Abstracts Biological Sciences

Transpiration by oil palm and rubber plantations: refining methods and delineating differences

by Furong Niu

Oil palm (Elaeis guineensis Jacq.) and rubber (Hevea brasiliensis Müll. Arg.) plantations cover large areas in tropical regions and may still expand further. In contrast, the area covered by natural forests has strongly declined. From environmental perspectives, this raises concerns not only with respect to biodiversity but also regarding the integrity of the hydrological cycle including potential changes in transpiration. For rubber plantations, high evapotranspiration rates were reported from mainland Asia and it was indicated that rubber tree transpiration responds sensitively to dynamics in environmental drivers. Prior to our studies, little scientifically based information was available for oil palm. The objective of this study was to test and if necessary adjust a sap flux measurement technique for the two species, to analyze influences of plantation age and environmental drivers on oil palm and rubber tree transpiration and to delineate differences between the two plantation types. The study was implemented in the lowlands of Sumatra, Indonesia. Methods include lab experiments with thermal dissipation probes and gravimetric reference measurements, three different sap flux methods applied in the field (thermal dissipation probes, heat field deformation and stem heat balance), and a monitoring of sap flux on a total of 25 plots in lowland Sumatra. Thus far, sap flux measurements had not been applied to oil palms. We first calibrated the thermal dissipation probe method (TDP) after Granier (1985) and tested a field sampling scheme. The probes were inserted into leaf petioles, as most likely the density of vessels is higher and vessels are distributed more homogenously than in the trunk. In the laboratory, we tested this set-up against gravimetric measurements and derived new, oil palm specific parameters for the calibration equation. In the field, in a 12-year-old plantation, 56 leaves on 10 palms were equipped with TDP sensors. We did not find significant influences of a series of studied variables and thus took an un-stratified approach to determine an appropriate sampling scheme to estimate stand transpiration rates. The relative standard error of the mean was used as a measure for the potential estimation error of stand transpiration 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 relative standard errors of the mean. Oil palm transpiration rates were then monitored on 15 on-farm plots using this sap flux technique and sampling scheme in conjunction with eddy covariance measurements at two sites. Oil palm water use and transpiration increased from two to about eight-year old plantations and then leveled off up to an age of 22 years. Among medium-aged, 10 to 18 years old plantations, substantial spatial heterogeneity was found, with the highest oil palm stand transpiration rates (2.5 mm d–1) in a highly… Advisors/Committee Members: Hölscher, Dirk (advisor), Knohl, Alexander (referee), Kreft, Holger (referee).