The carbon (C) budgets of riparian forests are sensitive to climatic variability. Therefore, riparian forests are hot spots of C cycling in landscapes. Only a limited number of studies on continuous measurements of methane (CH4) fluxes from riparian forests is available. Here, we report continuous high-frequency soil and ecosystem (eddy-covariance; EC) measurements of CH4 fluxes with a quantum cascade laser absorption spectrometer for a 2.5-year period and measurements of CH4 fluxes from tree stems using manual chambers for a 1.5 year period from a temperate riparian Alnus incana forest. The results demonstrate that the riparian forest is a minor net annual sink of CH4 consuming 0.24 kg CH4-C ha-1 y-1. Soil water content is the most important determinant of soil, stem, and EC fluxes, followed by soil temperature. There were significant differences in CH4 fluxes between the wet and dry periods. During the wet period, 83% of CH4 was emitted from the tree stems while the ecosystem-level emission was equal to the sum of soil and stem emissions. During the dry period, CH4 was substantially consumed in the soil whereas stem emissions were very low. A significant difference between the EC fluxes and the sum of soil and stem fluxes during the dry period is most likely caused by emission from the canopy whereas at the ecosystem level the forest was a clear CH4 sink. Our results together with past measurements of CH4 fluxes in other riparian forests suggest that temperate riparian forests can be long-term CH4 sinks.

Department of Agricultural Sciences Environmental Soil Sciences University of Helsinki Latokartanonkaari 7 00014 Helsinki Finland; Institute for Atmospheric and Earth System Research University of Helsinki Viikinkaari 2a 00014 Helsinki Finland

Department of Geography Institute of Ecology and Earth Sciences University of Tartu Vanemuise 46 51014 Tartu Estonia

Department of Geography Institute of Ecology and Earth Sciences University of Tartu Vanemuise 46 51014 Tartu Estonia; Global Change Research Institute of the Czech Academy of Sciences Department of Ecosystem Trace Gas Exchange Belidla 986 4a 603 00 Brno Czech Republic

Department of Geography Institute of Ecology and Earth Sciences University of Tartu Vanemuise 46 51014 Tartu Estonia; Institute of Forestry and Rural Engineering Estonian University of Life Sciences Kreutzwaldi 1 51006 Tartu Estonia

Institute of Agricultural and Environmental Sciences Estonian University of Life Sciences Kreutzwaldi 5 51006 Tartu Estonia

Smithsonian Environmental Institute 647 Contees Wharf Road Edgewater MD 21037 0028 USA

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