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 (137.8MgCO2 ha-1 year-1 ) was an order of magnitude lower during the measurement period at OPmature (17.5MgCO2 ha-1 year-1 ). However, mean water table depth (WTD) was shallower (0.26m) than a typical drainage target of 0.6m suggesting our emissions may be a conservative estimate for mature plantations, mean WTD at OPnew was more typical at 0.54m. Reductions in net emissions were primarily driven by increasing biomass accumulation into highly productive palms. Further analysis suggested annual peat carbon losses of 24.9MgCO2 -Cha-1 year-1 over the first 6years, lower than previous estimates for this early period from subsidence studies, losses reduced to 12.8MgCO2 -Cha-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 12years with the remaining 8years 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.
Highlights
The need for economic development across South East Asia, and global demand for agro-industrial commodities such as palm oil, rubber and pulp wood have driven the expansion of industrial- scale agriculture and associated land-use change in recent decades
Values ± given throughout these results indicate the standard error of the mean (SEM)
Despite our results reporting lower peat carbon loss in the early years following conversion than subsidence studies might have suggested, there is no doubt that these emissions remain extremely significant and peat swamp forests (PSFs) conversion to agriculture results in very large net emissions of CO2
Summary
The need for economic development across South East Asia, and global demand for agro-industrial commodities such as palm oil, rubber and pulp wood have driven the expansion of industrial- scale agriculture and associated land-use change in recent decades. In the absence of field studies of oil palm peatland conversions across the entire cultivation lifetime, emission factors determined for tropical forest conversion to agriculture on peatland have so far had to rely on very limited data In deriving their Tier 1 emission factor of 40 Mg CO2 ha−1 year−1 for conversions to oil palm on drained peatland, the IPCC list only eight direct studies, of which six were soil flux chamber studies and two were based on subsidence measurements. We present annual net ecosystem CO2 fluxes from individual measurement years at both sites and partition them into photosynthetic uptake and whole ecosystem respiration We combine both data sets into a single chronosequence over a 151-month period and use a mass balance approach (incorporating estimates of biomass accumulation and forest residue decomposition) to calculate changes in soil carbon stocks. We investigate the relationship between soil water drainage and carbon loss in the context of previous emission coefficients and consider the potential impact of changes in plantation drainage targets
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