Massive methane emission from tree stems and pneumatophores in a subtropical mangrove wetland

Purpose: Increasing evidence indicates that trees could emit methane (CH) from soils into the atmosphere. However, inter-species and seasonal variations in the magnitude of tree-mediated CH fluxes within coastal mangrove wetlands have not yet been clarified. Methods: We measured CH emission fluxes in three independent mangrove communities, Avicennia marina, Aegiceras corniculatum, and Kandelia obovata, within a subtropical mangrove wetland during tree dormancy and growth seasons using static chambers. Tree-stem, pneumatophore, and soil–atmosphere-interface CH fluxes were simultaneously measured, thus enabling an estimate of the contributions from each pathway to ecosystem CH fluxes. Results: Pneumatophore and tree-stem CH fluxes were much higher than the soil–atmosphere-interface CH flux. In mangrove communities with pneumatophores (A. marina), the pneumatophore CH fluxes accounted for 84% of the ecosystem CH flux, whereas tree-stem and soil–atmosphere-interface CH fluxes accounted for 9% and 7%, respectively. In contrast, in mangrove communities without pneumatophores (A. corniculatum and K. obovata), the tree-stem CH fluxes dominated (75–79%) the ecosystem CH fluxes, whereas the soil–atmosphere-interface CH fluxes accounted for 21–25%. Ecosystem CH fluxes, as well as pneumatophore, tree-stem, and soil–atmosphere-interface CH fluxes were higher during the growth season than the dormancy season. However, the partitioning of ecosystem CH fluxes did not significantly change between the two seasons. Conclusion: Tree stems and pneumatophores are important conduits for CH emissions in mangrove wetlands. Ecosystem CH emissions may offset 53% (with pneumatophores) or 10–13% (without pneumatophores) of the total C burial rates in global mangrove wetlands.