BG3.27

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.

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.