Climatic sensitivity of rubber (Hevea brasiliensis) plantations in Malaysia

Hevea brasiliensis is well known as a latex producing tree and has been planted commercially for more than a century in major rubber-producing countries such as Malaysia. Rubber plays a fundamentally important role in the Malaysian economy, being responsible for providing income to hundreds of thousands of small landholders. Because of competition for land use, H. brasiliensis is increasingly being planted in marginal areas where climate is less suitable and where risks of heat and water stress are expected to intensify in the future. In these areas, H. brasiliensis is often grown for timber as well as latex. In this context, it is important to understand the sensitivity of rubber clones planted in these marginal areas to climatic stress. This thesis aimed to understand the climatic sensitivity of adult H. brasiliensis clones in Malaysia using an array of complementary approaches. Firstly, using GIS and a static bioclimatic model of Hevea growth, I reveal in Chapter 2 that future climate change scenarios would alter the environment to be more favourable for cultivating rubber in many parts of Peninsular Malaysia. However, only approximately 27% of the current rubber area is located in suitable agro-climatic zones (Class 1). In Chapter 3, I evaluate differences in growth and yield among nine rubber clones and their response to contrasting precipitation conditions. I find evidence of a trade-off between growth and yield and also strong evidence of climatic control on both yield and growth. In Chapter 4, I evaluate the hydraulic properties of nine rubber clones and find strong controls of hydraulic traits on stem growth of such that clones with low hydraulic safety margins and xylem that is more vulnerable to embolism have higher growth rates. In Chapter 5, I explore difference in the thermal sensitivity of H. brasiliensis using photosystem-II (PSII) chlorophyll fluorescence quenching measurements on leaves warmed to different temperatures. I find that leaf thermal tolerance (T50) of H. brasiliensis, declines with leaf thickness, leaf mass per area (LMA) and leaf dry matter content (LDMC). I observe no association between T50 and hydraulic traits (P50, HSM50 and Pmin). Finally, I introduce a drought sensitivity metric in Chapter 6 based on seasonal foliar loss. In this chapter, I computed seasonal changes in NDVI used Sentinel-2 satellite images and related these to inter-year differences in maximum cumulative water deficit (MCWD) and found that clones characterised by low hydraulic safety margins were more sensitive in terms of phenology than those with higher safety margins. When taking into account the multivariate axes of growth, yield and climate resistance, we do not find any evidence of a ‘superclone’ that performs best in all categories but identify one clone (RRIM 929) that performs moderately well in each of these categories.