Team : MECA

PhD student : ENGONGA Arnauld

Thesis supervisors :  GRIL Joseph ,  BADEL Eric , Rostand Moutou Pitti

Dates : February 2019 - March 2023

Description

The rubber tree (Hevea brasiliensis) is the main source of natural rubber production, yielding over 13 million tonnes per year. This production is based on industrial and village plantations in humid tropical regions. To harvest the latex, a cut is made in the bark of the tree to sever the latex-producing cells and allow the latex to flow out. Unfortunately, during their economic life on the plantation (between 35 and 40 years), rubber trees are susceptible to wind damage, particularly trunk breakage. It is estimated that over 30 years, losses due to wind damage to trees amount to nearly 40%. Industrial rubber tree plantations are organised into monoclonal plots. The plants are selected for their productivity, disease resistance, etc., but not for their mechanical behaviour or susceptibility to breakage, as breeders lack the appropriate tools to evaluate these criteria. Field observations show interclonal differences in susceptibility to wind breakage, which makes the evaluation of mechanical criteria all the more important. Among the known causes of these differences in rubber trees are competition between latex production and growth, the shape of the crown, and the presence of tension wood. In this study, we focused on the mechanical behaviour of the stems of two rubber tree clones, one of which (IRCA825) is known to be very susceptible to breakage and the other (IRCA41) to be resistant. We studied biomechanical parameters such as the bending stiffness (EI) of the stems, the stiffness of green wood (E) and its breaking strength (MOR). To evaluate these parameters, we conducted in-situ bending tests, until the trunks broke, in clonal fields located in Côte d'Ivoire, followed by wood characterisation tests in the laboratory. Comparative analysis of bending tests on standing trees shows that clones IRC825 and IRCA41 have equivalent stem bending stiffness. However, IRCA41 differs from IRCA825 in that it has higher green wood stiffness and breaking strength, which is the main reason for its greater wind resistance. Comparative analysis of wood structure showed that these differences in wood stiffness and breaking strength are mainly due to higher wood density in IRCA41 than in IRCA825, with little difference in microfibril angle and specific modulus. In addition, we studied the thigmomorphogenic capacity of the clones, i.e. their ability to acclimatise to their windy environment, using a tree bracing system. The preliminary results of this trial have shown, for the first time, the ability of these two rubber tree clones to respond to mechanical stimulation from the wind after only six months. However, the mechanical benefit of this acclimatisation, as well as its interclonal variation, remains to be evaluated.

Link

https://hal.science/tel-04197692v2