BG3.18

Forests are often considered to be able to play a significant role in tackling global warming. To fully understand their potential in mitigating climate change and to develop more accurate ecosystem GHG flux budgets and process-based models of forests, we require more knowledge of methane (CH₄) and nitrous oxide (N₂O) exchange in forests, their underlying processes, environmental controls and responses to forest management. In recent years, it is becoming evident that not only soils but also the trees themselves may significantly contribute to CH₄ and N₂O fluxes in forest ecosystems.

Our research mainly focussed on greenhouse gas (GHG) exchange in temperate UK forests on both mineral and organic soils. We will primarily concentrate on CH₄ fluxes as N₂O fluxes were often relatively low in these forests and, by including CO₂ fluxes, we will put them into the context of the overall ecosystem GHG exchange. A range of flux methods at different scales were used in our field studies to be able to capture the often high temporal and spatial variability of the GHG exchange between the atmosphere and either soils, tree stems or entire trees aboveground, and to identify potential drivers of the fluxes. The impact of management practices including clear fell, drainage and the resulting micro-topography, and forest-to-bog restoration on CH₄ fluxes from organic soils following the first forest rotation will also be described. We regularly used novel automated and chamber approaches and technologies, and the advantages and limitations of the different flux approaches and their use to upscale fluxes to the landscape scale will be evaluated.

How to cite: Toet, S., Ma, R., Barrop, W., Keane, B., Stockdale, J., Andersen, R., Anderson, R., McNamara, N., Xenakis, G., Yamulki, S., and Morison, J.: Greenhouse gas exchange in temperate forest ecosystems in the UK - A quest for key components and drivers, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8813, https://doi.org/10.5194/egusphere-egu22-8813, 2022.

Methane production in plant foliage under aerobic conditions remains a cryptic and poorly constrained component of the global methane cycle. While several in-vitro studies reported light-dependent production of methane from plant biomolecules, thus far no studies have investigated methane fluxes at plant shoots during diel cycles. Here, we show that methane emissions from Scots pine shoots follow a distinct diurnal pattern and we demonstrate how these cycles allow estimating an upper limit of shoot methane emissions from ecosystem-atmosphere methane fluxes measured by the eddy covariance method. We present data from three measurement campaigns in forest, garden, and greenhouse settings that quantified methane fluxes of the shoots of Scots pine saplings and adult trees using manual and automated shoot chamber flux measurements systems, two distinct of trace gas analysers (Los Gatos Research UGGA, Picarro G2301). Despite the methodological differences, all campaigns found average methane flux rates between 0.05 and 0.20 nmol g^-1 foliar dry weight h^-1 in all campaigns. In the garden and greenhouse campaigns, where 24-hour measurement campaigns were possible, shoot methane fluxes exhibited pronounced diurnal cycles with a strong light dependent emission during daytime and low fluxes (mostly below the detection limit) during nighttime. Based on these strong light-dependent diurnal cycles, we were able to calculate an upper limit for shoot methane emissions at the ecosystem level. For this, we quantified the light-dependent and light-independent components of ecosystem-atmosphere methane fluxes measured by eddy covariance, with the light-dependent component tentatively indicating shoot-level methane fluxes. The monthly averages of the so-quantified light-dependent component accounted for 0.0-0.4 nmol methane m^-2 sec^-1 (range of monthly averages), which corresponds to ~0-1 nmol methane g^-1 foliar dry weight h^-1. This component is approximately 10-fold higher than shoot-level fluxes, indicating that other processes beside shoot emissions may contribute to light-dependent methane emissions. Nevertheless, even this higher estimate of shoot methane emissions correspond with the low end of the range reported by Keppler et al. (2006; 0.75–55 nmol g^-1 d.w. h^-1) and fall within the range reported by Fraser et al. (2015; 0.03–2 nmol g^-1 d.w. h^-1). Taken together, our results show how combining shoot and ecosystem level measurements can help constraining shoot emissions sufficiently for incorporating these fluxes in regional and global methane budgets. Taken together, our results show how combining shoot and ecosystem level measurements can help constraining shoot emissions sufficiently for incorporating these fluxes in regional and global methane budgets.

