Long-term dynamics of soil, tree stem and ecosystem methane fluxes in a riparian forest.

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 (CH) fluxes from riparian forests is available. Here, we report continuous high-frequency soil and ecosystem (eddy-covariance; EC) measurements of CH fluxes with a quantum cascade laser absorption spectrometer for a 2.5-year period and measurements of CH 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 CH consuming 0.24 kg CH-C ha y. Soil water content is the most important determinant of soil, stem, and EC fluxes, followed by soil temperature. There were significant differences in CH fluxes between the wet and dry periods. During the wet period, 83% of CH 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, CH 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 CH sink. Our results together with past measurements of CH fluxes in other riparian forests suggest that temperate riparian forests can be long-term CH sinks.