Methane (CH₄) emissions from a peatland are a balance between CH₄-producing and consuming processes in the peat.Certain graminoid and other herbaceous plant species provide a pathway for CH₄ to escape from the peat profile without becoming available to CH₄-consuming microbes (eg. Riutta et al. 2020). Whether also shrubs such as Betula nana and trees such as B. pubescens that are abundant in peatlands provide such a pathway is at least partly an open question.

We measured CH₄ fluxes of B. nana shoots both at a field site on a north boreal fen and in a climate-controlled (temperature, PAR) cabinet setup. In the field, sporadic measurements were made with manual chambers. Both chamber setups enclosed one shoot of the plant. In the climate-controlled setup, flux was measured multiple times per hour with an automated chamber system, and CH₄ concentration in the soil was monitored with gas samples. PAR and temperature were monitored by the automated setup. ¹³C-labelled CH₄ was injected into the soil in the climate-controlled setup to confirm that the source of the CH₄ in the chamber was the soil and not possible aerobic CH₄ production in the shoot, nor external contamination. Potential microbial CH₄ production within B. nana is evaluated through PCR detection and sequencing of the methanogenic mcrA gene (in progress). In the climate-controlled setup, fluxes were measured similarly also from saplings of B. pubescens and B. pendula – a related species that does not grow in peatlands. A computerized tomography (CT) scan of the plants was performed to assess the presence or absence of aerenchymatic tissues, which could act as a pathway for the CH₄ in the plant.

CH₄ emission from B. nana shoots was observed both in the field and in the climate-controlled setup. No PAR dependence of the emission was found, excluding the possibility of aerobic production of CH₄ in the shoot. Further, the labelling experiment confirmed that B. nana acted as a conduit for CH₄ originating from the soil. B. pubescens showed little flux and B. pendula no flux, even though CH₄ was present in the soil. The CT scan confirmed presence of aerenchymatic tissue in the shoots B. nana and to a lesser extent in B. pubescens but not in B. pendula.

References

Riutta, T., Korrensalo, A., Laine, A.M., Laine, J., Tuittila, E.-S., 2020. Interacting effects of vegetation components and water level on methane dynamics in a boreal fen. Biogeosciences 17, 727–740.

How to cite: Koskinen, M., Kohl, L., Grönholm, T., Polvinen, T., Putkinen, A., Laiho, R., Pihlatie, M., and Mäkiranta, P.: Betula shrubs as conduits for soil methane on peatlands, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5144, https://doi.org/10.5194/egusphere-egu21-5144, 2021.

Plant shoots can emit methane (CH₄) which is produced by an unknown aerobic, non-enzymatic process within the plant. Only a few publications report shoot CH₄ fluxes outside a laboratory setting, and those of boreal trees come to contradictory results (Machacova et al., 2016; Sundqvist et al., 2012).  Resolving the CH₄ fluxes of boreal trees is needed in order to understand the role of boreal forests in the global methane budget.

We conducted shoot chamber measurements on Scots pine (Pinus sylvestris) and Norway spruce (Picea abies) between April and May 2019, to find out if the shoots of boreal conifer trees are a source of aerobic CH₄ during the early growing season. The experiment was done with potted 2-3 year old nursery saplings in a common garden experiment, to enable regular measurements over a period of six weeks. CH₄ fluxes were measured 2-3 times per day, on two days per week from seven saplings (four P. sylvestris and three P. abies, respectively). We also conducted two around the clock campaigns where we measured the saplings hourly throughout the day and night. The CH₄ and carbon dioxide (CO₂) exchange were quantified with a portable LGR online greenhouse gas analyser connected in closed loop to custom-made, transparent shoot chambers. Photosynthetically active radiation (PAR) was measured concurrently with a PP Systems EGM-4 monitor.

Our measurements show emissions of CH₄ from both tree species, ranging from 0.25 to 7.64 and -0.45 to 6.42 g^-1 needle dry weight h^-1 (inter-quartile range) from P. sylvestris and P. abies shoots, respectively. The shoot CH₄ emissions from both species correlated positively with PAR. During the around the clock measurements the emissions showed a diurnal pattern. Our experiment demonstrates that the shoots of both P. sylvestris and P. abies can be a source of CH₄ in the spring and that the source process is likely driven by solar irradiation.

