Micrometeorological measurement of methane flux above a tropical peat swamp forest

Tropical peat swamp forest is a unique ecosystem, in which both swamp forest and peat soil have coexisted over millennia and accumulated a significant amount of soil carbon as peat. Owing to the huge soil carbon stock and high groundwater level (GWL), tropical peatlands potentially represent a significant source of methane (CH4) to the atmosphere. However, a few studies of CH4 flux by the soil chamber technique have reported that annual CH4 emissions from tropical peat swamp forest were very low as compared to mid- and high-latitude peatlands. Recently, it has been reported that some tree species growing in peat swamp forest emit CH4 from their stems. It is impossible to continuously measure ecosystem-scale CH4 flux including both soil and plant-mediated CH4 emissions by the chamber technique. Thus, we have measured net ecosystem CH4 exchange (FCH4) above a tropical peat swamp forest in Sarawak, Malaysia using the eddy covariance technique from February 2014 to July 2015 (18 months). The mean (±1 standard deviation) of half-hourly measured FCH4 was 24.0 ± 42.2 nmol m-2 s-1. Monthly mean FCH4 was always positive during the 18 months, even in the driest month with mean GWL of -30 cm. FCH4 was positively associated with GWL or soil moisture in a quadratic form. Annual FCH4 from March 2014 through February 2015 was 7.5-10.8 g C m-2 yr-1. The annual FCH4 was much higher than annual soil CH4 emissions from tropical peatlands, because the FCH4 included aboveground CH4 emissions mainly from tree stems. However, the annual FCH4 was relatively low in comparison with those measured by the eddy covariance technique in mid- and high-latitude peatlands.