Forests cover about 4 billion ha globally. They are important regulators of carbon dioxide (CO₂) fluxes, whereas the comprehensive understanding of their overall greenhouse gas (GHG) budgets, especially for methane (CH₄) and nitrous oxide (N₂O), are still largely unknown.

Wetland forest soils are commonly recognized as emitters of CH₄, whereas upland forest soils tend to consume CH₄. However, several studies demonstrate that trees can emit a large amount of CH₄ especially from tree stems and substantial amounts also from canopies through poorly studied and partly unidentified aerobic processes. Moreover, tree stems can have substantial concentrations of CH₄ inside, which can originate from soil or be produced by methanogens within the wood, while canopy CH₄ emissions are mostly abiotic and driven by light and temperature. Thus, forest vegetation can be a significant CH₄ source.

Various soil microbiological, chemical and physical properties influence N₂O fluxes in forests. In general, N₂O emissions from tropical wetland forest soils are significantly higher than those from tropical upland forests, temperate and boreal forests. High nitrogen (N) availability, coupled with high moisture content, makes tropical peatland soils especially likely to emit N₂O. Similarly, forests on drained N-rich peatland soils in temperate and boreal areas can be significant N₂O sources. In temperate zone, a considerable part of such emissions appears in winter.

Understanding spatial and temporal dynamics of GHG emissions is crucial for adequate modelling and mitigation of emissions in forests. In comparison with CO₂ fluxes, which are clearly temperature dependent, temporal and spatial variation of soil, tree stem and canopy CH₄ and N₂O emissions is more complex and poorly studied. Soil N₂O emissions in wetland and upland forests are mainly determined by soil moisture (soil oxygen concentration), and N₂O shows bell-shaped (unimodal) dependence on soil water content. In the wet periods, stem flux of CH₄ can be the main source for ecosystem exchange, whereas in the dry periods, emission from canopy adds to the total fluxes from soil and stems. N₂O fluxes from the soil and stems are normally low during the dry periods and peak during the wet periods and the freeze-thaw cycles.

Only a few examples are available on ecosystem-level CH₄ and N₂O budgets (fluxes from the soil, tree stems and shoots + eddy covariance (EC) measurements above the canopy). Nevertheless, estimation of the GHG balance in different forest ecosystems under various environmental conditions is essential for understanding their impact on the Earth’s climate.

In this presentation, we will bring results from ecosystem-level CH₄ and N₂O flux studies in forests growing on both organic and mineral soils in temperate and tropical zones.

How to cite: Mander, Ü., Ranniku, R., Schindler, T., Espenberg, M., Escuer-Gatius, J., Machacova, K., Pärn, J., Masta, M., Kazmi, F. A., Melling, L., Fachin Malaverri, L. M., Pihlatie, M., Kuusemets, L., Kasak, K., and Soosaar, K.: Hot spots and hot moments of methane and nitrous oxide fluxes in forests: from soil to ecosystem, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3653, https://doi.org/10.5194/egusphere-egu24-3653, 2024.

Release of CH₄ from shoots remains the least understood and most enigmatic process of tree-mediated CH₄ fluxes. While stem emissions of trees derive from transported or local, biotic pathways, CH₄ emissions from shoots likely originate dominantly from abiotic, aerobic production within the canopies. The estimates of the global source strength of aerobic CH₄ emissions suffer from large uncertainties, due to insufficient understanding of the source processes.

In this contribution, we show the environmental drivers, temporal patterns, and physiological determinants of the aerobic CH₄ emissions from the shoots of Scots pines and discuss the contribution of aerobic canopy emissions to the boreal forest CH₄ cycles. We present shoot-level CH₄ fluxes from saplings and mature Scots pine trees, measured in various settings, outdoors and in the greenhouse. We used chamber enclosure methods with online greenhouse gas analysers in both manual and automated measurement settings. For the automated measurements we built a custom measurement system that allowed continuous measurements of greenhouse gas fluxes.

The results from CH₄ flux measurements under different light sources indicate that aerobic CH₄ emissions are to the most part determined by the intensity and spectral composition of light, and that the emissions are most prominent under direct solar radiation. Hence, the diurnal variations exhibited by these emissions are associated with the diurnal cycle of sunlight, but also vary depending on the cloud conditions. By exposing Scots pine saplings to drought, we further distinguished that the light-driven CH₄ emissions from shoots are not a byproduct of photosynthesis-related biochemical reactions. Rather, these emissions result from abiotic thermal and photodegradation of plant compounds.