References

Machacova, K., Bäck, J., Vanhatalo, A. et al. 2016. Pinus sylvestris as a missing source of nitrous oxide and methane in boreal forest. Scientific Reports, 6(September 2015), 1–8.

Sundqvist, E., Crill, P., Mlder, M. et al. 2012. Atmospheric methane removal by boreal plants. Geophysical Research Letters, 39(21), 10–15.

How to cite: Tenhovirta, S., Kohl, L., Koskinen, M., Patama, M., and Pihlatie, M.: Methane emissions from Scots pine and Norway spruce in the spring, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5189, https://doi.org/10.5194/egusphere-egu21-5189, 2021.

Peatland soils are considered the dominating source of nitrous oxide (N₂O) and methane (CH₄) to the atmosphere. However, there are high spatio-temporal uncertainties regarding the budgets of these greenhouse gases (GHG) from peatlands due to complex dynamics between the chemical, physical and biological variables occurring in the soil. GHG fluxes from peatland soils are relatively well studied, however, tree stems have received far less attention and are often overlooked in GHG models and assessments. It is necessary to study relationships between stem and soil fluxes, and their chemical, physical and biological drivers to understand the fluxes' origin.

Our ongoing project focuses on measuring GHGs from tree stems and soil in the Agali Birch Forest Research Station in Estonia, representing a drained peatland with Downy Birch (Betula pubescens) and Norway Spruce (Picea abies) trees. Twelve representative sub-sites were selected in the study area. One half consist of an adjacent set of a Downy Birch and a Norway Spruce tree with manual tree stem chambers, plus one automatic dynamic soil chamber. The remaining sub-sites are set pairs of birch trees and soil chambers. Six birch trees and all six spruce trees have stem chambers installed at 10, 80 and 170 cm above the ground to measure stem fluxes' vertical profile. Chambers on the six remaining birch trees were only installed at the lowest height. During the weekly ongoing sampling campaigns that started in October 2020, we use manual static gas extraction from rigid stem chambers to analyse hourly changes in chamber headspace concentrations of CH₄ and N₂O. The gas samples are analysed in the laboratory within two weeks of collection using gas chromatography. Automated soil chambers collect CH₄ and N₂O flux data every two hours per chamber, and a connected Picarro measuring unit analyses the gas samples in-situ.

When extrapolated, our results can help understand stem and soil GHG emissions on an ecosystem level and acknowledge the role of tree stems for local and regional GHG budgets. Within a larger research framework, these GHG flux data will be joined with detailed soil biogeochemistry and microbial dynamics to further improve process-based modelling of peatland GHG emissions. We plan to continue our measurements for one full year to understand the seasonal changes in CH₄ and N₂O emissions patterns.

How to cite: Ranniku, R., Schindler, T., Lehtme, E., Mander, Ü., Machacova, K., and Soosaar, K.: Greenhouse gas dynamics in drained peatlands: CH4 and N2O fluxes from tree stems and soil, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15441, https://doi.org/10.5194/egusphere-egu21-15441, 2021.

Methane (CH₄) has high global warming potential, and its atmospheric concentration is increasing rapidly at the present rate of 0.3% yr^-1. Forest ecosystems cover a large part of the biosphere and play a significant role in climate change. Upland forest soils are considered as important terrestrial sinks for atmospheric CH₄; however, the complex interactions between microbial processes of CH₄ production and oxidation, and environmental drivers are not well understood. Balance of CH₄ in the forest ecosystems depends on two main natural processes, i.e., anaerobic methanogenesis and aerobic methanotrophy, driven by multiple environmental factors. A forest ecosystem's ability to exchange CH₄ depends on the soil type, environmental conditions, species composition, living trees and deadwood, age and health conditions of the tree stand, and their CH₄ balance can vary between seasons and years.

