JULES-Palm: Explicit representation of oil palm plantations in a land surface model for estimating land-use change carbon emissions in Indonesia

Abstract. Expansion of oil palm plantations is a dominant driver of land-use change in equatorial Asia and has contributed substantially to deforestation in Indonesia. Most dynamic global vegetation models (DGVMs), however, still represent oil palm plantations as generic cropland, thereby contributing to uncertainty in carbon emissions from land-use change (ELUC). We develop a new oil palm plant functional type (PFT) using the JULES land surface model, called JULES-Palm. This PFT incorporates oil palm-specific physiological, structural and plantation-management traits, including rotation-based harvest/replanting cycles, and is parameterised from literature-derived values before calibration using eddy covariance (EC) and allometric observations, including leaf area index (LAI), above-ground biomass and canopy height. This calibration improves simulated photosynthesis and the vegetation structure. We then ran JULES-Palm to quantify the contribution of oil palm plantations to Indonesian ELUC, using land-use drivers from the Mapbiomas Indonesia 2.0 land cover maps (2000–2022). We investigated three scenarios: (i) oil palm represented as a generic crop, (ii) oil palm omitted and treated as natural vegetation and (iii) explicit oil palm PFT. The mean and interannual variability of Indonesian ELUC (without peat emissions) is 0.058 ± 0.012 PgC yr^-1 with the oil palm PFT and 0.052 ± 0.010 PgC yr^-1 when oil palm is omitted; representing oil palm as a generic cropland gives 0.066 ± 0.011 PgC yr^-1. Cumulatively, the expansion of oil palm plantations by 10.5 Mha since 2000 contributed approximately 0.14 PgC, accounting for 10.3 % of Indonesia’s total ELUC including legacy fluxes, but approximately 89 % of land-use change attributable to conversions occurring during 2000–2022. Representing oil palm as a dedicated PFT reduces the estimated oil-palm-related ELUC relative to the generic-cropland representation by capturing plantation-specific biomass accumulation and carbon turnover. Thus, using a specific PFT for oil palm is preferable to providing a more accurate, process-based representation of plantation carbon dynamics, but treating oil palm as cropland still captures a first-order estimation of ELUC when a dedicated PFT is unavailable.