References:

Keppler, F., Hamilton, J., Braß, M. et al. Methane emissions from terrestrial plants under aerobic conditions. Nature 439, 187–191 (2006). https://doi.org/10.1038/nature04420

Fraser, W. T., Blei, E., Fry, S. C., et al.. Emission of methane, carbon monoxide, carbon dioxide and short-chain hydrocarbons from vegetation foliage under ultraviolet irradiation. Plant, cell & environment, 38(5), 980–989 (2015). https://doi.org/10.1111/pce.12489

How to cite: Kohl, L., Tenhovirta, S., Koskinen, M., Putkinen, A., Patama, M., Polvinen, T., Marmarella, I., Robson, T. M., Dominguez, M., Adamczyk, B., and Pihlatie, M.: Shoot methane emissions follow pronounced diurnal cycles that allow constraining aerobic methane production at the ecosystem-level, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3922, https://doi.org/10.5194/egusphere-egu22-3922, 2022.

Plants can emit methane (CH₄) produced by an unknown aerobic, non-enzymatic process, driven by plant stressors like UV-radiation, elevated temperatures and wounding. In ambient spring conditions in Finland, CH₄ emissions from the shoots of Scots Pine (Pinus sylvestris) correlated with solar radiation independently of temperature (Tenhovirta et al., in revision). The spring-time shoot CH4 emissions also had a diurnal pattern with the highest emissions during noon. It remains unknown whether these emissions are driven directly by solar radiation or indirectly via its effect on tree physiological processed such as photosynthesis or stomatal conductance. Characterizing the ecophysiology of the CH₄ fluxes of tree canopies is a crucial step in order to understand the role of forests in the global CH₄ cycle.

To test whether shoot CH₄ emissions are driven by tree physiological activity (e.g. stomatal conductance), we conducted a measurement campaign in greenhouse conditions during which Scots pine saplings were exposed to drought. During this 3-month-long campaign, CH₄, carbon dioxide (CO₂) and water vapour (H₂O) fluxes from tree shoots were measured with an automated shoot trace gas flux measurements system (ShoTGa-FluMS)(Kohl, Koskinen et al., 2021). This system is capable of replacing the CO₂ assimilated by the shoots, removing transpired water and cooling the chambers to near ambient temperatures. The experimental setup consisted of six 2-3 year old nursery saplings each with a shoot enclosed inside an automated shoot chamber, alternating (a) in closed loop with a Picarro G2301 cavity ring-down spectroscopy (CRDS) greenhouse gas concentration analyser (CH₄ and CO₂ measurements), (b) in a flow-through setup with a Li-cor 850 CO₂-H₂O analyser (photosynthesis and transpiration measurements), or (c) flushed with ambient air. The saplings were exposed to a daily 9-hour photoperiod of ~ 600-800 µmol s^-1 m^-2 photosynthetically active radiation (PAR), and irrigated automatically. Drought was induced by stopping the irrigation and continued to the point where net uptake of CO₂ no longer occurred.

Our experiment produced a unique dataset of continuous measurements of shoot-level CH₄, CO₂ and H₂O fluxes over a period of several weeks. Our preliminary results show small but consistent CH₄ emissions from the shoots of Scots Pine during daylight, supporting our earlier findings of the dependency of shoot CH₄ emissions on light. The data furthermore allows to analyse the effects of drought on tree physiological activity and shoot CH₄ fluxes providing much needed process understanding of shoot CH₄ emissions from boreal trees.

References

Kohl, Koskinen et al. 2021. An automated system for trace gas flux measurements from plant foliage and other plant compartments. Atmospheric Measurement Techniques 14: 4445–4460.