In this study, we present long-term CH₄ fluxes (from 2015 to 2019) in a 60-200-year-old coniferous forest site of Scots pine (Pinus sylvestris) grown on loose sandy soil in Soontaga research station (58°01'N 26°04'E) in Estonia. The fluxes of CH₄ were measured every two weeks, using a manual static soil chamber (n = 6) and gas chromatography method. Air temperature, precipitation and humidity, and soil moisture and temperature (10 cm depth) were measured continuously. The average annual temperature and precipitation recorded were 7.3 + 1.0 °C and 54.3 + 3.9 mm, respectively.

The results showed that mature pine forest soil was an annual net sink of CH₄: −21.14 + 0.59 g ha^−‍1 yr^-1 (mean + SE). No significant difference (p < 0.05) was found between the soil CH₄ uptake and tree age. Methane uptake correlated negatively (r² = 0.61, p < 0.05) with soil temperature and showed similar seasonal dynamics being highest during the vegetation period (Apr-Oct) and lowest during the non-vegetation period (Nov-Mar). The highest CH₄ uptake (−36.93 g ha^−‍1) was observed in July 2018, the warmest and driest month during the overall period. Even though soil moisture was only weakly correlated (r² = 0.15, p < 0.05) with CH₄ uptake, the CH₄ flux was affected by precipitation. As a result of this, it is noticed that CH₄ uptake in the cold and wet conditions decreased with increasing precipitation in winter and increased with warming during warm and dry conditions in summer.

Concluded, our coniferous pine forest was sequestering CH₄ during the investigated five years. The soil CH₄ uptake could be explained by CH₄ oxidation at optimal temperature in the water-unsaturated surface soil regulating the soil's microbial activity.

How to cite: Sardar Ali, M. K., Mander, Ü., Schindler, T., Machacova, K., Rannik, K., Uri, V., and Soosaar, K.: Long-term soil methane uptake in a mature pine forest in Soontaga Forest Station, Estonia, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15979, https://doi.org/10.5194/egusphere-egu21-15979, 2021.

Ombrotrophic, naturally open peatlands are increasingly susceptible to invasion by scrub and trees due to human disturbance, N deposition and climate change. There is limited research on the effect these trees have on ecosystem functions and their removal can be costly, making decisions over best management practice challenging. The adverse growing conditions associated with many of these peatlands can result in stunted tree growth meaning that complete enclosure of a tree remains a practical possibility. In this study we aim to quantify the CH₄ and CO₂ fluxes from whole trees growing on a disturbed peatland and assess their significance relative to the fluxes between the vegetated peat surface and atmosphere. We also aim to identify if the establishment of trees impacts CH₄ and CO₂ fluxes from the vegetated peat surface, as compared to adjacent uninvaded peatland.

We have developed a removable chamber capable of enclosing whole trees of up to 3 metres high, making it suitable for use on juvenile or stunted trees. Being able to enclose an entire tree removes potential errors caused by estimating whole tree fluxes by upscaling measurements from a subsample of tree surfaces. The chamber is constructed with a transparent membrane and removable cover so that light and dark measurements can be taken. We use the chamber to take CH₄ and CO₂ flux measurements on a site with approximately 20-year-old silver birch trees (Betula pendula) of an average height of 2-3 metres. Flux measurements have been taken from the trees and ground collars at different times of year. We have also studied diurnal variation.

Our initial results have shown that the trees on our site are emitters of CH₄, although this emission is small in comparison to that produced by the rest of the habitat. The vegetated peat surface in the wooded area had lower CH₄ emission but reduced CO₂ uptake as compared to the open area. The diurnal study on one tree indicates that methane emissions increase at night. A further diurnal study is planned to explore this further. This study extends the limit on the size of vegetation that can be sampled by a manually operated flux chamber.

How to cite: Barrop, W., Anderson, R., Andersen, R., and Toet, S.: Complete Enclosure of Stunted Trees to Study Greenhouse Gas Fluxes in Birch-Invaded Peatland, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12093, https://doi.org/10.5194/egusphere-egu21-12093, 2021.

