Trees are overlooked sources and sinks in global forest methane budgets.
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.
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.
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.
BER Program Manager
Terrestrial Ecosystem Science
Smithsonian Environmental Research Center
Edgewater, MD 21037-0028
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.
Covey, K.R., and J.P. Megonigal. “Methane production and emissions in trees and forests.” New Phytologist 222(1), 35–51 (2019). [DOI:10.1111/nph.15624]
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