they were also discharging 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 Vargas.

On the whole, the tree trunks acted as a source of carbon dioxide and as a small source of methane, but the magnitude of gases released varied based on the species.

Tulip poplar was one species that emitted plenty of carbon dioxide and methane, while beech trees emitted the highest amount of methane within the forest but released very little carbon dioxide.

“It might be some species-specific trait that’s controlling the flux,” said Warner.

Temperature threshold

Temperature also played a key role in controlling 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.

Rodrigo Vargas, Assistant Professor, University of Delaware

Warner said it is difficult to form a temperature relationship with methane because there are two methods that oppose each other.

“You have things in the soil producing methane—known as methanogenesis—things consuming it—known as methanotrophy—and so as you warm up, it’s more kind of like a shot gun where the magnitudes of methane scatter out more as it gets warmer; suggesting that other factors beyond temperature regulate methane emissions,” said Warner.

They noticed that outside a threshold of 17 °C for soil temperature, the inconsistency of methane consumption expands radically.

“Under 17 degrees, temperature is a key driver of methane flux but above 17 degrees, there are other drivers that will influence methane production,” said Vargas.

Soil hot spots

As for where the methane began, Warner said it is still an area to be explored; however, this research offers sufficient clues to provide the researchers some theories.

The first theory is that methane is generated 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. We know that happens in wetlands but in uplands, maybe it happens in one specific spot and nowhere else.

Daniel Warner, Doctoral Student, University of Delaware

The other mechanism that could be producing methane fluxes from trunks is infection or internal rotting inside the tree, which results in an environment where methanogenic bacteria can live and then methane disperses 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,” said Vargas.

Next steps

Both Warner and Vargas agreed that the following steps should be to analyze the generality of these observations across various forests, and find the mechanisms of methane production and transport in tree trunks. Finally, they recommend that global and ecosystem models should consider methane generated from tree trunks as a new source of methane to the atmosphere.

“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. 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,” said Warner.

Methane emissions in upland forests

Dan Warner, doctoral student at the University of Delaware, talks about his field research on methane gas emissions from trees. (Credit - UNIVERSITY OF DELAWARE/JEFFREY CHASE)

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