Impact of forest plantation on methane emissions from tropical peatland

Abstract

Tropical peatlands are a known source of methane (CH ₄) to the atmosphere, but their contribution to atmospheric CH ₄ is poorly constrained. Since the 1980s, extensive areas of the peatlands in Southeast Asia have experienced land‐cover change to smallholder agriculture and forest plantations. This land‐cover change generally involves lowering of groundwater level (GWL), as well as modification of vegetation type, both of which potentially influence CH ₄ emissions. We measured CH ₄ exchanges at the landscape scale using eddy covariance towers over two land‐cover types in tropical peatland in Sumatra, Indonesia: (a) a natural forest and (b) an Acacia crassicarpa plantation. Annual CH ₄ exchanges over the natural forest (9.1 ± 0.9 g CH ₄ m ⁻² year ⁻¹) were around twice as high as those of the Acacia plantation (4.7 ± 1.5 g CH ₄ m ⁻² year ⁻¹). Results highlight that tropical peatlands are significant CH ₄ sources, and probably have a greater impact on global atmospheric CH ₄ concentrations than previously thought. Observations showed a clear diurnal variation in CH ₄ exchange over the natural forest where the GWL was higher than 40 cm below the ground surface. The diurnal variation in CH ₄ exchanges was strongly correlated with associated changes in the canopy conductance to water vapor, photosynthetic photon flux density, vapor pressure deficit, and air temperature. The absence of a comparable diurnal pattern in CH ₄ exchange over the Acacia plantation may be the result of the GWL being consistently below the root zone. Our results, which are among the first eddy covariance CH ₄ exchange data reported for any tropical peatland, should help to reduce the uncertainty in the estimation of CH ₄ emissions from a globally important ecosystem, provide a more complete estimate of the impact of land‐cover change on tropical peat, and develop science‐based peatland management practices that help to minimize greenhouse gas emissions.

Abstract

We measured methane exchanges at the landscape scale using eddy covariance towers over a natural forest and an Acacia crassicarpa plantation in tropical peatland in Sumatra, Indonesia. Groundwater level (GWL) controls diurnal and seasonal variability in methane emissions. Annual methane emissions over the natural forest were around twice as high as those of the Acacia plantation. Lower methane emissions over the Acacia plantation may be attributed to lower GWL. Results highlight that tropical peatlands are significant methane sources, and probably have a greater impact on global atmospheric methane concentrations than previously thought.