Effects of drought on the methane emissions of the shoots of young scots pine saplings

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.