17:15 〜 19:15
[BCG05-P08] Spatial and temporal variability in methane emissions from trees and sediment in a subtropical mangrove forest
キーワード：mangrove, methane, CH4 flux
Mangrove forests are among the most carbon-rich tropical forests, storing carbon in tree biomas, both above and belowground, and in sediments. Although their distribution is limited to coastal areas in southern Japan, they are considered effective carbon sinks and the carbon buried in these systems is often refers to as blue carbon. However, because methane (CH4) gas has a much higher global warming potential than CO2, it has recently been recognized that CH4 emissions can partially offset the burial of blue carbon in mangrove forest sediments. Mangrove forests exhibit a complex spatial structure with various tidal amplitudes and local riverine inputs of freshwater, which vary seasonally with rainfalls. Estimating CH4 fluxes at the ecosystem scale is challenging due to large spatiotemporal heterogeneity. We hypothesized that terrain attributes, including topography, hydrology, sediment and water quality, and species distribution, can be reliable predictors of sediment CH4 fluxes and tree emissions related to the transfer of CH4 produced in the sediment.
The study site is a mangrove forest extending about 1500 m along the intertidal banks of the Miyara River estuary on Ishigaki Island (Japan, 24°21'N 124°12'E), which are inundated twice daily (mixed semidiurnal tides). We measured FCH4 from sediments, and trunks and aerial roots of trees using closed chambers connected to a cavity-enhanced absorption spectroscopy gas analyser at four occasions over an 8-month period. Sediment and water quality were characterized.
Sediment FCH4 was high upstream and decreased from upstream to downstream with increasing soil poral water salinity. However, the salinity difference explained only a small percentage of the total spatial variance. Moreover, although seasonal variability was limited, very large temporal variations were observed at the daily scale.
Tree FCH4 was higher on roots than on stems, with a decreasing trend with height along the stems. FCH4 was higher on knee roots than on buttress roots in Bruguiera gymnorhiza, the dominant species. Higher FCH4 were also measured at lower than at higher positions on prop roots of Rhizophora stylosa. Large spatial and temporal variations were also observed for tree FCH4
Accurate estimation of CH4 budgets of mangrove forests required a better understanding of FCH4 variability from mangrove trees and sediments, and its controls at different time scales, as well as species-specific relationships between stem diameter aerial root area, coupled with inventories of aerial roots.
The study site is a mangrove forest extending about 1500 m along the intertidal banks of the Miyara River estuary on Ishigaki Island (Japan, 24°21'N 124°12'E), which are inundated twice daily (mixed semidiurnal tides). We measured FCH4 from sediments, and trunks and aerial roots of trees using closed chambers connected to a cavity-enhanced absorption spectroscopy gas analyser at four occasions over an 8-month period. Sediment and water quality were characterized.
Sediment FCH4 was high upstream and decreased from upstream to downstream with increasing soil poral water salinity. However, the salinity difference explained only a small percentage of the total spatial variance. Moreover, although seasonal variability was limited, very large temporal variations were observed at the daily scale.
Tree FCH4 was higher on roots than on stems, with a decreasing trend with height along the stems. FCH4 was higher on knee roots than on buttress roots in Bruguiera gymnorhiza, the dominant species. Higher FCH4 were also measured at lower than at higher positions on prop roots of Rhizophora stylosa. Large spatial and temporal variations were also observed for tree FCH4
Accurate estimation of CH4 budgets of mangrove forests required a better understanding of FCH4 variability from mangrove trees and sediments, and its controls at different time scales, as well as species-specific relationships between stem diameter aerial root area, coupled with inventories of aerial roots.