Variability in methane emissions from stems and buttress roots of Bruguiera gymnorrhiza trees in a subtropical mangrove forest

Trees in wetlands emit a significant amount of methane (CH4) into the atmosphere which is produced in the soil where anaerobic conditions prevail. However, only very few studies have considered the role of trees in the CH4 budget of mangrove forests. How tree CH4 emissions (F-CH4) vary between and within individuals, and how these emissions vary with the day/night alternation and the tidal cycle, have not yet been elucidated. We measured F-CH4 from buttress roots and stems on individuals of Bruguiera gymnorrhiza of different sizes. We also measured F-CH4 from trees and sediments over a 24-h cycle. Higher FCH4 was measured on the buttress roots than on the stems, with a decreasing trend with height along the stems on a majority of trees. F-CH4 exhibited large variations over a 24-h cycle which were observed for the first time in this study. F-CH4 from sediment was twice higher at low tide compared to just before or after the flood period. F-CH4 from non-submerged stem positions at high tide was four times higher during the day than during the night. On buttress roots and on stem portion which were submerged, high F-CH4 were measured at the beginning of the ebb tide even at night. A better understanding of F-CH4 variability from mangrove trees and its controls, at different time scales and along spatial gradients is needed for accurate estimation of CH4 budgets of mangrove forests.