The Science

Scientists have only recently understood that trees can emit or consume methane from the atmosphere. This is the first comprehensive review of the literature on trees and forests as methane sources and sinks.

The Impact

Until recently it was assumed that all exchange of methane between forests and the atmosphere takes place at the soil surface. This review demonstrates that all surfaces in a forest—living wood, dead wood, leaves, branches, and epiphytes—can exchange methane, a fact that will change this study’s approach to building forest methane budgets.

Summary

Forest ecosystem methane (CH₄) research has focused on soils, but trees are also important sources and sinks in forest CH₄ budgets. Living and dead trees transport and emit CH₄ produced in soils; living trees and dead wood emit CH₄ produced inside trees by microorganisms; and trees produce CH₄ through an abiotic photochemical process. Here, researchers review the state of the science on the production, consumption, transport, and emission of CH₄ by living and dead trees, and the spatial and temporal dynamics of these processes across hydrologic gradients inclusive of wetland and upland ecosystems. Emerging research demonstrates that tree CH₄ emissions can significantly increase the source strength of wetland forests, and modestly decrease the sink strength of upland forests. Scaling from stem or leaf measurements to trees or forests is limited by knowledge of the mechanisms by which trees transport soil-produced CH₄, microbial processes that produce and oxidize CH₄ inside trees, a lack of mechanistic models, the diffuse nature of forest CH₄ fluxes, complex overlap between sources and sinks, and extreme variation across individuals. Understanding the complex processes that regulate CH₄ source-sink dynamics in trees and forests requires cross-disciplinary research and new conceptual models that transcend the traditional binary classification of wetland versus upland forest.

Principal Investigator

Pat Megonigal
Smithsonian Environmental Research Center
megonigalp@si.edu

Program Manager

Daniel Stover
U.S. Department of Energy, Biological and Environmental Research (SC-33)
Environmental System Science
daniel.stover@science.doe.gov

Funding

Funded by a grant to J. Patrick Megonigal by the Terrestrial Ecosystem Science (TES) program of the Office of Biological and Environmental Research (BER), within the U.S. Department of Energy (DOE) Office of Science, titled Sources, Sinks and Processes Regulating Cryptic Methane Emissions from Upland Ecosystems(DE-SC0008165) and the Smithsonian Institution.

References