Merten, J., A. Röll, T. Guillaume, A. Meijide, S. Tarigan, H. Agusta, C. Dislich, C. Dittrich, H. Faust, D. Gunawan, J. Hein, . Hendrayanto, A. Knohl, Y. Kuzyakov, K. Wiegand, and D. Hölscher. 2016. Water scarcity and oil palm expansion: social views and environmental processes. Ecology and Society 21(2):5.http://dx.doi.org/10.5751/ES-08214-210205

Oil palm (Elaeis guineensis Jacq.) plantations cover large and continuously increasing areas of humid tropical lowlands. Landscapes dominated by oil palms usually consist of a mosaic of mono-cultural, homogeneous stands of varying age, which may be heterogeneous in their water use characteristics. However, studies on the water use characteristics of oil palms are still at an early stage and there is a lack of knowledge on how oil palm expansion will affect the major components of the hydrological cycle. To provide first insights into hydrological landscape-level consequences of oil palm cultivation, we derived transpiration rates of oil palms in stands of varying age, estimated the contribution of palm transpiration to evapotranspiration, and analyzed the influence of fluctuations in environmental variables on oil palm water use. We studied 15 two- to 25-year old stands in the lowlands of Jambi, Indonesia. A sap flux technique with an oil palm specific calibration and sampling scheme was used to derive leaf-, palm- and stand-level water use rates in all stands under comparable environmental conditions. Additionally, in a two- and a 12-year old stand, eddy covariance measurements were conducted to derive evapotranspiration rates. Water use rates per leaf and palm increased 5-fold from an age of 2 years to a stand age of approx. 10 years and then remained relatively constant. A similar trend was visible, but less pronounced, for estimated stand transpiration rates of oil palms; they varied 12-fold, from 0.2 mm day⁻¹ in a 2-year old to 2.5 mm day⁻¹ in a 12-year old stand, showing particularly high variability in transpiration rates among medium-aged stands. Comparing sap flux and eddy-covariance derived water fluxes suggests that transpiration contributed 8 % to evapotranspiration in the 2-year old stand and 53 % in the 12-year old stand, indicating variable and substantial additional sources of evaporation, e.g., from the soil, the ground vegetation and from trunk epiphytes. Diurnally, oil palm transpiration rates were characterized by an early peak between 10 and 11 a.m.; there was a pronounced hysteresis in the leaf water use response to changes in vapor pressure deficit for all palms of advanced age. On the day-to-day basis this resulted in a relatively low variability of oil palm water use regardless of fluctuations in vapor pressure deficit and radiation. We conclude that oil palm dominated landscapes show some spatial variations in (evapo)transpiration rates, e.g., due to varying age-structures, but that the temporal variability of oil palm transpiration is rather low. The stand transpiration of some of the studied oil palm stands was as high or even higher than values reported for different tropical forests, indicating a high water use of oil palms under yet to be explained site or management conditions. Our study provides first insights into the eco-hydrological characteristics of oil palms as well as a first estimate of oil palm water use across a gradient of plantation age. It sheds first light on some of the hydrological consequences of the continuing expansion of oil palm plantations.

Oil palm (Elaeis guineensis Jacq.) water use was assessed by sap flux density measurements with the aim to establish the method and derive water-use characteristics. Thermal dissipation probes were inserted into leaf petioles of mature oil palms. In the laboratory, we tested our set-up against gravimetric measurements and derived new parameters for the original calibration equation that are specific to oil palm petioles. In the lowlands of Jambi, Indonesia, in a 12-year-old monoculture plantation, 56 leaves on 10 palms were equipped with one sensor per leaf. A 10-fold variation in individual leaf water use among leaves was observed, but we did not find significant correlations to the variables trunk height and diameter, leaf azimuthal orientation, leaf inclination or estimated horizontal leaf shading. We thus took an un-stratified approach to determine an appropriate sampling design to estimate stand transpiration (E_s, mm day⁻¹) rates of oil palm. We used the relative standard error of the mean (SE_n, %) as a measure for the potential estimation error of E_s associated with sample size. It was 14% for a sample size of 13 leaves to determine the average leaf water use and four palms to determine the average number of leaves per palm. Increasing these sample sizes only led to minor further decreases of the SE_n of E_s. The observed 90-day average of E_s was 1.1 mm day⁻¹ (error margin ± 0.2 mm day⁻¹), which seems relatively low, but does not contradict Penman–Monteith-derived estimates of evapotranspiration. Examining the environmental drivers of E_s on an intra-daily scale indicates an early, pre-noon maximum of E_s rates (11 am) due to a very sensitive reaction of E_s to increasing vapor pressure deficit in the morning. This early peak is followed by a steady decline of E_s rates for the rest of the day, despite further rising levels of vapor pressure deficit and radiation; this results in pronounced hysteresis, particularly between E_s and vapor pressure deficit.