Tree stems can be a source of the greenhouse gas methane (CH₄) and locally as regionally important to the overall GHG budget. Stem emissions even hold the potential of narrowing down knowledge gap in the global methane budget. However, assessments of the global importance of stem CH₄ emissions are complicated by a lack of research and high variability between individual ecosystems. Here, we determined the contribution of emissions from stems of mature black alder (Alnus glutinosa (L.) Gaertn.) to overall CH₄ exchange in two temperate peatlands. We measured emissions from stems and soils using closed chambers in a drained and an undrained alder forest over 2 years. Furthermore, we studied the importance of alder leaves as substrate for methanogenesis in an incubation experiment. Stem CH₄ emissions at the undrained alder forest were very variable in time and only persisted for a few weeks during the year. Generally the drained alder forest did not soil nor stem CH₄ emissions. Different upscaling approaches were assessed and all approaches showed that stem CH₄ emissions contributed less than 0.3 % to the total ecosystem CH₄ budget. However, stem CH₄ seem to depend strongly on the hydrological regime and therefore vary strongly between ecosystems. Hence, every ecosystem must be consdidered attentively with respect to their stem CH₄ emissions.

How to cite: Köhn, D., Günther, A., and Jurasinski, G.: Short lived peaks of stem methane emissions in temperate black alder forest - irrelevant for ecosystem methane budgets?, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2809, https://doi.org/10.5194/egusphere-egu21-2809, 2021.

Methane (CH₄) is the second most important anthropogenic greenhouse gas (GHG) after carbon dioxide (CO₂), globally responsible for more than 20% of the additional radiative forcing since 1750. Well-drained forest soils are considered as one of the most important biological CH₄ sinks. Net CH₄ exchange in forest soils depends on the balance of two contrasting microbial processes, i.e. CH₄ production and CH₄ oxidation, which are controlled by the population and activity of methanogens and methanotrophs, respectively. Additionally, recent reports have shown that living stems and shoots of trees in forests may produce and emit substantial quantities of CH₄, which can offset CH₄ consumption by soils; potentially switching the forest from a net CH₄ sink to a net source.

Tree-emitted CH₄ in forests may result from biological production in soils which is subsequently absorbed by roots and then transported in stems and emitted from stems and leaves. However, there is also evidence that CH₄ emissions from living tree stems may be biologically produced in situ within tree stems themselves. Long-term and high-frequency measurements of stem CH₄ flux in various individuals, tree species and forest ecosystems are needed to unravel the potential underlying mechanisms and pathways of stem CH₄ exchange.

This research compares CH₄ exchange from tree stem fluxes and soil in forest stands of English oak (Quercus robur) and Japanese larch (Larix kaempferi) and tries to understand the underlying mechanism of stem CH₄ exchange in temperate forests. High-frequency measurements of tree stem CH₄ exchange across various individuals were carried out in 2020 and the results will be presented from this study to help inform forest management and how to promote this globally important forest CH₄ sink under climate change.

How to cite: Ma, R., Stockdale, J., P. McNamara, N., Morison, J., Yamulki, S., and Toet, S.: Methane exchange in forests, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4706, https://doi.org/10.5194/egusphere-egu21-4706, 2021.

Tree stems can exchange CH₄ with the atmosphere at rates that can strongly affect GHG budgets at regional scales. However, we do not know those fluxes’ sensitivity to different components of climate change, such as the increase in concentrations of atmospheric CO₂. An increase in CO₂ concentrations might result in a change of water use efficiency, reducing transpiration fluxes, which may enhance soil methanogenesis due to an associated increase in soil water content. Additionally, an increase of CO₂ could also stimulate net primary production, increasing the supply of fresh carbohydrates to the rhizosphere, and thus stimulating CH₄ production in soil anaerobic microsites which may themselves become larger or more numerous due to the additional oxygen demand placed by the fresh carbohydrate on the soil atmosphere. Given the positive relation between soil and stem CH₄ exchange processes, any increase in soil CH₄ production may result in higher stem emissions. However, that effect of soil production on stem fluxes might decrease with stem height, with lower fluxes or even CH₄ uptake at higher stem heights. In this study, we present preliminary data on spatial and temporal variability of stem CH₄ fluxes measured in mature oak trees growing under both control and elevated CO₂ concentrations (~150 ppm above atmospheric concentrations) in a FACE experiment (free air CO₂ enrichment; BIFoR-FACE). These data may be crucial for informing processed based models on how forests GHG fluxes might behave under predicted future climate conditions.

