Micrometeorological measurement of methane flux above a tropical peat swamp forest

Abstract 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 (CH 4 ) to the atmosphere. However, a few studies of CH 4 flux by the soil chamber technique have reported that annual CH 4 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 CH 4 from their stems. It is impossible to continuously measure ecosystem-scale CH 4 flux including both soil and plant-mediated CH 4 emissions by the chamber technique. Thus, we have measured net ecosystem CH 4 exchange (FCH 4 ) 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 FCH 4 was 24.0 ± 42.2 nmol m –2  s –1 . Monthly mean FCH 4 was always positive during the 18 months, even in the driest month with mean GWL of –30 cm. FCH 4 was positively associated with GWL or soil moisture in a quadratic form. Annual FCH 4 from March 2014 through February 2015 was 7.5–10.8 g C m –2  yr –1 . The annual FCH 4 was much higher than annual soil CH 4 emissions from tropical peatlands, because the FCH 4 included aboveground CH 4 emissions mainly from tree stems. However, the annual FCH 4 was relatively low in comparison with those measured by the eddy covariance technique in mid- and high-latitude peatlands.