Diseased trees are source of climate-changing gas

Diseased trees in forests may be a significant source of methane that causes climate change, according to a study by researchers at the Yale School of Forestry & Environmental Studies (F&ES) published in Geophysical Research Letters.

Sixty trees sampled at Yale Myers Forest in northeastern Connecticut contained concentrations of methane that were as high as 80,000 times ambient levels. Normal air concentrations are less than 2 parts per million, but the Yale researchers found average levels of 15,000 parts per million inside trees.

''These are flammable concentrations,'' said Kristofer Covey, the study's lead author and a Ph.D. candidate at Yale. ''Because the conditions thought to be driving this process are common throughout the world's forests, we believe we have found a globally significant new source of this potent greenhouse gas.''

The estimated emission rate from an upland site at the Yale forest is roughly equivalent to burning 40 gallons of gasoline per hectare of forest per year. It also has a global warming potential equivalent to 18 percent of the carbon being sequestered by these forests, reducing their climate benefit of carbon sequestration by nearly one-fifth.

''If we extrapolate these findings to forests globally, the methane produced in trees represents 10 percent of global emissions,'' said Xuhui Lee, a co-author of the study and the Sara Shallenberger Brown Professor of Meteorology at Yale. ''We didn't know this pathway existed.''

The trees producing methane are older - between 80 and 100 years old - and diseased. Although outwardly healthy, they are being hollowed out by a common fungal infection that slowly eats through the trunk, creating conditions favorable to methane-producing microorganisms called methanogens.