How to cite: Barba, J. and Gauci, V.: Methane Stem Fluxes Under Elevated CO2, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10079, https://doi.org/10.5194/egusphere-egu21-10079, 2021.

Tree planting has the potential to increase carbon sequestration and is used as a common management strategy on former landfill sites to improve their visual appeal and manage issues such as leachates from decomposing organic matter. Tree stems mediate methane (CH₄) emissions to the atmosphere from anaerobic soils, bypassing bacterial populations that would otherwise break down CH₄ before it is released to the atmosphere. This process has been observed in wetland forests but has yet to be measured in a landfill context. We examined whether trees emitted more CH₄ and carbon dioxide (CO₂) on a closed UK landfill site relative to a more natural, comparable control site to determine the importance of this natural phenomenon in a managed environment. CH₄ and CO₂ fluxes from tree stem and soil surfaces were measured using flux chambers and an off-axis integrated cavity output spectroscopy analyser. Temporal and seasonal variations in greenhouse gas emissions from landfill tree stems were also investigated, as well as the impact of different landfill management techniques including site closure methods and tree species planted. Analyses showed that tree stem emissions from landfill were larger than from trees in the non-landfill control site. However, there was high variability in the greenhouse gas fluxes from trees on the landfill. Findings from this investigation suggest that conditions associated with landfill construction may increase CH₄ emissions from trees planted on their surface after closure of the site. Trees planted on former landfill sites may therefore result in additional CH₄ emissions to the atmosphere.

How to cite: Fraser-McDonald, A., Boardman, C., Gladding, T., Burnley, S., and Gauci, V.: Tree stem greenhouse gas emissions from forested closed landfill sites, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2405, https://doi.org/10.5194/egusphere-egu21-2405, 2021.

Trees are known to be sources of methane (CH₄), an important greenhouse gas, into the atmosphere. However, still little is known about the seasonality of the tree stem CH₄ fluxes, particularly for the dormant season, and about the impact of environmental parameters on this gas exchange. This makes the estimation of net annual ecosystem CH₄ fluxes difficult.

We determined seasonal dynamics of CH₄ exchange of mature European beech stems (Fagus sylvatica) and of adjacent forest floor in a temperate montane forest of White Carpathians, Czech Republic, from November 2017 to December 2018. We used static chamber methods and gas chromatographic analyses. We aimed to understand the unknown role in seasonal changes of CH₄ fluxes of these forests, and the spatiotemporal variability of the tree fluxes.

The beech stems were net annual sources for atmospheric CH₄, whereas the forest floor was a predominant sink for CH₄. The stem CH₄ emissions showed high inter-individual variability and clear seasonality following the stem CO₂ efflux. The fluxes of CH₄ peaked during the vegetation season, and remained low but significant to the annual totals during winter dormancy. By contrast, the forest floor CH₄ uptake followed an opposite flux trend with low CH₄ uptake detected in the winter dormant season and elevated CH₄ uptake during the vegetation season. Based on our preliminary analyses, the detected high spatial variability in stem CH₄ emissions can be explained neither by the CH₄ exchange at the forest floor level, nor by soil CH₄ concentrations, soil water content and soil temperature, all measured in vertical soil profiles close to the studied trees.

European beech trees, native and widely spread species of Central Europe, seem to markedly contribute to the seasonal dynamics of the ecosystem CH₄ exchange, and their CH₄ fluxes should be included into forest greenhouse gas emission inventories.

Acknowledgement

This research was supported by the Czech Science Foundation (17-18112Y), National Programme for Sustainability I (LO1415), CzeCOS (LM2015061), and 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 Libor Borák and Leszek Dariusz Laptaszyński for their technical and field support.

