Tree stem and soil methane fluxes in temperate, upland forests: sources, sinks & drivers

Methane (CH4) and carbon dioxide (CO2) are two important greenhouse gases (GHG). Mineral soils in temperate forests are one of the most significant biological sinks for CH4. However, recent studies reported tree stem CH4 emissions may offset soil CH4 sink and contribute to the global CH4 budget. Current knowledge is still limited on the pathway and underlying mechanism of stem CH4 emissions and uptake. Therefore, this thesis investigated the exchange of CH4 and CO2 fluxes from tree stems and soils in temperate upland forests. The presence of ECM mycelium exhibited a fluctuation effect on soil CH4 uptake and soil respiration during the short-term 1-2 years, but significantly decreased the cumulative CH4 uptake during the long-term 8.5-9.5 years. Biochar addition did not have any significant effect on soil CH4 uptake and mostly no effect on soil respiration over the entire study. Tree stem CH4 flux did not show any significant differences between English oak (Quercus robur) and Japanese larch (Larix kaempferi) trees during a growing season. The seasonal pattern of stem CH4 flux was not found, but large daytime and intra-specific variations in oak and larch stem CH4 fluxes were observed showing both CH4 uptake and emission. High-frequency measurements (1.5 hourly frequency) of three white poplar (Populus alba) tree stem CH4 and CO2 fluxes showed large hour-to-hour variation in stem CH4 flux with both CH4 uptake and emissions, and a clear diurnal pattern of stem CO2 flux with larger flux during night-time than daytime. Wood incubation experiments showed the highest rates of potential CH4 production and CH4 oxidation were from bark. Combined with the results of lower internal stem CH4 concentration and soil as a net CH4 sink, it suggests that biologically in situ tree stem CH4 production is the major source of stem CH4 emission in temperate upland forests.