5:15 PM - 6:45 PM
[BCG04-P06] Clarifying the origin of CH4 emission hot spots in upper tree trunks: Local genesis or axial transport
Keywords:Methane, trunk CH4 flux, upland forest, transport phenomena in stem, methanogenesis
Methane (CH4) is the second most potent anthropogenic greenhouse gas. Upland soils serve as the largest biological sink for CH4, in contrast to wetlands, which constitute the largest natural source of CH4. It is known that wetland plants, including trees, emit CH4 produced in the soil, acting as conduits for CH4. However, recent studies revealed that tree trunks in upland forests emit CH4, and at least a part of the emission results from microbial CH4 production under anoxic conditions inside the trunk.
Since endogenous CH4 production may occur also in the upper part of the trunk if the anoxic condition is created, emissions from that part of the trunk should not be ruled out in upland forests. However, most previous studies have measured the emission at easily accessible locations on the trunk, typically from the lower 2 m of the trunk. Along with the local CH4 production, the transport of CH4 within the trunk including molecular diffusion and advection by xylem sap flow is also likely to be an important process in determining the vertical pattern of emission from the trunk. This study investigated the vertical pattern of trunk CH4 emission up to 12 m, focusing on the processes behind that, i.e. the CH4 production and transport inside the trunk.
All investigations were conducted in a cool-temperate upland forest at the Ashiu Forest Research Station of Kyoto University (35.34 N, 135.76E). Six trees of three species (Aesculus turbinata, Cryptomeria japonica, and Fagus crenata) standing on a well-drained soil were selected and trunk CH4 emission was measured at six heights along the trunk up to 12 m, for once each in July 2022 and October 2023. To find out whether local methanogenesis or axial CH4 transport along the trunk is important in determining the vertical emission pattern, (a) the vertical distribution of endogenous CH4 production was investigated up to 12 m, and (b) radial and axial CH4 transport inside the trunk was modeled, and the vertical pattern of emission at steady state was simulated using the observed heartwood CH4 concentration as an input.
The upper part of the trunk emitted CH4 consistently over two measurements in 2022 and 2023, with the emission peaking at a height of 6 to 7 m above the ground in two trees. The magnitude of the emission decreased dramatically from the measurement in 2022 to that in 2023 in one individual. The CH4 emission and internal CH4 production showed similar vertical patterns along trunk height in two species, and the simulated vertical pattern of emission did not change much when varying parameters regarding axial transport. Therefore, we concluded that the distribution of CH4 production was more important compared to axial CH4 transport in determining the vertical pattern of emission in our case. Furthermore, we suggested that the hot spot of CH4 production inside the trunk is related to either wood rot or the limitation of O2 supply into the trunk depending on the species. This study illustrated the complexity of the intra-individual spatial variability of trunk CH4 emission and provided new insights into the processes behind the emission, encouraging the future development of the process-based model for trunk CH4 emission.
Since endogenous CH4 production may occur also in the upper part of the trunk if the anoxic condition is created, emissions from that part of the trunk should not be ruled out in upland forests. However, most previous studies have measured the emission at easily accessible locations on the trunk, typically from the lower 2 m of the trunk. Along with the local CH4 production, the transport of CH4 within the trunk including molecular diffusion and advection by xylem sap flow is also likely to be an important process in determining the vertical pattern of emission from the trunk. This study investigated the vertical pattern of trunk CH4 emission up to 12 m, focusing on the processes behind that, i.e. the CH4 production and transport inside the trunk.
All investigations were conducted in a cool-temperate upland forest at the Ashiu Forest Research Station of Kyoto University (35.34 N, 135.76E). Six trees of three species (Aesculus turbinata, Cryptomeria japonica, and Fagus crenata) standing on a well-drained soil were selected and trunk CH4 emission was measured at six heights along the trunk up to 12 m, for once each in July 2022 and October 2023. To find out whether local methanogenesis or axial CH4 transport along the trunk is important in determining the vertical emission pattern, (a) the vertical distribution of endogenous CH4 production was investigated up to 12 m, and (b) radial and axial CH4 transport inside the trunk was modeled, and the vertical pattern of emission at steady state was simulated using the observed heartwood CH4 concentration as an input.
The upper part of the trunk emitted CH4 consistently over two measurements in 2022 and 2023, with the emission peaking at a height of 6 to 7 m above the ground in two trees. The magnitude of the emission decreased dramatically from the measurement in 2022 to that in 2023 in one individual. The CH4 emission and internal CH4 production showed similar vertical patterns along trunk height in two species, and the simulated vertical pattern of emission did not change much when varying parameters regarding axial transport. Therefore, we concluded that the distribution of CH4 production was more important compared to axial CH4 transport in determining the vertical pattern of emission in our case. Furthermore, we suggested that the hot spot of CH4 production inside the trunk is related to either wood rot or the limitation of O2 supply into the trunk depending on the species. This study illustrated the complexity of the intra-individual spatial variability of trunk CH4 emission and provided new insights into the processes behind the emission, encouraging the future development of the process-based model for trunk CH4 emission.