Impact of forest plantation on methane emissions from tropical peatland

Abstract

Tropical peatlands are a known source of methane (CH₄ ) to the atmosphere, but their contribution to atmospheric CH₄ 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 CH₄ emissions. We measured CH₄ 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 CH₄ exchanges over the natural forest (9.1 ± 0.9 g CH₄ m^-2 year^-1 ) were around twice as high as those of the Acacia plantation (4.7 ± 1.5 g CH₄ m^-2 year^-1 ). Results highlight that tropical peatlands are significant CH₄ sources, and probably have a greater impact on global atmospheric CH₄ concentrations than previously thought. Observations showed a clear diurnal variation in CH₄ exchange over the natural forest where the GWL was higher than 40 cm below the ground surface. The diurnal variation in CH₄ 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 CH₄ 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 CH₄ exchange data reported for any tropical peatland, should help to reduce the uncertainty in the estimation of CH₄ 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.