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.

the recent expansion of oil palm (op, Elaeis guineensis) plantations into tropical forest peatlands has resulted in ecosystem carbon emissions. However, estimates of net carbon flux from biomass changes require accurate estimates of the above ground biomass (AGB) accumulation rate of op on peat. We quantify the AGB stocks of an OP plantation on drained peat in Malaysia from 3 to 12 years after planting using destructive harvests supported by non-destructive surveys of a further 902 palms. Peat specific allometric equations for palm (R 2 = 0.92) and frond biomass are developed and contrasted to existing allometries for op on mineral soils. Allometries are used to upscale AGB estimates to the plantation block-level. Aboveground biomass stocks on peat accumulated at ~6.39 ± 1.12 Mg ha −1 per year in the first 12 years after planting, increasing to ~7.99 ± 0.95 Mg ha −1 yr −1 when a 'perfect' plantation was modelled. High inter-palm and inter-block AGB variability was observed in mature classes as a result of variations in palm leaning and mortality. Validation of the allometries defined and expansion of non-destructive inventories across alternative plantations and age classes on peat would further strengthen our understanding of peat op AGB accumulation rates.Global demand for palm oil has risen such that the land area supporting oil palm (OP, Elaeis guineensis) plantations has increased to ~25 Mha globally; making OP the 12 th largest edible crop by land area 1 . The rapid expansion of OP in Insular Southeast Asia during the last quarter decade has resulted in the conversion of 3.1 Mha of tropical peatlands 2 . The carbon emissions from the oxidation of soil organic matter following the conversion of peat swamp forest to OP are relatively well known, yet the net carbon emission of peat swamp forest conversion to OP across the life of a plantation remains poorly constrained 3-6 . In part, uncertainty is attributed to a scarcity of literature which addresses the rate at which OP on peat accumulates carbon in biomass over time [6][7][8][9][10] . The majority of OP standing biomass is stored as aboveground biomass (AGB) constituting 84% of biomass stocks, with the reminder (16%) stored as belowground biomass (BGB); consequently, efforts here focus primarily on AGB quantification [11][12][13] .Recent efforts to quantify the AGB stocks of forests and plantations have increasingly used remote sensing techniques 14,15 . However, remote sensing estimates ultimately rely on direct ground-based measurement of AGB stocks either for calibration or validation 15,16 . Forest and plantation vegetation is destructively harvested to obtain the vegetation dry-weight (DW) and infer biomass carbon stocks (~47.4% of dry biomass) 17,18 . These destructive measurements are essential but are costly in terms both of time and resources; allometric equations which relate AGB stocks to non-destructive or semi-destructive measurements of vegetation structural characteristics are therefore invaluable 18,19 . Destructive and non-destructive AGB ...

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