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 CH₄ budget. The ecosystem-scale CH₄ fluxes were analysed with regards to the CH₄ emissions of water bodies within the EC footprint. For this purpose, 17 chamber measurements were conducted on the waterbodies every two weeks for two weeks periods and EC data were divided into such subperiods accordingly. 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 results showed, 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^-2 day^-1 over the period May 2022 – May 2023. 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 sector 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 median total CH₄ flux = 156 mg CH₄ m^-2 day^-1 from April 2022 to May 2023. Ebullition was the dominant pathway of CH₄ release throughout the whole monitored period. The relation of water area/footprint area (%) of 17 floating chamber measurements ranged from 3 to 6% and fluxes coming from water bodies contributed to the EC fluxes significantly. From 17 subperiods, 15 of them were characterised by higher EC fluxes than fluxes coming from the water bodies. Two periods showed opposite result, which might indicate on additional sources of methane.

Finally, two models were applied to compare gapfilled data and answer on the question how the methane budget changes if we use different models.

The overall aim of this project is to arrive at a complete picture of all measured sinks and sources of CH₄ in the studied ecosystem.