Methane, one of the most potent greenhouse gases, has increased in the atmosphere by roughly 150% since the preindustrial era. Although much methane comes from wetlands and agriculture activities, it also arises from mountainous upland forests, which are traditionally thought to be a net sink for atmospheric methane. Here Wang et al. present one of the first mechanistic explanations for why some tree species in mountain forests can produce and emit methane at varying rates: The amount of gas a tree produces depends on the water in its core, or heartwood.
In the past, scientists thought of low-elevation, wetland forests as primarily net sources of methane gas, whereas mid- to high-elevation, mountainous upland forests were thought to be net methane sinks. Furthermore, scientists have focused mainly on ground methane flux in these areas—living-tree stem-based methane processes and fluxes are poorly understood. To determine the importance of these methane processes and fluxes, the researchers turned to large swathes of China’s mountainous upland forests.
Previous studies have shown that methane originates in a tree’s heartwood, the dense, dead inner wood of trees that provides support but no longer transports sap or water, and is released into the atmosphere through its outer rings of sapwood and bark. After randomly selecting plots of trees, the team drilled holes into trees that reached their heartwood, plugging the holes tightly to avoid gas exchanges with the atmosphere. They extracted samples of the gas and/or water and brought them to a lab for further analysis. They also performed in situ experiments, in which they enclosed the trees in plastic chambers and captured the gas as it escaped.
After gathering data based on a lot of field investigation and in situ measurements in summer and autumn 2016, the team found a strong, positive relationship between the amount of water in a tree’s heartwood and the amount of methane production. According to the team, this study suggests that water is a critical variable in controlling methane production in the heartwood of living trees when temperature is not a limiting factor for methane production. (Journal of Geophysical Research: Biogeosciences, https://doi.org/10.1002/2017JG003991, 2017)
—Emily Underwood, Freelance Writer