In a well-aerated temperate forest soil, the response of stem methane emissions to variations in soil water content depends on the tree species and stem height measured

Methane (CH₄) is the second most significant anthropogenic greenhouse gas after CO₂, contributing to 20% of global warming. Low CH₄ emission by stems in well-aerated forest soils can influence global carbon cycles, mainly by reducing the CH₄ sink capacity of forests. Methane could be produced in anoxic soil zones, mainly when soil water is high, from late autumn to early spring. Methanogenesis occurs not only in soil since methanogenic archaea have been identified in the heartwood of trees where high concentrations of CH₄ were recorded. However, these concentrations do not lead directly to high emissions, as the CH₄ can be oxidised by the communities of methanotrophs also present in the tree and/or transported elsewhere.

To explore seasonal variations in methane fluxes (F_CH4) and the factors involved, methane fluxes were manually recorded 13 times at four heights (0.5, 1.3, 2 and 4 m) on the stems of three common temperate tree species (Carpinus betulus, Fagus sylvatica, and Quercus robur) in the Hesse forest (ICOS site, NE of France) from April 2023 to March 2024, using a trace gas analyser. The three species at this site have different root depth profiles, with the root system of Q. robur being deeper.

Over the sampling period, tree were low methane emitters for all species (mean ± SE, 0.71 ± 0.34 µg CH₄ h⁻¹), with notable variability, particularly for Q. robur, ranging from - 12.47 to 15.64 µg CH₄ h⁻¹. Methane fluxes especially below 1.5 meters varied along the year in relation to the 3 levels in soil moisture defined according to the water table level (wet: water table above 0.55 m, dry: water table below 1 m and moderate: intermediate and the two above). Methane emitted by the three species differed with soil moisture. Q. robur exhibited lower emissions only during driest soil conditions, as its deep root system allows access to methane production zones deeper in the soil. In contrast, F. sylvatica showed reduced emissions at both drier (moderate and dry) levels, likely due to its shallower root system's limiting access to methane-rich soil layers.But methane emissions did not decrease with height or with decreasing soil water content in all trees, indicating that methane could be produced inside the wood. In fact, in one third of the wood cores sampled, potential methanogenic production was recorded.

Our study confirmed that methane emissions from trees are influenced both by soil methane and by internal production processes. Our work has shown that the differences in emissions between species could be explained by the root profile.