How to cite: Tenhovirta, S., Kohl, L., Koskinen, M., and Pihlatie, M.: Effects of drought on the methane emissions of the shoots of young scots pine saplings, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11588, https://doi.org/10.5194/egusphere-egu22-11588, 2022.

Methane (CH₄) is the second largest contributor to global warming and the importance of reducing CH₄ emissions was recently highlighted through the Global Methane Pledge launched in 2021. Forest soils can act both as sinks and sources of CH₄, largely depending on the hydrological status of the soil, and both direction and magnitude of CH₄ fluxes often vary considerably even across small spatial and temporal scales. Thus, projected changes in precipitation patterns can be expected to affect both total CH₄ budgets and the spatiotemporal distribution of sinks and sources. Forest management – for example clear cutting and nitrogen (N) fertilization – also affects CH₄ cycling in forests with the potential to turn CH₄ sinks into CH₄ sources, but little is currently known about the mechanisms and to what extent fluxes are affected by forest management.

In the ForBioFunCtioN project, we have set up an extensive climate and management manipulation experiment across five Norwegian spruce dominated bilberry forest sites spanning from a recent clear-cut to mature managed (80 years) and old unmanaged (140 years) stands. Treatments include warming with open-top chambers, simulated increased precipitation and additions of N fertilizer and biochar in a total of 12 different treatment combinations (n = 144). We utilize state-of-the-art technology (LI-7810 Trace Gas Analyzer, LI-COR^®) for measurements of soil-atmosphere and deadwood-atmosphere exchange of CO₂ and CH₄.

Here, we present the experimental setup and soil and deadwood flux measurements of CO₂ and CH₄ from June to December 2021. Initial results show that soil CH₄ fluxes vary considerably both between and within sites yet indicate short-term responses of CH₄ fluxes to addition of biochar and N fertilizer in particular.

How to cite: Johannesson, C.-F., Larsen, K. S., and Nordén, J.: Forest soil and deadwood CH4 fluxes in response to climate change and forest management, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7061, https://doi.org/10.5194/egusphere-egu22-7061, 2022.

Freshwaters (rivers, streams, ponds, reservoirs) are a well-recognized source of methane (CH₄) characterised by large spatiotemporal variability. However, the determination of the role of water bodies for net CH₄ exchange on forest ecosystem scale is very scarce. Our study aimed to determine the importance of individual emission pathways for total CH₄ fluxes from streams and to verify the possible identification and quantification of CH₄ fluxes from water bodies on the floodplain forest ecosystem level. In 2020, we measured CH₄ fluxes by diffusion and ebullition from a lowland stream flowing through the lowland broadleaf mixed temperate forest at Lanžhot in the Czech Republic (Central Europe). For this purpose, 18 bubble traps were installed at three stream sites and periodically sampled for gas volume and its CH₄ content from April to December. Diffusive CH₄ fluxes from water were measured at 14 days intervals with a floating chamber connected to a portable GHG analyser. Simultaneously, CH₄ exchange was determined on the forest ecosystem scale using the eddy covariance method (EC). We hypothesized initially that due to a relatively small area of water bodies in the EC footprint and a high probability of CH₄ consumption by soils, CH₄ emissions will be detectable by EC only in case that water bodies will create the potential CH₄ emission hotspots in the studied ecosystem. We found that the investigated stream was a significant source of CH₄ (mean 260 ± 107 mg CH₄ m^-2 day^-1) with ebullition as a dominant pathway (55 – 85%) of CH₄ release throughout the whole monitored time period. Furthermore, first EC results showed that the whole ecosystem is a small but constant CH₄ source as we observed an average emission flux of 16 ± 18 mg CH₄ m^-2 day^-1 over the period June to November 2021. In-depth investigations of the potential CH₄ sources and sinks within the studied ecosystem should answer the question of how the relative proportion of water surfaces and related CH₄ emission corresponds to whole ecosystem CH₄ fluxes.