How to cite: Machacova, K., Warlo, H., Svobodová, K., Agyei, T., Uchytilová, T., Horáček, P., and Lang, F.: European beech is a net annual source of methane (CH4), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4335, https://doi.org/10.5194/egusphere-egu21-4335, 2021.

Forest soils in Central Europe received massive atmospheric deposition of SO₂ and NO_x during the second half of the 20^th century. The resulting fast acidification of the soils was accompanied by massive forest dieback and problematic nutrient imbalances at some sites. After the emissions of SO₂ have been reduced in the 80´s and 90´s, the situation of acidic deposition has been gradually improving. Yet, the deposition of N compounds remains high and still has an impact on forest ecosystems. Natural soil development and “regeneration” is a slow process, which is why other options were investigated to recover heavily affected forest soils. A well-known means to mitigate the observed effects of the anthropogenic acidification surges is liming, i.e. the application of minerals such as CaCO₃ and CaMg(CO₃)₂ that are able to buffer strong acids. Liming directly affects soil pH which is a “master variable” of the soil. Soil pH, and thus, liming, affects and interacts with many soil processes from mineralization of organic matter and humification, to (de-) stabilization soil structure, nutrient availability and mobility, plant growth and more.

Several study sites were established in the 1980 in Baden-Wuerttemberg to study long term effects of liming on soil structure and forest growth. At all sites a “control” plot and a “limed” plot were established next to each other. The limed plots were treated with approx. 3 t ha^-1 of CaCO₃ in the 1980´s and 6 t ha^-1 of Ca/MgCO₃ in 2003. Here we report on results from three sites (Bad Waldsee, Hospital, Herzogenweiler) with Spruce stands (70-110 years), where long term effects of liming on the physical soil structure and soil gas profiles (2017-2019) were studied (Jansone et al., 2020). Liming resulted in a reduction of the thickness of the humus layer and a blurring of the previously clearly separated boundary between the mineral soil and the humus layer. Even though total pore space in the top soil was slightly reduced at the limed plots, soil gas diffusivity was higher at a given air-filled pore-space. This indicates a better connectivity in the air-filled pores, that means more larger pores connecting the atmosphere at the soil surface and the mineral soil. Soil CO₂ concentrations showed clear seasonal patterns and a typical increase with depth. Higher CO₂ concentrations tend to be found in the un-limed control plots. Soil CH₄ concentrations at the soil–humus interface were closer to atmospheric concentrations in the limed plots compared to the control plots. This can be interpreted as an effect of the decrease in the thickness of the humus layer and the increase in the soil gas diffusivity (better aeration) or in a reduced activity of the methanotrophic community.

Acknowledgement

This research was financially supported by Bundesministerium für Ernährung und Landwirtschaft (BMEL), grant number 28W-B-4-075-02 (2018–2021).

Literature

Jansone, L., von Wilpert, K. and Hartmann, P., 2020. Natural Recovery and Liming Effects in Acidified Forest Soils in SW-Germany. Soil Systems, 4(38): 1-35.

How to cite: Maier, M., Gartiser, V., Lang, V., Habel, R., Jansone, L., and Hartmann, P.: Effects of liming on soil structure and GHG fluxes at three spruce sites in SW Germany, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4223, https://doi.org/10.5194/egusphere-egu21-4223, 2021.

Knowledge regarding processes, pathways and mechanisms that may moderate methane (CH₄) sink/source behaviour along the sediment - tree stem - atmosphere continuum remains incomplete. Here, we applied stable isotope analysis (δ¹³C-CH₄) to gain insights into axial CH₄ transport and oxidation in two common and globally distributed subtropical lowland forest species (Melaleuca quinquenervia and Casuarina glauca). We found consistent trends in CH₄ flux (decreasing with height) and δ¹³C-CH₄ enrichment (increasing with height) in relation to stem height from the ground. The average lower tree stem (0-40 cm) δ¹³C-CH₄ of M. quinquenervia and C. glauca flooded forests (-53.96 ‰ and -65.89 ‰) were similar to adjacent flooded sediment CH₄ ebullition (-52.87 ‰ and -62.98 ‰), suggesting that CH₄ is produced mainly via sedimentary sources. Upper stems (81-200 cm) displayed distinct δ¹³C-CH₄ enrichment (M. quinquenervia -44.6 ‰ and C. glauca -46.5 ‰ respectively) compared to lower stems. Coupled 3D photogrammetry and novel 3D measurements on M. quinquenervia revealed that distinct hotspots of CH₄ flux and isotopic fractionation were likely due to bark anomalies where preferential pathways of gas efflux were likely enhanced. By applying a  fractionation factor (derived from previous lab based tree stem bark experiments), diel experiments revealed greater δ¹³C-CH₄ enrichment and higher oxidation rates in the afternoon relative to the morning. Overall, we estimate CH₄ oxidation rates between the lower to upper stems across both species ranged from 1 to 69 % (average 33.1 ± 3.4 %), representing a substantial tree-associated CH₄ sink occurring during axial transport.