In ambient conditions, we show median aerobic CH₄ emissions of 5.41 ng CH₄ g^-1 DW h^-1under direct sunlight and 2.52 ng CH₄ g^-1 DW h^-1 during variable cloudiness. These emissions are 1-2 % of the emission factor used in most of the global upscale estimates of aerobic CH₄ emissions from vegetation. Therefore, Scots pine canopies in boreal climates are likely a CH₄ source of only minor importance on a global scale. These emissions may, however, decrease the sink strength of the boreal upland forest soils: our conservative estimate is that the canopy emissions of CH₄ may decrease this soil sink strength by 2.1 – 4.6 %. This estimate may yet underestimate the significance of canopy fluxes on the ecosystem scale due to the high spatiotemporal variation of both the canopy CH₄ emissions and the CH₄ uptake rates of boreal upland soils. To further refine the estimates of the source strength of aerobic CH₄ emissions of tree canopies it is, therefore, important to gain more data of shoot-level CH₄ fluxes from field measurements.

How to cite: Pihlatie, M., Tenhovirta, S., Kohl, L., and Koskinen, M.: Resolving the aerobic methane emissions from Scots pine shoots, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-22027, https://doi.org/10.5194/egusphere-egu24-22027, 2024.

The cycling of greenhouse gases in forest ecosystems is significantly influenced by tree stems. Yet, little is known about the variability and drivers of stem-atmosphere greenhouse gas fluxes, especially in managed boreal riparian ecosystems where environmental conditions vary substantially at small spatial scales and throughout the year. Here, we report magnitudes and drivers of tree stem-atmosphere fluxes of carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O) in a riparian buffer zone of a Swedish boreal forest that has been subject to recent forest clearcutting and historic ditching. For two full years, we conducted CO₂ and CH₄ flux chamber measurements on a monthly basis in 14 spruce trees (Picea abies) and 14 birch trees (Betula pendula) that grew between one and fifteen meters from a headwater stream. We also performed N₂O flux measurements during three occasions. All trees were net emitters of CO₂ and CH₄ over the majority of the year, while N₂O fluxes were close to zero. CO₂ fluxes correlated strongly and positively with air temperature and followed distinct seasonal cycles peaking in summer. CH₄ fluxes correlated modestly with air temperature and solar radiation and peaked in late winter and summer. Trees with larger stem diameter released more CO₂ and less CH₄, and trees that were nearer the stream released more CO₂ and CH₄. The CO₂ and CH₄ fluxes did not differ between spruce and birch in general, but correlations of CO₂ fluxes with stem diameter and distance to stream differed between the tree species. The absence of distinct vertical trends in the CO₂ and CH₄ fluxes along the stems and their lack of correlation with groundwater levels and groundwater greenhouse gas concentrations point to tree internal production as the primary source of the tree stem gas emissions. Upscaled to the ecosystem, the tree stem CO₂, CH₄ and N₂O emissions represented 52% of the forest floor CO₂ emissions and 2.5% and 11.3% of the forest floor CH₄  and N₂O uptake, respectively, during the snow-free season. The snow cover season contributed 15% and 35% to annual tree stem CO₂ and CH₄ emissions, respectively. In contrast to other riparian zone studies, the stem gas fluxes in our study generally exhibited characteristics of an upland rather than a wetland ecosystem, likely because of historical ditching and subsequent groundwater level declines.

How to cite: Klaus, M., Öquist, M., and Macháčová, K.: Tree stem-atmosphere greenhouse gas fluxes in a boreal riparian forest, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6221, https://doi.org/10.5194/egusphere-egu24-6221, 2024.

Tree stems are known to emit greenhouse gases CH₄, CO₂ and N₂O to the atmosphere but the processes and drivers behind these fluxes are still contested. Soil water is taken up by tree roots and moves up the xylem due to a negative pressure gradient caused by transpiration through the leaves. Consequently, dissolved gases in the soil water move up the stem and are potentially diffused to the atmosphere through the bark. Periods of soil freeze-thaw in the spring are crucial hot-moments of GHG release from the soil, as well as stems. As birch trees go through a sap running period between the thawing of the soil and bud break, they provide an opportunity to study stem GHG fluxes during the peak time of emissions, together with the concentrations of dissolved gases in the birch sap.

