1) Research Objectives,
Oil palm (Elaeis guineensis Jacq.) plantations are of major research interest due to its high economic relevance on the one hand and its potential ecological impacts on the other hand. Besides ecological impacts, also alterations of the water budget at landscape level are expected. A recent study compared the transpiration and evapotranspiration fluxes in two oil palm plantations of different ages and suggests an important role for water storage in and on the plant for partitioning the water fluxes. (Roll, et al., 2015). Atmospheric forcings (vapour pressure deficit (VPD) and global radiation) were not sufficient to explain the dynamics of evapotranspiration and transpiration at the stand level. Stable water isotope measurement (δ D and δ 18O) may provide the missing information needed to disentangle the different evaporative sources.
2) Study site
The study location will be in a 12-years old oil palm plantation in the PTPN6 site near the city of Jambi, Sumatra (Indonesia).
A 22 meters flux tower is in place and it is operated by the University of Göttingen in the framework of the CRC project (subproject A03).
3) Study goal
With the present study, we aim to quantitatively understand the partitioning of evapotranspiration into evaporation and transpiration in the oil palm plantation. Answering this question could provide a better description of the plantation water cycle and budget (A02 and A03) which would then help informing and improving biophysical and biogeochemical oil palm models (A07).
4) Methodology and concept
Task 1 - Measuring the water vapor isotopic composition along a vertical profile through the oil palm plantation.
Two weeks of synchronous measurements of water vapor fluxes using the eddy covariance technique (Baldocchi, 2003) and isotopic measurement (δ D and δ 18O) and concentration of water vapour using the Wavelength-Scanned Cavity Ring Down Spectroscopy (CRDS, Picarro® Inc. 2120i, Sunnyvale, CA, USA) will be performed. The water vapor isotopes measurements will be carried out continuous, and at high frequency (0,5 Hz) at 5 different heights from 1 meter above soil level to the top of the flux tower, at 22 meters, using a set of heated Teflon tubes coupled to a valves switching system. Eddy covariance measurements (10 Hz) will run in parallel and will be used to validate the water vapor concentration measured by the CRDS. Sampling height will switch every 12 minutes to ensure an hourly cycle and the first 2 minutes of every sampled air will be discarded to minimize the memory effect between samples.
Instrumental calibration will be carried out before, halfway through and at the end of the sampling campaign. Due to the short measurement campaign we do not expect large drifts in the isotopic signal.
Task 2 - Direct measurements of the isotopic composition of leafs, twigs, soil, precipitation and water collected from the axillas.
In order to quantify the partitioning of the water vapor sources through the stand we will collect rain samples using custom made pluviometers (3 pluviometers will provide the required replicates). Additionally, we will sample leaf, stem and soil on which we will perform cryogenic extractions of the water content in a dedicated laboratory at Göttingen University. Collected samples will be stored in glass vials and the caps sealed with Parafilm to prevent evaporation. Samples will be stored at room temperature during transport.
The water extracted will then be analyzed using a similar CRDS in use at the University of Göttingen. An attempt to sample the water collected in the butts of the petioles (axillas) will be performed using cotton balls because it is known that the material contained in this "pockets" is composed by a mixture of organic material and water and the resultant extracted water could contain a relativlely high amount of organic compounds that could affect the isotopic measurements.