How to cite: Kowalska, N., Bednarik, A., and Jocher, G.: Quantification of methane fluxes from water bodies on the floodplain forest ecosystem level, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6985, https://doi.org/10.5194/egusphere-egu22-6985, 2022.

Tree stems emit CO₂ and can exchange CH₄ with the atmosphere (either emitting or uptaking), with a significant contribution to the C budgets from local to regional scales. However, there is still a need to better understand the spatial and temporal variability of stem CO₂ and CH₄ fluxes to quantify the role of vegetation on the C cycle and how these fluxes will behave under future environmental conditions such as atmospheric elevated CO₂. An increment of atmospheric CO₂ concentrations might result in higher photosynthetic rates, which would spin the C cycle in the trees, potentially increasing stem CO₂ emissions due to higher stem respiration and higher soil-derived CO₂ contribution. Higher photosynthetic rates might also stimulate fine roots exudation, which could stimulate methanotrophic or methanogenic communities. Additionally, elevated CO₂ would increase water use efficiency at the leaf level, reducing the amount of water transpired, and potentially increasing soil moisture, which would favour conditions for CH₄ production. In this study, we present one year of monthly measurements of stem CO₂ and CH₄ fluxes from mature oaks (Quercus robur) growing under elevated CO₂ (~150 ppm above atmospheric concentrations) and ambient conditions, in a second-generation FACE experiment (Free Air CO₂ Enrichment; BIFoR-FACE UK). Trees growing under ambient conditions emitted 76% more CO₂ than those under elevated atmospheric CO₂, which was not what we hypothesized. Despite stem CH₄ fluxes have been reported in multiple upland ecosystems for lots of tree species, our preliminary results did not show clear evidence of CH₄ stem fluxes (emissions or uptake) for the oaks at our study site. Similar measurements in other FACE experiments are needed to determine if our results on the effect of elevated CO₂ on stem CO₂ and CH₄ fluxes could be extrapolated to other ecosystems and species. 

How to cite: Barba, J., Curioni, G., and Gauci, V.: Temporal and spatial effects of elevated CO2 on greenhouse gas fluxes from tree stems in an upland temperate forest, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12003, https://doi.org/10.5194/egusphere-egu22-12003, 2022.

Besides soils, tree stems are known to emit nitrous oxide (N₂O) into the atmosphere. However, it seems, stems of some tree species might also take up this important greenhouse gas from the atmosphere under certain conditions. Even though tree leaves dominate the tree surface area, they are entirely excluded from field N₂O flux measurements, and their role in forest N₂O exchange is still unknown.

We aimed to investigate the contribution of leaf fluxes to the forest N₂O exchange. We determined N₂O exchange of stems and leaves of mature European beech (Fagus sylvatica), and adjacent soil in a typical temperate upland mixed forest in Southern Germany, using non-steady-state chamber methods and a system of scaffold towers reaching the top of tree crowns in 35 m. The measurements were accompanied by a parallel determination of stem, leaf and soil CO₂ exchange and numerous environmental characteristics (soil N₂O and CO₂ concentrations and water content in vertical soil profiles, soil and air temperature).

We found out that the beech stems and especially the leaves were net sinks of N₂O from the atmosphere (–1.07 ± 3.47 and –249.9 ± 84.3 mg N₂O ha⁻¹ ground area h⁻¹, respectively), whereas the soil was a net N₂O source into the atmosphere (24.0 ± 10.8 mg N₂O ha⁻¹ h⁻¹). The never studied tree leaves were identified as a key player in ecosystem N₂O exchange, taking up in fact 10 times more N₂O than the soil emits at the same time. Therefore, native Central European and widely spread European beech trees seem to contribute to forest N₂O uptake markedly.

For the first time, we revealed tree leaves being substantial N₂O sinks. Our results clearly show that the current and ongoing exclusion of tree leaves from forest N₂O flux measurements can lead to a severe underestimation of the overall tree and forest N₂O exchange and, therefore, global forest greenhouse gas flux inventories.

