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

• Published in: The Science of the total environment 2022-02, Vol.809 (C), p.151723-151723, Article 151723
• Main Authors: Mander, Ülo, Krasnova, Alisa, Schindler, Thomas, Megonigal, J. Patrick, Escuer-Gatius, Jordi, Espenberg, Mikk, Machacova, Katerina, Maddison, Martin, Pärn, Jaan, Ranniku, Reti, Pihlatie, Mari, Kasak, Kuno, Niinemets, Ülo, Soosaar, Kaido
• Format: Article
• Language: English
• Subjects: Automated chambers; Carbon Dioxide - analysis; Ecosystem; Eddy covariance; Forests; Methane; Quantum cascade laser absorption spectrometer; Soil; Soil fluxes; Soil water content; Stem fluxes; Trees
• ISSN: 0048-9697; 1879-1026
• DOI: 10.1016/j.scitotenv.2021.151723

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. During the 2.5 –yr study the forest was a CH4 sink. However, in the wet period (Sep-Dec 2017) 83% of CH4 was emitted from the tree stems whereas in the dry period CH4 was consumed in the soil and stem emission was very low. [Display omitted] •Significant difference in CH4 fluxes between the wet and dry periods•Wet period: 83% of ecosystem CH4 emission moves through the tree stems•Dry period: CH4 consumed in the soil and emitted from the canopy•Strong positive correlation of soil and stem CH4 fluxes with soil water content•In a long term, riparian alder forests constitute a minor CH4 sink