Bark Bacteria May Limit Tree Greenhouse Gas Emissions

Paperbark Trees in Coombabah Lake Conservation Park. Photo by Silverish Lily via Wikimedia Commons. CC BY-SA 4.0.

Wetland trees are an unexpected but major source of global methane emissions. Methane-oxidizing bacteria recently discovered in tree bark could inform how climate scientists and legislators approach future methane mitigation strategies.

Methane comprises 10% of greenhouse gas emissions in the United States, but can be 32-87 times more potent than carbon dioxide at warming Earth’s atmosphere. Natural sources produce millions of metric tons of methane every year because of climate change feedback systems, with wetland forests contributing about one-third of total methane emissions worldwide. This may be startling to some, as trees are widely known to offset greenhouse gas emissions, but this isn’t a new discovery.

In 1907, Francis Bushong was surprised to discover that he was able to burn a sample of cottonwood tree sap. Upon further analysis, he found that methane made up about 60% of the gas produced by cottonwood trees. Bushong published his research in the Chemical and Physical Papers journal in 1907. Interest in Bushong’s findings waned throughout the century, but today, as the world’s eyes focus on climate change, increasing numbers of scientists are measuring methane emissions from trees.

U.S. methane emissions mostly come from coal, natural gas, and oil production and transportation; livestock; and agriculture, according to a 2019 EPA report. Wetland forests contribute about one-third of global methane emissions, yet global climate and greenhouse gas mitigation policies have ignored them as a main methane emitter, as reported by a 2017 paper by the Potsdam Institute of Climate Impact Research. Methane emissions from wetlands have also increased due to changing global wetland area and the temperature-sensitivity of methane. 

Global monthly averages of methane emissions. Figure via National Oceanic and Atmospheric Administration.

Fortunately, certain wetland tree bark contains bacteria that can reduce the tree’s methane emissions, according to a discovery reported earlier this spring in Nature. For the first time, scientists were able to show that tree bark may be a host to methane-oxidizing bacteria, which consume methane as their energy source and produce other forms of carbon as a byproduct. The team examined the bark of Melaleuca quinquenervia, a wetland tree also known as the paperbark tree, and found that bark-dwelling, methane-oxidizing bacteria reduce paperbark tree methane emissions by almost 36%.

The researchers state that “if [methane-oxidizing bacteria] are a ubiquitous feature within the bark of methane emitting trees, our conceptual understanding of the global methane cycle may need revision.” Many corporations are demonstrating their environmental responsibility by planting trees to offset their carbon emissions. While in most cases, trees’ carbon absorption outweighs their methane emissions, we may need a better and more nuanced understanding of the exact dynamic.

Although further research is needed, especially with regard to different species of trees that hold methane-oxidizing bacteria, this research is a stepping stone for scientists and policymakers to better understand and mitigate tree methane emissions, particularly considering oft-overlooked wetland emissions.

This article was edited by Lydia Guertin and Ashley Schefler.