TRANSPIRATION, GROWTH AND LATEX PRODUCTION OF A HEVEA BRASILIENSIS STAND FACING DROUGHT IN NORTHEAST THAILAND: THE USE OF THE WaNuLCAS MODEL AS AN EXPLORATORY TOOL

In order to get the benefit of the growing world demand for natural rubber, Hevea brasiliensis is increasingly planted in drought-prone areas, such as in the southern part of northeast Thailand. Modelling can be a useful approach in identifying key points of improvement for rubber tree cultivation in such water-limited areas. The first objective of this study was to test the possibility of using the Water Nutrients and Light Capture in Agroforestry Systems (WaNuLCAS) model as an exploratory tool to simulate water use, growth and latex production in a pure stand on a daily basis. The second was to evaluate the relative accuracy of predictions with the current model version. Finally, the third aim of this study was to identify particular parameterisations that may be adapted to improve overall prediction quality. Model outputs were compared to measurements recorded in a mature rubber tree stand of RRIM 600 clones growing in the water-limited area of northeast Thailand. The period of analysis concerned seven months of full foliation, from May to November, including a severe drought spell. Whole-tree transpiration was estimated by xylem sap flow measurement from 11 trees. The results show that the model was able to simulate daily and seasonal change of soil water content, tree transpiration, girth increment and latex production within plausible ranges. However, under detailed scrutiny, the predictions show large inaccuracies compared to the observations: soil water content (determination coefficient (R²) = 0.461, relative root mean square error (RMSEᵣₑₗ) = 35%), tree transpiration (R² = 0.104, RMSEᵣₑₗ = 94%), tree girth increment (R² = 0.916, RMSEᵣₑₗ = 208%) and latex production (R² = 0.423, RMSEᵣₑₗ = 169%). As soil water content was overestimated during the driest periods, no water stress was predicted and transpiration, growth and latex production were logically overestimated during such periods. However, tree transpiration was also largely overestimated in conditions of non-limiting soil water availability with high evaporative demand. Hence, two key points of parameterisation and improvement are identified for better simulation in our conditions: the soil water balance and particularly the ratio between water infiltration and run-off, and the regulation of transpiration under high evaporative demand. In conclusion, the WaNuLCAS model is usable as an exploratory model to simulate water use, growth and production for a pure rubber tree stand. However, in our conditions of much degraded soil and high evaporative demand, the modules of soil water balance and tree transpiration require particular parameterisations and improvement.