Methane (CH4) is a powerful greenhouse gas. There is much less of it in the atmosphere than carbon dioxide (its concentration is measured in parts per billion), and it lingers there only eight to 12 years. But it is far better at trapping heat emanating from Earth. Over a century, a gram of methane can have 20 times the impact on Earth’s climate as a gram of CO2.
Wetlands rank among the world’s largest methane emitters because methanogens—microbes that emit methane—thrive in flooded, anoxic soils. However, methanotrophs—microbes that consume methane—live in the aerobic area of all soils, where they intercept methane before it escapes to the atmosphere. In wetland soils, they can reduce the amount of methane lost to the atmosphere by up to 90 percent. In upland soils, they consume 100 percent of the soil-produced methane, and also remove it from the atmosphere. This seemed to suggest a simple conclusion: Wetlands emit methane, and uplands consume it.
But this simple conclusion may be more complicated in forests. Earlier research on methane fluxes in forests has focused on uptake by soils, assuming that the trees themselves had no role in the process. It is now clear that trees are actively influencing the methane budget of forests as well.
The Biogeochemistry Lab is now studying the role of trees as conduits of methane emissions to the atmosphere. With funding from the Department of Energy, SERC postdoc Lisa Schile and Johns Hopkins graduate student Scott Pitz are trying to pin down if, when and where trees emit methane. The question of why trees emit methane is perhaps our most important question.
Schile is measuring methane emissions coming from wetland trees, while Pitz focuses on upland trees in the SERC ForestGEO plot. In 20-by-20 meter plots, they have placed closed chambers on the soil surface and on tree trunks to measure the methane flux. Each tree has several chambers at different heights, to see whether different parts of the trunk emit more methane than others. They are also watching the impact of the seasons, soil moisture, tree species and tree diameter.
Four main questions guide our research:
- Do upland trees release methane to the atmosphere?
- What determines the size and timing of upland tree methane emissions?
- What is the source of methane emitted by trees?
- How do trees affect the methane budget of forest ecosystems?
Read more: Which kinds of wetlands emit the most methane?