Effects of irrigation on oil palm transpiration during ENSO-induced drought in the Brazilian Eastern Amazon

Oil palm plantations are rapidly expanding in the Brazilian eastern Amazon. Further expansion of irrigated oil palm plantations may occur because they may improve productivity during drought events compared to rainfedonly plantations. To investigate the importance of irrigation to alleviate drought-induced stress on palm trees, and its potential as a management alternative, we monitored transpiration (T, mm day(-1)) in a five-year-old oil palm plantation during an extreme drought caused by the El-Nino Southern Oscillation (ENSO) event in the Amazon basin. Our study spanned over the dry season (DS) and the wet season (WS), using an ongoing irrigation experiment (control with no irrigation, drip and sprinkler irrigation), to identify how differences in water supply affect the oil palm T during a period of reduced precipitation induced by ENSO. Our results indicate that independently of the irrigation system, T was higher in the DS, compared to the WS. Higher soil moisture in both the irrigation treatments, relative to the control plot, allowed for 23% higher average T relative to the control plot. The vapor pressure deficit (VPD) variation presented a higher effect over oil palm transpiration than photosynthetic active radiation. In both irrigation systems, a higher rate of transpiration with increases in VPD was observed than in the control plot during ENSO drought. Additionally, within individual palm fronds, we also observed differences in VPD tolerance (higher for lower and older leaves) and avoidance (higher for upper and newer leaves). Results indicate that irrigation increased fruit production by 35% and 26% in the sprinkler and drip irrigation systems, respectively. Overall, these results demonstrate the potential role of irrigation to alleviate water stress and maintain productivity of palm oil plantations during periods of moisture stress, while highlighting the possible feedback of increasing transpiration on water balance and hydrologic regulation.