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Nutrient-rich northern peatlands are often drained to enhance forest productivity, turning peatland soils into sinks of methane (CH4) and sources of nitrous oxide (N2O). However, further attention is needed on CH4 and N2O dynamics during the winter period to fully understand the spatio-temporal variability of fluxes. Besides soil, tree stems can also emit CH4 and N2O. However, stem contribution is not considered in most biogeochemical models. We determined the temporal dynamics of winter-time CH4 and N2O fluxes in a drained peatland forest by simultaneously measuring stem and soil fluxes and exploring the relationships between gas fluxes and soil environmental parameters. During sampling (October 2020–May 2021), gas samples from Downy Birch (Betula pubescens) and Norway Spruce (Picea abies) trees were collected from different tree heights using manual static chambers and analysed using gas chromatography. Soil CH4 and N2O concentrations were measured using an automated dynamic soil chamber system. 
Tree stems were a net source of CH4 and N2O during the winter period. The origin of stem CH4 emissions was unclear, as stem and soil CH4 fluxes had opposite flux directions, and the irregular vertical stem flux profile did not indicate a connection between stem and soil fluxes. Stem N2O emissions may have originated from the soil, as emissions decreased with increasing stem height and were driven by soil N2O emissions and environmental parameters. Soil was a net sink for CH4, largely determined by changes in soil temperature. Soil N2O dynamics were characterised by hot moments – short periods of high emissions related to changes in soil water content. Tree stem emissions offset the soil CH4 sink by 14% and added 2% to forest floor N2O emissions. Therefore, CH4 and N2O budgets that do not incorporate stem emissions can overestimate the sink strength or underestimate the total emissions of the ecosystem.

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<p>Peatland soils are considered the dominating source of methane (CH<sub>4</sub>) and nitrous oxide (N<sub>2</sub>O) to the atmosphere. However, there are high spatio-temporal uncertainties regarding the soil greenhouse gas (GHG) fluxes due to complex dynamics between the soil chemical, physical and biological variables. Although GHG fluxes from peatland soils are relatively well studied, tree stem fluxes have received far less attention and are often overlooked in GHG models and assessments. Moreover, simultaneous year-long measurements of soil and tree stem CH<sub>4</sub> and N<sub>2</sub>O fluxes in peatland forests are missing, as previous studies have primarily focused on the growing season. We aim to determine the seasonal dynamics of CH<sub>4</sub> and N<sub>2</sub>O fluxes in drained peatland forests, as drainage can lead to release of the large amounts of carbon and nitrogen stored in peat into the atmosphere as GHGs.</p><p>Our research focuses on tree stems and soil GHG fluxes in the Agali Drained Peatland Forest Research Station in Estonia, dominated by Downy Birch (<em>Betula pubescens</em>) and Norway Spruce (<em>Picea abies</em>) trees. During the weekly sampling campaigns (November 2020–December 2021), we used manual static stem chambers to collect gas samples, which were later analysed for CH<sub>4</sub> and N<sub>2</sub>O in the laboratory using Shimadzu GC-2014 gas chromatography. We measured soil CH<sub>4</sub> and N<sub>2</sub>O fluxes using an automated dynamic soil chamber system connected to a Picarro G2508 analyser.