Mechanical Control of Root Growth: A Computer Simulation

The root system is very important for the stability of a tree. External wind forces must be transferred by the roots into the soil. Therefore, the tree root system must be large and strong enough to prevent breakage and uprooting. The shape of the root system is also important for the tree's stability. This paper investigates the shape of the root system with regard to the external loading forces and soil properties. The material properties of the soil are very important for the failure of the root-soil plate. The strength of soil depends on its resistance to shear stresses. Resistance to shear is provided by the friction between the soil particles and is proportional to the normal force between them (Mohr-Coulomb failure criterion). Root-reinforced soil can be compared with a composite material in which fibres (roots) of relatively high tensile strength are embedded in a matrix of lower tensile strength (soil). Plant root scan increase the shear resistance of the soil. At Forschungszentrum Karlsruhe the so-called SKO method was modified to predict root morphologies of trees for different loading and soil conditions.The predictions are based on the assumption that the optimum root design (in the purely mechanical sense!) is a design that guarantees an even distribution of the shear loading along the roots. This approach gives good qualitative agreement with the root formation observed in nature for cases considered here. It is clear that this mechanical optimum can be disturbed drastically by biological growth regulators.