Throughout their lifetimes, healthy forests produce more oxygen than they use, while taking in greenhouse gases via plants and soils. This ecosystem-wide service, called carbon sequestration, regulates global climate and is an essential component of climate models and goals. Forest health, however, influences carbon cycling, and when trees get sick, the net reduction of greenhouse gases may be more limited than previously thought.
New research conducted at the University of Notre Dame Environmental Research Center (UNDERC) suggests that upland forests harboring trees with a common and incurable fungal disease known as heart rot could actually be emitting more methane than they take in, therefore releasing more greenhouse gases than previously thought. Methane, a flammable natural gas, is more than 30 times more effective at trapping heat than carbon dioxide.
"Historically, upland forests were thought to be strong methane sinks because they have organisms in their dry soils that take up methane instead of releasing it to the atmosphere," said Adrian Rocha , an ecologist at the University of Notre Dame who supervised the research. "Heart rot disease has the potential to switch upland forests from being methane sinks to methane sources since diseased trees emit more methane than healthy trees."
The healthy trees that Rocha and colleagues investigated in the northwoods of Wisconsin and Michigan emitted less methane than nearby trees infected with heart rot, a slow-acting, internal disease caused by fungi that results in the decay of a tree's trunk and branches from the inside and affects hardwood trees globally. As the severity of the infection increases, so too does the amount of methane released from each tree.
The study, among the first to link methane venting to tree health, was published in New Phytologist , a leading international plant science journal.
To non-invasively measure the severity of the heart rot, researchers employed a technique called sonic tomography, which uses sound waves to map the location of the rot inside each trunk. Sound moves differently through rotted wood and healthy wood and, once captured by sensors placed onto the bark, is used to generate a map of disease severity.
From there, the researchers measured the carbon-based greenhouse gases flowing out of each tree. While carbon dioxide venting remained largely stable from tree to tree, regardless of disease state, methane emissions increased according to the level of heart rot severity in the tree.
Further, Chathuranga Senevirathne, a Notre Dame graduate student in Rocha's lab who led the study, pinpointed where each type of gas was coming from by drilling into the tree at regular intervals and taking new gas measurements as he went. In doing so, he found that carbon dioxide quantities peaked just underneath the bark, a section called the sapwood, while methane emissions topped out in the very center of the trunk, called the heartwood.
"While it's been established for a few decades that trees do give off some methane, even when in a healthy state, the connection between methane and heart rot hadn't been explored," said Rocha, who is an associate professor in the Department of Biological Sciences . "Everyone in the field had accepted that it was coming through the soil, but it turns out it's coming from the center of the tree itself."
To rule out soil transport, the researchers sampled the carbon dioxide and methane flows in the soil around the base of each tree studied. Regardless of the disease progression of the tree, the soils released small amounts of carbon dioxide and absorbed small amounts of methane.
Despite the apparent correlation between heart rot and gas emission, the fungi that cause the disease are not directly responsible for the elevated methane levels observed, as heart-rot fungi taken from a diseased tree did not produce methane in the lab. Instead, the fungi are aided in breaking down heartwood by methanogens, a group of methane-producing single-celled microorganisms called archaea, whose presence the researchers verified by removing samples of wood from the heart of each tree and analyzing them with genomic sequencing.
"Decomposition is a complex process which involves both the heart-rot fungi and methanogens, since methanogens 'eat' the wood to produce methane," said Rocha, who is a faculty affiliate of Notre Dame Energy and Notre Dame's Environmental Change Initiative . "The fungi are not directly responsible for the methane emissions, but at the same time, heart rot creates an ideal microenvironment for the archaea to thrive."
One characteristic of this microenvironment is bark fractures, which appear on the surface of the tree as the interior deteriorates. Fractures in a tree's skin also permit the more efficient release of methane from the heartwood to the exterior. As trees become sicker and sicker with heart rot, methanogen production receives a boost, while proliferating bark fractures create methane emission "hot spots" on the surface of the tree.
"With the progression of heart rot, diseased trees become methane hotspots on the forest level, while bark fractures act as hotspots at the tree level," Senevirathne said. "With the discovery of these new emissions, there's a good chance that the amount of methane upland forests take in has been overestimated in ecosystem models."
"Identifying the sources and sinks of methane is one of the biggest mysteries and hottest topics in forest science," said Nathan Swenson, a forest ecologist and the Gillen Director of the University of Notre Dame Environmental Research Center . "The work from Rocha's laboratory has elegantly demonstrated the important role disease plays in the carbon cycle."
Rocha and Senevirathne's future work at UNDERC will investigate these flows on an ecosystem level and aim to determine the tipping point where upland forests could transition from carbon sink to carbon source, which could challenge the widely accepted impact of forests on climate.
"The outstanding natural setting and scientific infrastructure at UNDERC uniquely position the center to host cutting-edge research like that performed by Senevirathne and colleagues, integrating genomic analysis to ecosystem gas flux," Swenson said.
Funding from the National Science Foundation and NASA supported this research. Senevirathne was funded by the Merrilee Clark Redmond Endowment, and field work was supported by the Hank Family Endowment.
