Dying trees and low water reserves are two of the surprising culprits behind the rising levels of the greenhouse gas methane, according to two separate studies released by Yale and Washington State University this week.

This is mostly because they provide ideal environments for methane-producing microbes to thrive.

Researchers at the Yale School of Forestry and Environmental Studies looked at around 60 trees at Yale Myers Forest in northeastern Connecticut and found that they contained concentrations of methane that were a hundred times greater than normal air concentration.

The trees with the greatest concentration of methane were older – between 80 and 100 years old – and diseased. They were being hollowed out by a common fungal infection that slowly eats through the trunk, creating conditions favorable to methane-producing microorganisms called methanogens.

Normal air concentrations of methane are less than 2 parts per million, but the Yale researchers found average levels of 15,000 parts per million inside the trees. The estimated emission rate from the study site was roughly equivalent to burning 40 gallons of gasoline per hectare of forest per year.

The methane concentrations also had a detrimental effect to the forest’s ability to act as a carbon sink. The global warming potential equivalent of the recorded methane levels was equal to 18 percent of the carbon being sequestered, reducing the 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 Sara Shallenberger, Brown Professor of Meteorology at Yale. "We didn't know this pathway existed."

Meanwhile, the Washington State University researchers looked at dams, the water reservoirs behind them, and surges of greenhouse gases as water levels go up and down.

They measured dissolved gases in the water column of Lacamas Lake in Clark Country and found methane emissions jumped 20-fold when the water level was drawn down.

Measuring sampled bubble rising from the lake mud found a 36-fold increase in methane when the water level was down.

“Reservoirs have typically been looked at as a green energy source,” said Bridget Deemer, a doctoral student at Washington State University-Vancouver. “But their role in greenhouse gas emissions has been overlooked.”

The research could lead to different ways of managing drawdowns, said John Harrison, assistant professor of Earth and environmental Sciences. Dam managers can consider the optimal time to take out a dam, as emissions may be higher in summer months, when warmer temperatures and low oxygen conditions in bottom waters stimulate the microbial activity that produces greenhouse gases.

The findings of both studies would also be important for climate and atmospheric scientists seeking to better understand global greenhouse gas levels and its possible climate change effects.

Methane is a greenhouse gas that is 25 times more effective than carbon dioxide at trapping heat in the atmosphere. It has a net lifetime of about 10 years. – K.R. Jalbuena

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