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In order to assess water balance of rubber plantations at plot scale, accurate dynamic measurements of tree transpiration is required. Sapflow was measured in a 12 years old plantation (RRIM 600) in eastern Thailand, by heat dissipation method (Granier 1985, 1987) using home-made 20 mm-long radial probes, continuously heated (0.2 W), and connected to a data logger. The calibration of the homemade probes was checked in the laboratory with reference to the gravimetric method. We adjusted the experimental devices to rubber field conditions and evaluated the different sources of variability to design the appropriate monitoring process. Natural thermal gradient was not significant. Vertical variation along the trunk below canopy was low. Conversely a significant azimuthal variation was recorded, though without specific trend. Accordingly 2 probes (one on the North face, one on the South face), located above the tapping panel were used for monitoring. Radial distribution of sapflow was recorded, using a long probe, and our results showed significant variations of sapflow along the radius. A function was derived from these radial variations to estimate total tree sapflow from measurements within the outer 2 cm of sapwood. Finally to evaluate stand transpiration (mm h-1), trees were sampled according to trunk diameter. Such data will be used to partition evapotranspiration measured on the same plot by eddy-correlation between tree transpiration, T, and soil and understorey evaporation, E. (Texte intégral)

Carbon sequestration potential of rubber plantations may provide opportunities to increase the profitability and acceptability of plantations. Thus, carbon exchange of the plantations has to be assessed accurately. A complete system has been installed in eastern Thailand (Chachoengsao Rubber Research Center, RRIT-DOA) in 2006. Carbon fluxes of rubber plantation ecosystem are continuously measured by eddy-correlation method (CO2 exchanges between the ecosystem and the atmosphere). Evapo-transpiration (ETR) are measured by eddy-correlations and water balance together and partitioned between tree transpiration and soil evaporation. Meanwhile, amounts of C stored in the trees will be evaluated by measuring biomass increment along the life cycle of the plantation, in combination with estimations of the carbon content of the different compartments. These measurements will provide the annual balance of C within plantations at different ages. Energy balance will be assessed by measurements of net radiation (Rn) and estimation of energy partitioning among heat fluxes and heat storage. Results obtained at ecosystem scale by these methods will be compared to gas exchanges measured at the level of the different compartments (canopy, trunk, root system, soil...). Thereby, validated CO2 and H2O fluxes will be used to model gas exchanges of rubber plantation ecosystem according to climate and other environmental parameters as well as crop management. (Texte intégral)

After two years of tapping in Chachoengsao province, uprooting of 12 trees (clones RRIM 600, PB 235, GT1, 4 trees per clone) provided estimations of the relations existing between the total dry biomass of rubber trees and their trunk girth. These relations were found linear at least inside the studied range (trunk girth varying from 45 cm to 65 cm), but different between clones. A general model, able to estimate in one location the biomass of all clones might perhaps be set up by taking into account not only the trunk girth but also some other variables describing canopy structure, as the canopy biomass cannot be described by the trunk girth only. Nevertheless, finding and parameterizing such a model requires further survey. Thanks to the linear regression models obtained between biomass and trunk girth, biomass of the trees could be estimated at the moment of opening and after two years of tapping. Biomass partition variables could be calculated, allowing estimation of the harvest index and k factor, accounting for rubber production efficiency. Nevertheless, as these models rely on the uprooting of only 4 trees per clone, an extended survey is required to confirm their accuracy. Competition between growth and production was found linear for all three clones, confirming numerous previous studies. Clone GT1 showed a rather bad behavior regarding the competition between growth and rubber production. Clones PB 235 and RRIM 600 showed much better behaviors with lower competition between growth and production. Moreover, RRIM 600 seemed to maintain a significant growth compared to other clones during the tapping stop period (February to April 2000, dry season), sign of a better water use effectiveness. Rubber breeders in rather dry areas should aim at such a characteristic .and systematic survey of clonal girth increments during the dry season could provide a reliable picture of clonal susceptibility to draught and water stress.