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Recent evidence shows that the shoots of Scots pine trees are a source of small emissions of methane (CH4), but it remains unclear if these emissions significantly reduce the CH4 sink strength of boreal forests. Progress in the shoot methane research is hindered by the technical challenges of shoot-level trace-gas flux measurements requiring closed-loop, non-steady-state chamber systems, as well as the difficulties in producing sufficient measurement data from hard-to-reach canopies in remote field sites. We have identified possible sources of disturbance by VOCs and water vapour in spectral, laser-based online measurement systems, and developed ways to manage them (Kohl et al., 2019; Tenhovirta et al., 2022). Extended measurement campaigns in ambient conditions were made possible by conducting the measurements on tree saplings instead of mature trees, resulting in the finding that CH4 emissions from shoots increase with solar radiation and are enhanced by temperature (Tenhovirta et al., 2022). Our automated chamber measurement system in the greenhouse (Kohl, Koskinen et al., 2021) has enabled continuous measurements and control/treatment experiments. These measurements reveal that the CH4 emissions from Scots pine shoots follow diel cycles (Kohl, Tenhovirta, et al., in prep.), and rule out soil production or photosynthesis as a source for these emissions (Tenhovirta et al., in prep). In addition, developing this system has provided solutions to common issues associated to chamber measurements, including excessive heating, water vapour accumulation and leakage. In this contribution we show how some of these issues are solved in manual and automatic chamber systems to achieve reliable measurements, how to choose detection limits, and what solutions we propose for the remaining challenges. References Kohl L, Koskinen M, Polvinen T, Tenhovirta SAM, Rissanen K, Patama M, Zanetti A, Pihlatie M. 2021. An automated system for trace gas flux measurements from plant foliage and other plant compartments. Atmospheric Measurement Techniques 14: 4445–4460. Kohl L, Koskinen M, Rissanen K, Haikarainen I, Polvinen T, Hellén H, Pihlatie M. 2019. Technical note: Interferences of volatile organic compounds (VOC) on methane concentration measurements. Technical note: Interferences of volatile organic compounds (VOC) on methane concentration measurements: 1–18. Tenhovirta SAM, Kohl L, Koskinen M, Patama M, Lintunen A, Zanetti A, Lilja R, Pihlatie M. 2022. Solar radiation drives methane emissions from the shoots of Scots pine. New Phytologist 235: 66–77.

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/nature04420Fraser, 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

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. ReferencesRiutta, 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.

Plant shoots can emit methane (CH4) from multiple source processes (microbial methanogenesis in soils and core wood, aerobic CH4 production in foliage). We constructed a chamber system to isolate these processes and study how leaf level CH4 emissions respond to environmental factors like dark-light-cycles, temperature, drought, or CO2 concentrations. Tree samplings are located in a FITOCLIMA D 1200 plant growth chamber for PAR, temperature and humidity control and equipped with a measurement chamber to quantify CH4 exchange in a closed loop setup with a Picarro G2301 CH4 analyser. The system was further customized to control temperature, CO2, and humidity in the measurement chamber. The system allows the detection of CH4 flux rates of on the order of 1 nmol CH4 h-1 and can conduct high frequency (< 15 min) measurements of CH4 emissions rates from small shoots (These experiments demonstrated that the shoots of different tree species emit CH4 from distinct sources. Scots pine shoots emitted CH4 produced within the shoot, likely through aerobic CH4 production, which showed a strong diurnal cycles that follows irradiation and photosynthesis rates. Shoot from some birch species, in contrast, showed emissions of soil-borne CH4 that remained constant throughout day and nighttime. We expect that future experiment with this unique setup will allow to further disentangle shoot CH4 emissions and characterize their response to environmental conditions including light, temperature, and relative humidity.

Plant shoots can act as sources or sinks of trace gases including methane and nitrous oxide. Accurate measurementsof these trace gas fluxes require enclosing of shoots in closed non-steady state chambers. Due to plant physiological activity, this type of enclosures, however, lead to CO2 depletion in the enclosed air volume, condensation of transpired water, and warmingof the enclosures exposed to sunlight, all of which may bias the flux measurements. Here, we present PlasTraGAS, ab novel measurement system designed for continuous and automated measurements of trace gas and volatile organic compound (VOC) fluxes from plant shoots. The system uses transparent shoot enclosures equipped with Peltier cooling elements and automatically replaces fixated CO2 and removes transpired water from the enclosure. The system is designed for measuring trace gasfluxes over extended periods, capturing diurnal and seasonal variations and linking trace gas exchange to plant physiologicalfunctioning and environmental drivers. Initial measurements show daytime CH4 emissions two pine shoots of 0.056 and 0.089 nmol g−1 foliage d.w.h−1or 7.80 and 13.1 nmol m−2 h−1. Simultaneously measured CO2 uptake rates were 9.2 and 7.6 mmol m−2 sec−1 and transpiration rates of 1.24 and 0.90 mol m−2 h−1. Concurrent measurement of VOC emissionsdemonstrated that potential effects of spectral interferences on CH4 flux measurements were at least ten-fold smaller than themeasured CH4 fluxes. Overall, this new system solves multiple technical problems that so far prevented automated plant shoottrace gas flux measurements, and holds the potential for providing important new insights into the role of plant foliage in the global CH4 and N2O cycles.