We quantified the fluxes of CH₄, CO₂ and N₂O from Downy birch (Betula pubescens), as well as Norway spruce (Picea abies) for comparison, in a temperate nutrient-rich drained peatland forest in April and May 2023. In addition, we studied the relationship between birch stem fluxes and dissolved gas concentrations inside the xylem sap. Stem fluxes were determined using static chambers attached to the tree stems and automatic LI-COR gas analysers. Dissolved gas samples were extracted from the collected birch sap and soil water after water-atmosphere equilibration, and were analysed in the lab using gas-chromatography. In addition, we analysed the relationships between the chemical and microbiological composition of the soil and soil and stem GHG fluxes.

Birch stem CH₄, CO₂ and N₂O fluxes peaked in the end of April, following the the temporal trend of soil and air temperature, with higher fluxes during warmer days, likely related to increased microbial activity in the soil. Dissolved CH₄ concentrations in the birch sap peaked with a delay in relation to peak stem emissions, indicating that xylem sap flow rate needs to be studied to comprehend the water dynamics inside the stem. Relationships between stem fluxes and dissolved gas concentrations were strongest at the bottom part of the tree. A more detailed analysis together with examination of the underlying soil chemistry and microbiology will be presented to further explain the processes behind soil and tree stem GHG flux dynamics.

How to cite: Ranniku, R., Truupõld, J., Espenberg, M., Escuer-Gatius, J., Ali Kazmi, F., Mander, Ü., and Soosaar, K.: Greenhouse gas fluxes from Downy Birch stems during the spring sap-run period and their dependence on dissolved gas concentrations in xylem sap, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14147, https://doi.org/10.5194/egusphere-egu24-14147, 2024.

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. 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 June to December 2021. 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 sectors 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 average
CH4 fluxes of 260 ± 107 mg CH4 m-2 day-1, respectively, over the period from April to 
December 2021. Ebullition was the dominant pathway of CH4 release throughout the whole 
monitored time period. Results from the stem and soil CH4 flux measurements identified 
hornbeam stems and soil as net sinks for CH4 (-0.025 and -0.999 mg CH4 m-2
day-1, respectively). Finally, after putting all pieces together we will arrive at a holistic view of CH4
dynamics within the studied floodplain forest ecosystem with the potential of transfer of 
knowledge to ecosystem of similar kind elsewhere.

How to cite: Kowalska, N., Jocher, G., Bednařík, A., Warlo, H., Soosaar, K., and Macháčová, K.: Ecosystem-scale floodplain forest methane exchange, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7519, https://doi.org/10.5194/egusphere-egu24-7519, 2024.

Many national governments, organisations and environmental groups have pledged to plant trees in an effort to increase carbon sequestration and mitigate climate change. Tree planting is commonly used as a restoration strategy for former landfill sites, and it is likely that many urban and urban-fringe areas, including closed landfills, will continue to be prioritised for tree planting in the coming years. Trees growing in natural and managed environments have the capacity to act as conduits for the transport of methane (CH₄) produced belowground to the atmosphere. This process has also been observed in natural ecosystems for nitrous oxide (N₂O) and we examined whether trees growing on closed landfills also mediate N₂O emissions to the atmosphere. We investigated whether trees on a closed UK landfill site emitted more N₂O than those on a comparable natural site. Measurements were made from stem and soil surfaces over a four-month period using flux chambers and Gas Chromatography. Results were then scaled up and the contributions of N₂O stem fluxes to the total surface fluxes in different environments were compared. Analyses showed that stem and soil N₂O fluxes from landfill were larger than from trees on the comparable non-landfill site. Tree stem N₂O emissions on the former landfill also showed seasonal patterns and decreased with higher sampling positions above ground level. Findings indicated that tree stem N₂O emissions accounted for less than 1% of the estimated total landfill surface flux, which was comparable to findings from a mesocosm study, but lower than estimates of the total N₂O ecosystem flux in dry and flooded boreal forests (8% and 18%, respectively). Overall, this investigation suggested that trees planted on closed landfill sites may result in additional N₂O emissions to the atmosphere, although the tree stem contribution to the total surface flux on the former landfill was a lower magnitude than that of fluxes previously reported from natural forested ecosystems.

How to cite: Fraser-McDonald, A., Boardman, C., Gladding, T., Burnley, S., and Gauci, V.: Nitrous oxide (N2O) emissions from a forested closed landfill site, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1443, https://doi.org/10.5194/egusphere-egu24-1443, 2024.

