Ecosystem-scale floodplain forest methane exchange

Floodplain forests play an important role in the exchange of methane (CH4) with the atmosphere. However, due to climate change and anthropogenic activities, main factors driving this exchange, such as ground water table and soil temperature, are constantly changing. The studied floodplain forest in Lanžhot, Czech Republic, represents nowadays relatively dry conditions.The main aims of our study were to quantify the CH4 emission on the floodplain forest ecosystem level using the eddy covariance (EC) method, with special emphasis on environmental conditions, turbulence development and footprint, as well as to probe all potential CH4 sinks and sources within the studied ecosystem for arriving at a complete CH4budget. The ecosystem-scale CH4 fluxes were analysed with regards to the CH4 emissions of water bodies within the EC footprint. CH4 fluxes from a stream located within the footprint of the EC tower were measured using floating chambers and bubble traps. Studies were complemented by the analysis of the contribution of trees to the CH4 exchange. For this purpose, stem chambers measured CH4 fluxes on hornbeam trees, one of the main tree species at the study site and in Central Europe. Additionally, CH4 fluxes from the soil were included in the analysis to capture all potential CH4 sources and sinks within the studied ecosystem.We initially hypothesized that ecosystem-scale CH4 exchange will be negligible. Our resultsshowed, however, that the whole ecosystem is a small but constant CH4 source as we observed an average emission flux of 11.7 mg CH4 m-2day-1 over the period June to December 2021. In addition, we observed variability of the CH4 fluxes in relation to the wind direction and to u*(friction velocity, indicator for turbulence development). Further analysis shall answer on the question if more water bodies within a particular wind sectors means higher fluxes above the canopy and if higher turbulence is correlated with higher CH4 fluxes above canopy as hotspot emissions are better mixed up. The probed stream was a substantial source of CH4 with averageCH4 fluxes of 260 ± 107 mg CH4 m-2 day-1, respectively, over the period from April to December 2021. Ebullition was the dominant pathway of CH4 release throughout the whole monitored time period. Results from the stem and soil CH4 flux measurements identified hornbeam stems and soil as net sinks for CH4 (-0.025 and -0.999 mg CH4 m-2day-1, respectively). Finally, after putting all pieces together we will arrive at a holistic view of CH4dynamics within the studied floodplain forest ecosystem with the potential of transfer of knowledge to ecosystem of similar kind elsewhere.