Implications of Agroecological Changes in Rubber-based Intercropping System on the Sustainability of Hevea Rubber Production

Abstract

Hevea rubber cultivations supply to meet the requirement of the world consumption and generate major incomes for millions of rubber smallholders. However, its conventional cultivation practices like monocropping, replanting, and long-term utilization of chemical fertilizer have accumulated negative impacts substantially on the environmental and socio-economic concerns of rubber growing regions. Thus, to reduce those impacts and develop sustainability in the natural rubber production, the rubber-based intercropping system, which improves the agroecology and livelihoods of smallholders, became the most recommended option for the smallholders. However, some combinations of the rubber-based intercropping were observed with adverse effects on the growth and yield of the crops. Thus, this thesis research studied the different rubber-based intercropping practices in terms of agroecology and tree physiology, and their implications for ensuring the sustainability of natural rubber production integrated with intercropping systems. Two experimental studies were set up in Songkhla province, southern Thailand. The first experiment was a case study to investigate the changes in agroecosystem components of a rubber-based intercropping farm and their interactions under integrated fertilizations mixed with organic soil amendments. The second experiment aimed to study the seasonal changes in leaf area index (LAI) and soil moisture content (SMC) under rubber-based intercropping farms, and their interrelations with the latex biochemical compositions, yield, and technological properties. The first experiment was conducted at a rubber-salacca intercropping farm and identified the consequences of the integrated fertilization combined with two organic soil amendments: humic acid (HSA); chitosan (CSA) compared to conventional chemical fertilization. The CSA application increased soil organic matter by 80%. In the 21-40 cm soil depth, the rubber roots treated with HSA and the salacca palm roots treated with CSA showed greater fine root length density. Under CSA, the physiological status of the rubber trees showed less stress. The treatments of HSA and CSA showed 145% and 72%, respectively, higher total production of salacca palm than that of the chemical fertilization. Improvements in the soil fertility, the root’s function, the crops’ yields, and the tree’s physiological status were consequences as complementarity in the system under the integrated fertilizations. The second experiment selected three rubber-based intercropping farms: rubber-bamboo (RB); rubber-melinjo (RM); rubber-coffee (RC), and one rubber monocropping farm (RR). Among the rubber-based intercropping farms, the mean relative humidity of RB and RM throughout the study period were higher than that of RR by 14% and 18%, respectively, whereas RC had a mere 6% higher than RR. However, regarding the mean temperature, RB and RM maintained only 4% less than RR, while RC had the same mean temperature as RR. Over the study period, RB, RM and RC exhibited significantly higher LAI values at 1.2, 1.05 and 0.99, respectively, while RR had a low LAI of 0.79. Increasing SMC trends by soil depths were pronounced in all rubber-based intercropping farms. RB and RM expressed less physiological stress and delivered latex yield on average 40% higher than RR. With higher molecular weight distributions, their rheological properties were comparable to those of RR. However, the latex in RB and RM significantly increased the Mg contents to 660 and 742 ppm, respectively, in S2. Their dry rubbers contained ash contents of more than 0.6% in S3. This research would contribute to the sustainability of natural rubber production integrated with rubber-based intercropping ensuring the complementarity benefits in the farm ecosystem leading to the superiority of Hevea rubber’s technological properties.