Sayan Sdoodee

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.

The mangosteen flowering date model in Nakhon Si Thammarat province, southern Thailand

Mangosteen (Garcinia mangostana L.) is one of the economically important fruits of Thailand. Recent studies show that the climatic variability affects the flowering period of tropical fruit trees. The objectives of this study are: 1) to investigate the correlation between climatic factors (in particular with, rainfall, maximum temperature, minimum temperature, relative humidity, and dry period before flowering) and mangosteen flowering date and 2) to develop the poisson regression model to predict the flowering date of mangosteen in Nakhon Si Thammarat Province, Thailand. This model is useful in guiding farmers to manage mangosteen orchards. The results showed that the rainfall, the maximum and minimum temperatures, the relative humidity, and the dry period before flowering dates affect mangosteen flowering dates. The model which will be used as a guideline for mangosteen flowering date prediction was log(μ)=10.85+0.0001x1-0.0564x2-0.0634x3-0.0232x4+0.0003x5 where μ where is the mean of mangosteen flowering date, x1 is rainfall, x2 is maximum temperature, x3 is minimum temperature, x4 is relative humidity, and x5 is dry period before flowering date.

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.