Acknowledgement

This research was supported by the Czech Science Foundation (17-18112Y) 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 Jan Hrdlička and Thomas Feuerbach for their technical support.

How to cite: Machacova, K., Schindler, T., Mander, Ü., Soosaar, K., and Grams, T. E. E.: Leaves of mature European beech consume nitrous oxide (N2O) from the atmosphere, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4171, https://doi.org/10.5194/egusphere-egu22-4171, 2022.

After a period where atmospheric methane (CH₄) levels were nearly steady, its levels have been rapidly raising since 2007, but the main reasons remains uncertain. Increases in wetlands emissions could be one possible reason, mainly at tropical regions like Amazonia, which host some of the largest wetlands/seasonally flooded areas on the globe. Based on 590 lower troposphere vertical profiles of CH₄ and carbon monoxide (CO) observations over four sites at Amazon (at the northeast, southeast, northwest-central and southwest-central regions) we estimated that Amazon region contributes with 8% of global CH₄ emissions, and wetlands are the mainly CH₄ source to the atmosphere (Basso et al., 2021). Vertical profiles are sampled using light aircraft, high-precision greenhouse gas and CO analysis of flask air, fortnightly between 2010 and 2018. We observed an unexpected east-west gradient in CH₄ emissions, with higher emissions in northeast Amazon region. The higher emissions are mainly from wetlands and are not explained by biomass burning and anthropogenic emissions (like enteric fermentation), but its causes remains unclear. In the other three sites located further downwind along the main air-stream the CH₄ emissions represents approximately 24-36% of what is observed in the northeast region. Our wetlands emission estimates of each region were compared to analogous fluxes from the WetCharts wetland model ensemble (Bloom et al., 2017). The estimates were similar except for the northeast region, where WetCharts does show substantial emissions, but still just 40% of our estimates based on the lower troposphere observations (Basso et al., 2021).

How to cite: Basso, L., Gatti, L., Marani, L., Miller, J., Gloor, M., Melack, J., Cassol, H., Tejada, G., Domingues, L., Arai, E., Sanchez, A., Corrêa, S., Anderson, L., Aragão, L., Correia, C., Crispim, S., and Neves, R.: Regional variability in Amazon methane emissions based on lower-troposphere observations, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12685, https://doi.org/10.5194/egusphere-egu22-12685, 2022.

Forests play an important role in the exchange of radiatively important gases with the atmosphere. Previous studies have shown that in both temperate and tropical wetland forests tree stems are significant sources of methane, yet little is known about tree stem trace greenhouse gas dynamics in drier, free-draining soils that dominate global forested areas. Here, we examine methane fluxes on tree stems spanning a climate gradient of upland forests and floodplain forest across 4 locations in the Amazon, Brazil (Cunia, Rios Negro, Solimoes and Tapajos), lowland tropical forest on free-draining soils in Panama, Central America (Barro Colorado Nature Monument), deciduous woodland in the United Kingdom (Wytham, Oxfordshire) and boreal forest in Sweden. We found that trees behaved as both methane sources (near the tree base) and sinks (higher up the tree stem) across tropical, temperate and boreal sites and are highly variable, yet we were able to identify a broad correlation between the size of tree stem methane uptake fluxes and mean annual temperature across the climate gradient. The vertical spatial patterns of flux up the individual measured trees and climate gradient temperature-methane flux relationship together with revised LiDAR derived tree surface allometry permitted global scaling of fluxes in upland forest. Results of this scaling together with the implications of this refined understanding of the global methane cycle under various scenarios are discussed.

How to cite: Gauci, V., Pangala, S., Shenkin, A., Barba, J., Bastviken, D., Figueiredo, V., Gomez, C., Enrich-Prast, A., Sayer, E., Stauffer, T., Welch, B., Allen, M., and Malhi, Y.: Methane source-sink behaviour in upland trees spanning a global climate gradient, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13360, https://doi.org/10.5194/egusphere-egu22-13360, 2022.