Charlchai Tanavud

A simple framework to analyze water constraints on seasonal transpiration in rubber tree (Hevea brasiliensis) plantations

Climate change and fast extension in climatically suboptimal areas threaten the sustainability of rubber tree cultivation. A simple framework based on reduction factors of potential transpiration was tested to evaluate the water constraints on seasonal transpiration in tropical sub-humid climates, according pedoclimatic conditions. We selected a representative, mature stand in a drought-prone area. Tree transpiration, evaporative demand and soil water availability were measured every day over 15 months. The results showed that basic relationships with evaporative demand, leaf area index and soil water availability were globally supported. However, the implementation of a regulation of transpiration at high evaporative demand whatever soil water availability was necessary to avoid large overestimates of transpiration. The details of regulation were confirmed by the analysis of canopy conductance response to vapor pressure deficit. The final objective of providing hierarchy between the main regulation factors of seasonal and annual transpiration was achieved. In the tested environmental conditions, the impact of atmospheric drought appeared larger importance than soil drought contrary to expectations. Our results support the interest in simple models to provide a first diagnosis of water constraints on transpiration with limited data, and to help decision making toward more sustainable rubber plantations.

Reclamation of Land Disturbed by Shrimp Farming in Songkla Lake Basin, Southern Thailand

Based on remotely sensed data, it was revealed that, over the period of 24 years, from 1982 to 2006, areas devoted to shrimp farming in Songkla Lake Basin, southern Thailand, increased by a total of 7,401 ha, equivalent to an increase of 212.0%. The dramatically rise of the land under shrimp culture was attributed to culture technological advances, government subsidies and strong demand for shrimp in global markets. However, with the continued expansion of culture ponds, most farms were abandoned after 5–7 years of operation due to production losses stemming from problems with water quality and associated outbreaks of disease. Characterization of the properties of soil materials collected from an abandoned shrimp pond indicated that the shrimp farmed soils possessed several adverse physical and chemical impediments to the establishment of vegetation. The major physical limitations were largely associated with low saturated hydraulic conductivity and low oxygen diffusion rate whereas high salinity levels were the main chemical constraints. Methods to overcome these physical problems were investigated by mixing the farmed soils with locally available amendment materials such as gypsum and vermiculite. Findings revealed that the incorporation of these amendments into the farmed soils improved their physical properties through increased saturated hydraulic conductivity and oxygen diffusion rate values. The elevation of the saturated hydraulic conductivity values could also facilitate the leaching out of salt from the farmed soils resulting in reduced salinity levels. The applicability of this amelioration procedure under field conditions merits further research.

A Simple Water Balance Model of Rubber Tree Plantations under Different Evaporative Demand Regimes

Drought constraints and transpiration of rubber (Hevea brasiliensis) plantations under different evaporative demand regimes were assessed by the simple water balance model. A lump water model, BILJOU (BILan hydrique JOUrnalier), is the daily water balance model. This model requires daily potential evapotranspiration (ETo) and rainfall as input climatic data, also requires site and stand parameters are maximum extractable soil water and leaf area index (LAI). The study was carried out two sites; namely Songkhla and Chachoengsao province, Thailand, traditional and new plantation area, respectively. The calibration of this model was done with sap flow measurements. Soil water derived by tensiometer for Songkhla and soil sampling for Chachoengsao were used to validate the model. Under non limiting soil water and full canopy, transpiration of rubber was influenced by evaporative demand. Consistently, under limited soil water represented as threshold of relative extractable water (REWc < 0.4), transpiration was influenced by REW. In the new plantation area; Chachoengsao, drought constraints were evident annually from the beginning of senescence until the new accomplished flushing; December to June. However, at Songkhla site, transpiration and soil water was mainly driven by evaporative demand. And the limitation of soil water represented shortly during the plateau stage of LAI.