Short‐ and long‐term carbon emissions from oil palm plantations converted from logged tropical peat swamp forest

Abstract

Need for regional economic development and global demand for agro‐industrial commodities have resulted in large‐scale conversion of forested landscapes to industrial agriculture across South East Asia. However, net emissions of CO2 from tropical peatland conversions may be significant and remain poorly quantified, resulting in controversy around the magnitude of carbon release following conversion. Here we present long‐term, whole ecosystem monitoring of carbon exchange from two oil palm plantations on converted tropical peat swamp forest. Our sites compare a newly converted oil palm plantation (OPnew) to a mature oil palm plantation (OPmature) and combine them in the context of existing emission factors. Mean annual net emission (NEE) of CO2 measured at OPnew during the conversion period (137.8 Mg CO2 ha−1 year−1) was an order of magnitude lower during the measurement period at OPmature (17.5 Mg CO2 ha−1 year−1). However, mean water table depth (WTD) was shallower (0.26 m) than a typical drainage target of 0.6 m suggesting our emissions may be a conservative estimate for mature plantations, mean WTD at OPnew was more typical at 0.54 m. Reductions in net emissions were primarily driven by increasing biomass accumulation into highly productive palms. Further analysis suggested annual peat carbon losses of 24.9 Mg CO2‐C ha−1 year−1 over the first 6 years, lower than previous estimates for this early period from subsidence studies, losses reduced to 12.8 Mg CO2‐C ha−1 year−1 in the later, mature phase. Despite reductions in NEE and carbon loss over time, the system remained a large net source of carbon to the atmosphere after 12 years with the remaining 8 years of a typical plantation's rotation unlikely to recoup losses. These results emphasize the need for effective protection of tropical peatlands globally and strengthening of legislative enforcement where moratoria on peatland conversion already exist.

Separating autotrophic and heterotrophic soil CO<sub>2</sub> effluxes in afforested peatlands

“…In other parts of the world, peatland drainage is still actively happening and studies from these sites show very high rates of peat oxidation during the first 5-10 years of conversion (e.g. McCalmont et al, 2021).…”

Net soil carbon balance in afforested peatlands and separating autotrophic and heterotrophic soil CO<sub>2</sub> effluxes

“…In other parts of the world, peatland drainage is still actively happening, and studies from these sites show very high rates of peat oxidation during the first 5-10 years of conversion (e.g. Cooper et al, 2020;McCalmont et al, 2021;Minkkinen et al, 2018;Prananto et al, 2020). In the UK, forest plantations on deep peat sometimes end in clear felling of the site and restoration of the peat.…”

Temporal Variability in Heterotrophic Carbon Dioxide Emissions From A Drained Tropical Peatland in Uganda

“…Southeast Asia provides a strong set of measurements from drained peatlands, in particular under oil palm plantations. Fluxes from SE Asia are highly variable, with an Rh of 18-56 t CO2 ha-1 yr-1 in drained forest and oil palm in Malaysia (31), 74-111 t CO 2 ha -1 yr -1 from a range of studies on drained oil palm plantations in SE Asia (32), and 24.9 t CO 2 ha -1 yr -1 in a young oil palm plantation in Borneo (33). The Intergovernmental Panel on Climate Change emissions factor for drained cropland and fallow in the tropics is 51 t CO 2 ha -1 yr -1 (34).…”