High frequency measurements reveal distinct sources of shoot methane emissions.

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 (<5g foliage biomass). Initial measurements were conducted with Scots pine and birch saplings. In addition, we measured conducted manual methane flux measurements on shoots of Scots pine saplings in two 24-hour campaigns.

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.