Spatial and temporal variability in methane emissions from tree stems of Fraxinus mandshurica in a cool-temperate floodplain forest

We performed field measurements on the spatial and temporal variability in CH4 emissions from stem surfaces of mature Fraxinus mandshurica Rupr. trees in a floodplain forest of northern Japan. Stem CH4 fluxes were measured by a static closed-chamber method at ca. 15 cm above ground on ten selected trees to test among-individual variability, and the diurnal and seasonal changes in three representative trees. Daytime stem CH4 emission rates varied between 81 and 1,305 A mu g CH4 m(-2) h(-1) among individual trees, and showed a spatial gradient apparently corresponding to the difference in water table depth at the experimental site. Stem CH4 fluxes were quite stable throughout a 24 h period for foliated trees in August and were similar for defoliated trees in November. Large differences were observed in the magnitude of seasonal changes in stem CH4 flux among individual trees; one sampled tree showed no clear seasonal changes in stem CH4 flux, while another tree exhibited drastic seasonal changes ranging larger than one order of magnitude. Results demonstrated the high variability in stem CH4 emissions in space and time, and suggested the importance of soil temperature, water table depth and porewater CH4 concentration as possible environmental factors controlling stem CH4 emissions from temperate forested wetlands.