The contribution of trees to the wetland methane (CH₄) budget remains highly uncertain. The water table level is an essential driver for stem CH₄ emissions, which vary across seasons and soil hydrological conditions. Exceptionally dry conditions are increasingly affecting forests because of climate change, and wetland trees can potentially switch from CH₄ sources to sinks. CH₄ production and oxidation potentially occur in the oxic/anoxic microsites within wetland tree stems, as in soil, balancing the net stem CH₄ emissions to the atmosphere. Yet, the microbial ecology behind these processes is still vastly unexplored, and understanding the role of microbial ecology is essential to predict stem CH₄ emission patterns.

Our study focused on characterising stem CH₄ fluxes using semi-rigid static chambers and assessing CH₄ oxidation and production activities through gas-enriched incubations in two forested wetland ecosystems: a temperate wetland in Flitwick Moor (UK) and tropical peat swamp forests in the Sebangau Forest (Kalimantan, Indonesia), both experiencing lower water table levels than previous years during the same period. Targeted tree species were measured at multiple height intervals and were Alnus glutinosa and Betula pubescens in Flitwick Moor and Shorea balangeran and Xylopia fusca in the Sebangau Forest, the same tree species that were investigated in earlier studies at these sites. DNA analysis from bark, wood, and soil involved two-step PCR and sequencing targeting the 16S rRNA gene, complemented by whole shotgun metagenomics (WGS) to explore the microbial composition and CH₄-cycling microorganisms.

Results from Flitwick Moor and the Sebangau Forest showed significantly reduced stem CH₄ emissions (<50 µg m^-2 hr^-1) compared to earlier studies, with trees adopting an upland-like behaviour, displaying heterogenous fluxes with no clear axial pattern or relation to wood properties, as well as CH₄ uptake. There was evidence of CH₄ oxidation in trees of both ecosystems in the range of 7-47 µg m^-3 hr^-1. The aerobic and facultative anaerobic bacteria population dominated in tree tissues, and the same number of methanotrophic genera were present in soil and trees, suggesting that microbial groups were recruited from the soil. In A. glutinosa tree tissues a significant positive relation existed between the CH₄ oxidising bacteria relative abundance and the oxidation activity. The methanotrophic fraction represented up to 5% of the bacteria in wood, confirming the hypothesis that methanotrophs are ubiquitous in trees of different ecosystems.

CH₄-cycling microorganisms are likely to adapt to a soilborne-CH₄ gradient up the tree stems; the reduced stem CH₄ fluxes in this study resulted from dry soil conditions and potentially from microbial oxidation inside the stem. Conversely, the small proportion of CH₄-cycling microorganisms compared to other microbial groups likely reflected the reduced stem CH₄ fluxes along the soil-tree continuum. The ratio of CH₄-cycling microorganisms might vary across seasons and different hydrological conditions; further long-term studies in forested wetlands will help elucidate the interplay between CH₄-cycling microorganisms and stem CH₄ fluxes and the importance of trees in balancing CH₄ emissions in potentially drier future scenarios.

How to cite: Gomez, C., Pangala, S., Gowing, D., Olsson-Francis, K., Page, S., and Gauci, V.: Wetland trees are a potential methane sink during dry soil conditions, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14034, https://doi.org/10.5194/egusphere-egu24-14034, 2024.

Soils, particularly in upland forests, are the largest biological sink for atmospheric methane (CH₄), providing a valuable ecosystem service. Rubber plantations have continually expanded in Southeast Asia, and it is known that converting forests to rubber plantations reduces soil CH₄ uptake. However, the effect of management practices, and in particular fertilization, on the methane balance of a rubber plantation has not yet been studied. Rubber plantations cover almost 10% of the country's surface area and almost all rubber plantations are fertilized, two thirds of them intensively or very intensively.

We measured net soil CH₄ fluxes over more than a year in a 9-ha experimental rubber plantation with four levels of fertilizer application. We observed a strong and significant reduction of net soil CH₄ uptake with increasing fertilisation, which was not explained by differences in CH₄ diffusion related to soil water content. Fertilisation not only decreased the methanotrophic activity but also stimulated methanogenic activities probably related to an increase in the availability of nitrogen and labile carbon substrates.

Our results show that intensive fertilization turned soil from methane sink to source, particularly during the rainy season. Given the areas cultivated with rubber trees in Thailand and more widely in South-East Asia, a transition towards rational fertilization of plantations would have a significant positive effect on national reporting greenhouse gas inventories.

How to cite: Epron, D., Chotiphan, R., Duangngam, O., Wang, Z., Shibata, M., Paul, S. K., Kasemsap, P., and Sajjaphan, K.: Fertilization turns a rubber plantation from sink to methane source, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3743, https://doi.org/10.5194/egusphere-egu24-3743, 2024.

