Stems of mature European hornbeam trees are low consumers of methane and low emitters of nitrous oxide
Authors: Machacova Katerina , Soosaar Kaido , Ranniku Reti , Warlo Hannes 
Published: February 2023

Floodplain forests play an important role in the exchange of greenhouse gases - methane (CH4) and nitrous oxide (N2O) - with the atmosphere. However, due to climate change and anthropogenic activities related i.a. to the construction of retention basins, the water regime of these forests has often changed (groundwater table lowering, severe decrease in flood events). Resulting alternations of various environmental parameters can also affect the greenhouse gas exchange.Soils are well-known as substantial sources and sinks of CH4 and N2O. However, besides soils, tree stems can also emit or take up these greenhouse gases under certain conditions. But due to limited knowledge of the role of trees in forest CH4 and especially N2O fluxes under varying conditions, the calculations of the forest ecosystems CH4 and N2O exchange have mostly been limited to trace gas exchange at the level of soil&#8211;atmosphere interface, thus excluding the exchange activity of trees. This approach can lead to a severe under- or overestimation of the CH4 and N2O ecosystem fluxes.We aimed to investigate the contribution of trees to the CH4 and N2O exchange of floodplain forests in danger of gradual drying. We determined CH4 and N2O fluxes of stems of mature European hornbeam (Carpinus betulus), and adjacent soil in a temperate floodplain forest in Southern Moravia, Czech Republic, in May and June 2022, using non-steady-state chamber methods and spectroscopic gas analysis. The measurements were accompanied by a parallel determination of stem and soil CO2 exchange and numerous tree and environmental characteristics (internal heartwood concentrations of CH4, N2O and CO2; soil CH4, N2O, CO2,and O2 concentrations and water content in vertical soil profiles; soil and air temperature).Our preliminary results identified hornbeam stems as net sinks of CH4 (&#8722;6.83 &#177; 0.53 &#181;g CH4 m&#8722;2 stem area h&#8722;1, mean &#177; standard error) and very low net emitters of N2O (0.241 &#177; 0.337 &#181;g N2O m&#8722;2 h&#8722;1). The adjacent soil was a strong sink of CH4 (&#8722;41.8 &#177; 2.96 &#181;g CH4 m&#8722;2 soil area h&#8722;1) and a source of N2O (2.16 &#177; 0.95 &#181;g N2O m&#8722;2 h&#8722;1). Even though the forest is classified as a floodplain forest, the soil volumetric water content was very low (0.281 &#177; 0.012 m3 m&#8722;3) and the soil O2 concentration was similar to the ambient concentration (19.1 &#177; 0.095%; both parameters at 10 cm soil depth).The European hornbeam, a native and widely spread tree species in Central Europe, seems to contribute markedly to the CH4 uptake of the studied floodplain forest under low soil water content.&#160;&#160;AcknowledgementThis research was supported by the Ministry of Education, Youth and Sports of CR within the CzeCOS program (LM2018123) and project SustES - Adaptation strategies for sustainable ecosystem services and food security under adverse environmental conditions (CZ.02.1.01/0.0/0.0/16_019/0000797). We thank Marian Pavelka and Manuel Acosta for field station access.