El Niño–Southern Oscillation (<i>ENSO</i>) event reduces CO₂ uptake of an Indonesian oil palm plantation

The El Niño–Southern Oscillation (ENSO) in 2015 was one of the strongest observed in almost 20 years and set the stage for a severe drought and the emergence of widespread fires and related smoke emission over large parts of Southeast Asia. In the tropical lowlands of Sumatra, which were heavily affected by the drought and haze, large areas of tropical rainforest have been converted into oil palm (Elaeis guineensis Jacq.) plantations during the past decades. In this study, we investigate the impact of drought and smoke haze on the CO2 exchange, evapotranspiration and surface energy budget in a commercial oil palm plantation in Jambi province (Sumatra, Indonesia) by using micrometeorological measurements, the eddy covariance method and a multi linear regression model (MLRM). With the MLRM we identify the contribution of meteorological and environmental parameters to net ecosystem CO2 exchange. During the initial part of the drought, when incoming shortwave radiation was elevated, CO2 uptake increased by 50 % despite a decrease in upper-layer soil moisture by 35 %, an increase in air temperature by 10 % and a tripling of atmospheric vapor pressure deficit. Emerging smoke haze decreased incoming solar radiation by 35 % compared to non-drought conditions and diffuse radiation became almost the sole shortwave radiation flux for two months resulting in a strong decrease in CO2 uptake by 86 %. Haze conditions resulted in a complete pause of oil palm carbon uptake for about 1.5 months and contributed to a decline in oil palm yield by 35 %. With respect to climate change and a pronounced drying trend over the western Pacific during El Niño, our model showed that an increase in drought may stimulate CO2 uptake while more severe smoke haze, in combination with drought, can lead to pronounced losses in productivity and CO2 uptake highlighting the importance of fire prevention.