Simulating the effects of rainforest to oil palm conversion on carbon, water and energy fluxes and carbon stocks

In Southeast Asia, the rapidly expanding oil palm monoculture and associated destruction of rainforests and peatlands have been in the spotlight for their prominent roles in greenhouse gas (GHG) emission and biodiversity loss. Industrial oil palm plantations have achieved the highest yield per hectare of oil production among major crops, providing substantial economic benefits and agricultural commodities such as biofuels, cosmetics and edible oil. A thorough quantification of the carbon (C) balance of oil palm plantations and the long-term and large-scale forest – oil palm replacement effects is necessary, however, still lacking, for understanding the climatic impacts of tropical land use change (LUC). This study presents a complete modeling of the C dynamics of oil palm plantations throughout development stages (from planting to rotation) and the associated nitrogen (N), water and energy exchanges between oil palms, soil and the atmosphere at ecosystem scale with a land surface modeling approach. A new modeling scheme for palm species (named CLM-palm) has been developed within the Community Land Model framework. CLM-Palm incorporates the agricultural model capacity for simulating the growth and yield of oil palm with unique phenology and allocation functions and a suit of new parameterizations on biogeophysics (e.g. radiative transfer and evapotranspiration (ET)) and on biogeochemistry (e.g. C and N dynamics, fertilization effects). An application employing these model developments exemplifies how LUC in Indonesia, specifically rainforests conversion to oil palm plantations, affects C sequestration and water and energy cycling of tropical lands. With reference to eddy covariance flux measurements, sap flow estimates and remote sensing-derived surface variables, the simulation results show clear distinctions between young and mature oil palm plantations and old-growth rainforest in C fluxes (e.g. gross primary production (GPP) and net ecosystem exchange (NEE)) and biophysical properties (e.g. ET, surface albedo and temperature). An oil palm plantation can quickly catch up and surpass the C assimilation and water use rates of a rainforest through growth development, but the plantation has a general warmer ground surface than the forested site even after maturity. A transient simulation spanning two rotation periods (each 25 years) showed that long-term oil palm cultivation is only able to restore about a half of the original C storage capacity of the forested site before clear-cut. More than 50% of the assimilated C by the oil palm plantation is not retained on the site but instead exported as oil products which are consumed by humans and reverted to CO2 soon. Soil C stock declines slowly and gradually due to limited litter return in the managed plantation. Overall, rainforest to oil palm conversion reduces long-term C stocks and C sequestration capacity and has potential warming effects on the land surface at the site scale, despite the fast growth and high C assimilation rate of the heavily fertilized plantation. (Texte intégral)