</p><p>Preliminary results show that on average, birch stem GHG fluxes were greater than spruce stem fluxes. Birch trees were a net annual source of both CH<sub>4 </sub>(0.38 ± 0.09 µg C m<sup>-2</sup> stem area h<sup>-1</sup>, mean ± SE) and N<sub>2</sub>O (0.94 ± 0.32 µg N m<sup>-2</sup> h<sup>-1</sup>). Spruce trees were a net source of CH<sub>4</sub> (0.08 ± 0.05 µg C m<sup>-2</sup> h<sup>-1</sup>) but a net sink of N<sub>2</sub>O (–0.08 ± 0.02 µg N m<sup>-2</sup> h<sup>-1</sup>). Temporal dynamics of birch stem CH<sub>4</sub> emissions were characterised by significant emission peaks in November and June. During the rest of the year smaller fluxes with fluctuations between emissions and uptake were observed. Spruce stem CH<sub>4</sub> fluxes followed a roughly similar pattern as birch fluxes. However, during the birch emission peak in June, spruce stems showed uptake of CH<sub>4</sub>. Birch stem N<sub>2</sub>O emissions remained very small for most of the year, with increased emissions in autumn months and March. Spruce stem N<sub>2</sub>O fluxes remained very low throughout the year.</p><p>Soils were a net annual sink of CH<sub>4</sub> (–6.44 ± 0.76 µg C m<sup>-2</sup> ground area h<sup>-1</sup>) and source of N<sub>2</sub>O (41.68 ± 3.15 µg N m<sup>-2</sup> h<sup>-1</sup>). CH<sub>4</sub> was taken up by the soil most of the year, however occasional emissions occurred. A substantial increase in CH<sub>4</sub> uptake was observed in June, peaking at –49.53 µg C m<sup>-2</sup> h<sup>-1</sup> at the end of July, and diminishing towards the end of summer. Hot moments – notably higher daily average emissions compared to the period average – characterised the temporal dynamics of soil N<sub>2</sub>O emissions.</p><p>Further results on soil meteorological and biogeochemical properties will help determine the possible drivers of stem and soil fluxes’ dynamics and their origin.</p>

<p>Floodplain forests play an important role in the exchange of greenhouse gases - methane (CH<sub>4</sub>) and nitrous oxide (N<sub>2</sub>O) - 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.</p> <p>Soils are well-known as substantial sources and sinks of CH<sub>4</sub> and N<sub>2</sub>O. 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 CH<sub>4</sub> and especially N<sub>2</sub>O fluxes under varying conditions, the calculations of the forest ecosystems CH<sub>4</sub> and N<sub>2</sub>O exchange have mostly been limited to trace gas exchange at the level of soil–atmosphere interface, thus excluding the exchange activity of trees. This approach can lead to a severe under- or overestimation of the CH<sub>4</sub> and N<sub>2</sub>O ecosystem fluxes.</p> <p>We aimed to investigate the contribution of trees to the CH<sub>4</sub> and N<sub>2</sub>O exchange of floodplain forests in danger of gradual drying. We determined CH<sub>4</sub> and N<sub>2</sub>O fluxes of stems of mature European hornbeam (<em>Carpinus betulus</em>), 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 CO<sub>2</sub> exchange and numerous tree and environmental characteristics (internal heartwood concentrations of CH<sub>4</sub>, N<sub>2</sub>O and CO<sub>2</sub>; soil CH<sub>4</sub>, N<sub>2</sub>O, CO<sub>2</sub>,<sub> </sub>and O<sub>2</sub> concentrations and water content in vertical soil profiles; soil and air temperature).</p> <p>Our preliminary results identified hornbeam stems as net sinks of CH<sub>4</sub> (−6.83 ± 0.53 µg CH<sub>4</sub> m<sup>−2</sup> stem area h<sup>−1</sup>, mean ± standard error) and very low net emitters of N<sub>2</sub>O (0.241 ± 0.337 µg N<sub>2</sub>O m<sup>−2</sup> h<sup>−1</sup>). The adjacent soil was a strong sink of CH<sub>4</sub> (−41.8 ± 2.96 µg CH<sub>4</sub> m<sup>−2</sup> soil area h<sup>−1</sup>) and a source of N<sub>2</sub>O (2.16 ± 0.95 µg N<sub>2</sub>O m<sup>−2</sup> h<sup>−1</sup>). Even though the forest is classified as a floodplain forest, the soil volumetric water content was very low (0.281 ± 0.012 m<sup>3</sup> m<sup>−3</sup>) and the soil O<sub>2</sub> concentration was similar to the ambient concentration (19.1 ± 0.095%; both parameters at 10 cm soil depth).</p> <p>The European hornbeam, a native and widely spread tree species in Central Europe, seems to contribute markedly to the CH<sub>4</sub> uptake of the studied floodplain forest under low soil water content.</p> <p> </p> <p> <em>Acknowledgement</em></p> <p><em>This 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.</em></p>