Trees are recently understood to emit large quantities of CH₄ through their stems, particularly in tropical wetland environments. There are still large uncertainties of the processes driving tree CH₄ emissions, however, the primary mechanism is thought to be through transfer of CH₄ produced in soil into tree biomass and then to the atmosphere. Another possible mechanism is via anaerobic decomposition of rotting tree biomass in stems. In the Brazilian Amazon, very little is known about sources and variability of tree CH₄ emissions and how they may vary across different flooded regions.

Across regions of the Amazon we aim to understand the variation of CH₄ emissions from trees. These regions are characterised as white water flooded forest (Várzea region) and black water flooded forest (Igapo region). Using two tree species of similar ages across two regions, we measured tree CH₄ emissions and surrounding porewater CH₄ concentrations for two flooded seasons. Across all study locations and tree species we found large but variable net CH₄ emissions ranging from 0.01 to 84 mg m^-2 hr^-1. These variations in emissions are significantly influenced by the tree species. Furthermore, we measured significantly different fluxes when measuring the same tree species across two regions, suggesting there could be vast alterations in flux when attempting to measure emissions across the Amazon region.

Our work also revealed that CH₄ emission was highest at the base of trees (30 cm) compared to measurements made higher up the stem (70 cm). This is consistent with radial diffusion of soil derived CH₄ up the stem and also stongly suggests the source of CH₄ is soil derived. Porewater concentrations of CH₄ throughout the soil column further supports tree CH₄ emission deriving from soil.

Furthermore, we analysed the stable isotopic carbon values of emitted CH₄ and demonstrate that this vertical reduction in emitted CH₄ is also in part a product of biological oxidation of CH₄ by methanotrophic bacteria located in woody material. The isotopic profile varied between two tree species and at the base of the tree compared with higher up the stem. We also noted individual tree species had isotopic variability across the two sites.

These results show significant CH₄ emissions from trees to atmosphere in the Amazon. By using common tree species of similar ages we demonstrate that the strength and variability of these emissions are strongly influenced by site specific variables that require further investigation.

How to cite: Blincow, H., McNaramara, N., Hoyt, A., Gomez, C., Elias, D., Lamb, J., De Sousa, R., Gris, D., Pequeno Reis, L., and Pangala, S.: Variability of tree methane emissions across regions of the Amazon rainforest, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17619, https://doi.org/10.5194/egusphere-egu24-17619, 2024.

Methane (CH₄) is a greenhouse gas with 35 times the warming potential of carbon dioxide. In the last 15 years, the concentration of atmospheric CH₄ has sharply increased and the signature of carbon stable isotope in CH₄ has become more negative suggesting biotic sources, such as tropical wetlands, might be partly responsible of the current atmospheric methane budget. Floodplains in the Brazilian Amazon have been found to release vast amounts of CH₄ but the methane dynamics in upland forests are not very well studied. We assessed the magnitude of CH₄ fluxes from soil and tree stem surfaces across dry and wet seasons in two contrasting ecosystems in the Central Amazon basin: the seasonally flooded varzea and the upland terra firme forest. Likewise, some potential drivers of such fluxes were assessed: tree diameter, stem height of measurement, tree species, water table depth, and air temperature. Methane fluxes were measured using chamber-based techniques in the period 2022-2023. Overall, greater fluxes were released from the trees stems of the varzea forest during the first half of the wet season (June-August). On the other hand, the stem surface of upland trees emitted very low CH₄ fluxes (< 1 mg m^-2 h^-1). Methane fluxes of most trees from the flooded forests decreased with stem height, a pattern not shown by tree fluxes in the upland forest. The fluxes from tree stem emissions varied by tree species in both forest types: Munguba tree (Pachira aquatica) and Jarana tree emitted more CH₄ fluxes than other species in varzea and upland forests, respectively. The next step of our research will be the assessment of the microbial role in the methane cycle of both forest types using a combination of isotopic and -omic techniques.

How to cite: del Aguila Pasquel, J., Sousa Moura, J. M., Junior, M. F., dos Santos Lima, K., Tapajos, R., Brechet, L. M., van Haren, J. L. M., and Saleska, S. R.: Methane fluxes from soil and tree stem surfaces in flooded and non-flooded forests in the Central Amazon basin., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14065, https://doi.org/10.5194/egusphere-egu24-14065, 2024.