Continuous measurement of canopy CO₂:H₂O fluxes of an oil palm plantation using the eddy correlation technique

The eddy correlation technique was used to measure directly the exchanges of carbon dioxide and water vapour between the atmosphere and the vegetation of an oil palm plantation. The plantation was at on an inland site at Sintok, Kedah that has a distinct dry period at the start and the end of each year (i.e. January-March and December). The DxP palms were planted in July 2000 and hence were about four years old when measured in 2004 when their canopy had not yet closed. The leaf area index for the palms in June 2004 was about 3.37 as measured by conventional techniques. An 8.7 meter tall scaffold-tower was installed in mid-February 2004 to support the instruments. Carbon dioxide and water vapor fluxes above the palm canopy were continuously monitored with a Campbell eddy correlation system and readings were recorded at hourly intervals. Power to the system was provided by two 12VI150Ah solar deep cycle gel batteries and three 75 watts crystalline solar panels. The battery voltage record showed that the batteries were able to provide sufficient power to the equipment at night, while the solar panels were able to recharge the batteries and power the equipments during the daytime. Solar and net radiations were measured using a silicon energy sensor and a Q7 net radiometer respectively. Soil heat flux was measured using soil heat flux plates buried 1 cm below the soil surface and installed at 1, 2 and 4 meter distances from the trunk of a palm. The mean solar radiation recorded during the dry period in February 2004 and the wet period in May 2004 was 19.6 and 17.2 MJlm²/day respectively. The mean hourly daytime CO₂ flux to the canopy, after adjustment for the simultaneous fluxes of sensible heat and water vapour, was about 0.54 glm₂/h in February 2004 and 1.92 glm₂/h in May. A very low rainfall of about 170.6 mm was recorded in February 2004 while the total rainfall received in May 2004 was over 2407mm. Uptake of CO₂ by the canopy is presented as a positive flux and release of CO₂ as negative flUX. The fluxes are not exclusively due to the gas exchange of the palms as CO₂ was also assimilated by other ground vegetation and released by respiration of weeds and soil micro-organisms. However, very little ground vegetation was observed during the dry months as compared to the wet months. Diurnal changes in the CO₂ flux did not always coincide with changes in radiation and often, CO₂ uptake fell during the latter part of each day, as vapour pressure deficit increased. In conclusion, the system was found to be suitable for the collection of continuous hourly CO₂ and water vapour fluxes in oil palm plantations.