Tropical peatlands are significant sources of methane (CH₄), but their contribution to the global CH₄ budget remains poorly quantified due to the lack of long-term, continuous and high-frequency flux measurements. To address this gap, we measured net ecosystem CH4 exchange (NEE-CH4) using eddy covariance technique throughout the conversion of a tropical peat swamp forest to an oil palm plantation. This encompassed the periods before, during and after conversion periods from 2014 to 2020, during which substantial environmental shifts were observed. Draining the peatland substantially lowered mean monthly groundwater levels from -20.0 ± 14.2 cm before conversion to -102.3 ± 31.6 cm during conversion and increased slightly to -96.5 ± 19.3 cm after conversion. Forest removal increased mean monthly soil temperature by 2.3 to 3.1 °C, reducing net radiation (Rn) and raising vapor pressure deficit (VPD). Following the tree removal, controlled burning temporarily warmed air temperature by 8 °C, increased VPD and significantly attenuated Rn, resulting in negative values owing to radiation interception by smoke and increased surface warming. Contrary to expectations that drainage would lower CH4 emissions, the site remained a consistent net source, with even higher emissions observed during and after conversion. The mean monthly NEE-CH4 during conversion (23.3 ± 8.6 mg C m-2 d-1) was about 2-times higher than before conversion (12.1 ± 5.3 mg C m-2 d-1) and about 1.5-times higher than after conversion (16.3 ± 4.1 mg C m-2 d-1). The heightened CH4 release is likely attributable to emissions from drainage ditches, underscoring their significant role in post-conversion CH4 dynamics. Despite its short duration, controlled burning substantially elevated NEE-CH4, ranging from 0.04 to 0.91 mg C m-2 s-1. Our findings highlight the substantial impact of land conversion on peatland CH4 dynamics, emphasizing the need for accurate flux measurements across various conversion stages to refine global CH4 budgets.
Tropical peat forests are a globally important reservoir of carbon, but little is known about CO2 exchange on an annual basis. We measured CO2 exchange between the atmosphere and tropical peat swamp forest in Sarawak, Malaysia using the eddy covariance technique over 4 years from 2011 to 2014. The CO2 fluxes varied between seasons and years. A small carbon uptake took place during the rainy season at the beginning of 2011, while a substantial net efflux of >600 g C/m2 occurred over a 2 month period in the middle of the dry season. Conversely, the peat ecosystem was a source of carbon during both the dry and rainy seasons in subsequent years and more carbon was lost during the rainy season relative to the dry season. Our results demonstrate that the forest was a net source of CO2 to the atmosphere during every year of measurement with annual efflux ranging from 183 to 632 g C m-2 year-1 , noting that annual flux values were sensitive to gap filling methodology. This is in contrast to the typical view of tropical peat forests which must have acted as net C sinks over time scales of centuries to millennia to create the peat deposits. Path analyses revealed that the gross primary productivity (GPP) and ecosystem respiration (RE) were primarily affected by vapour pressure deficit (VPD). Results suggest that future increases in VPD could further reduce the C sink strength and result in additional net CO2 losses from this tropical peat swamp forest in the absence of plant acclimation to such changes in atmospheric dryness.