How to cite: Jeffrey, L., Maher, D., Tait, D., Reading, M., Chiri, E., Greening, C., and Johnston, S.: Isotopic evidence for axial tree stem methane oxidation within subtropical lowland forests, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1923, https://doi.org/10.5194/egusphere-egu21-1923, 2021.

Tropical peatlands are a complex ecosystem with poorly understood biogeochemical regimes. An immense peat carbon stock and waterlogged-anaerobic conditions may possibly favor methane formation in this ecosystem. Methane is released to the atmosphere either from soil/water surface or through vegetation (both herbaceous plant and tree). Using the conventional flux chamber method, assessing spatiotemporal variability and vegetation-mediated methane emissions remains a practical challenge for scientists. Consequently, research related to ecosystem-scale methane exchange remains limited. Yet, published data display a large range of methane emission estimates and, hence, highlight a knowledge gap in our science on tropical peatland methane cycling.

In this context, we set out to measure the net ecosystem methane exchange (NEE-CH₄) from an unmanaged degraded peatland in the east coast of Sumatra, Indonesia. The measurements were conducted using the eddy covariance system, composed of a 3D sonic anemometer coupled with a LI-7700 open-path methane analyzer, above the vegetation canopy at 41 m tall tower for over 4 years period (October 2016-September 2020). Therefore, the measurements incorporated all existing methane sources and sinks within the flux footprint, i.e. soil surface, trunk of living tree, vascular plant, and water surface.

Our measurements indicate that unmanaged degraded tropical peatland emitted 54±12 kg CH₄ ha^-1 year^-1 to the atmosphere. The magnitude of daytime NEE-CH₄ were up to six times larger than those during the nighttime. This cautions that sampling bias (e.g. only daytime measurements) can overestimate the daily NEE-CH₄. The diurnal variation in NEE-CH₄ was correlated with associated changes in the canopy conductance to water vapor. Therefore, it was attributed to the vegetative transport of dissolved methane via transpiration. There was no clear relationship between NEE-CH₄ and soil temperature, while it decreased exponentially with declining groundwater level. Low groundwater level enhances methane oxidation in the upper oxic peat layer. Further, low groundwater level might relocate methane production below the root zone, resulting in insufficient methane in the root zone to be taken and transported to the atmosphere.

Our results, which are among the first eddy covariance exchange data reported for any tropical peatland, should help to reduce uncertainty in the estimation of methane emissions from a globally important ecosystem, and to better understand how land-use changes affect methane emissions.

How to cite: Deshmukh, C. S., Desai, A. R., Evans, C. D., Page, S. E., Susanto, A. P., Nardi, N., Nurholis, N., Hendrizal, H. M., Kurnianto, S., Suardiwerianto, Y., Asyhari, A., Sabiham, S., Agus, F., and Astiani, D.: Methane emissions from an unmanaged degraded tropical peatland: Interacting influences of plant-processes and groundwater level, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8119, https://doi.org/10.5194/egusphere-egu21-8119, 2021.

Airborne measurements of methane (CH₄) were recorded over three major wetland areas in Zambia in February 2019 during the MOYA (Methane Observations and Yearly Assessments) ZWAMPS field campaign. Enhancements of up to 600 ppb CH₄ were measured over the Bangweulu (11°36’ S, 30°05’ E), Kafue (15°43’ S, 27°17’ E), and Lukanga (14°29’ S, 27°47’ E) wetlands. Three independent methods were used to quantify methane emission fluxes; aircraft mass balance, aircraft eddy covariance, and atmospheric inversion modelling. Results yielded methane emission fluxes of 10-20 mg CH₄ m^-2 hr^-1, which were up to an order of magnitude greater than the emission fluxes simulated by various wetland process models (WetCHARTs ensemble and LPX-Bern). Independent column CH₄ observations from the TROPOMI instrument were used to verify these fluxes, and investigate their applicability for timescales longer than the duration of the MOYA flight campaign.

How to cite: Shaw, J., Allen, G., Barker, P., Pitt, J., Lee, J., Bauguitte, S., Pasternak, D., Bower, K., Daly, M., Lunt, M., Ganesan, A., Vaughan, A., Fisher, R., France, J., Lowry, D., Palmer, P., Bateson, P., and Nisbet, E.: Methane fluxes from Zambian tropical wetlands, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14938, https://doi.org/10.5194/egusphere-egu21-14938, 2021.

Tropical forests are the most productive terrestrial ecosystems, global centres of biodiversity and important participants in the global carbon and water cycles. The Amazon, which is the most extensive tropical forest, can contain more than 600 trees (diameter at breast height above 10 cm) and up to 200 tree species in only one hectare of forest. In upland forest, tropical soils are known to be a methane (CH₄) sink and a weak source of nitrous oxide (N₂O), which are both major greenhouse gases (GHG). Most of researches on GHG fluxes have been conducted on the soil compartment but recent works reported that tree stems of some tropical forests can be a substantial source of CH₄ and, a to lesser extend of N₂O. Tropical tree stems can act as conduits of soil-produced GHG but biophysical mechanisms controlling GHG fluxes and differences among tree species are not yet fully understood.

In order to quantify CH₄ and N₂O fluxes of different tropical tree species, we took gas samples in 101 mature tree stems of twelve species with the manual chamber technique during the wet season 2020, in a French Guiana forest. Tree species were selected because of their abundance and their habitat preference. We chose trees belonging to two contrasted forest habitats, the hill-top and hill-bottom, which are respectively characterized by aerobic conditions and seasonal anaerobic conditions. Simultaneously with sampling GHG, we measured bark moisture and tree diameter. Four tree species were found in both habitats whereas the eight others were only present in one of these two habitats.

Among the 101 tree stems, 78.6% were net sources of CH₄ with a greater proportion in hill-bottom than hill-top. Overall, stem CH₄ fluxes were significantly and positively correlated with the wood density (χ² = 28.0; p < 0.01; N = 75) but neither with the habitat, bark moisture or tree size. We found a significant effect of the tree species on stem CH₄ fluxes (F = 3.7, p < 0.001) but no interactions between the tree species and habitats.

Among 43.0% of the stem N₂O fluxes that were different from zero, half were from trees that were net sources of N₂O mainly located in hill-top. Stem N₂O fluxes are not significantly correlated with habitat, as also with the tree size, wood density or bark moisture. Unlike stem CH₄ fluxes, tree species did not significantly influence stem N₂O fluxes.

Our study revealed that, in tropical forest, spatial variations in GHG fluxes would not only depend on soil water conditions, but also on tree species. Specific tree traits such as the wood density can favour stem CH₄ emissions by providing more or less effective pore space for CH₄ diffusion but seems to have a limited influence on stem N₂O fluxes maybe because of the lower diffusive and ebullitive transport of N₂O compared to CH₄. Further investigation linking tree species traits and tree GHG fluxes are, however, necessary to elucidate the processes and mechanisms behind tree CH₄ and N₂O exchanges.

How to cite: Daniel, W., Stahl, C., Burban, B., Goret, J.-Y., Cazal, J., Janssens, I., and Bréchet, L.: Inter-specific variations in tree stem methane and nitrous oxide exchanges in a tropical rainforest, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13260, https://doi.org/10.5194/egusphere-egu21-13260, 2021.