Evaluating the effect of plant root architecture on soil reinforcement and slope stability is an important issue for the improved mitigation of shallow landslides. Nevertheless, few studies have examined tree root architecture on steep slopes where root systems tend to be asymmetric. Using a modelling approach based on the finite element method, we aim at examining how tree root system asymmetry affects soil strength and slope stability. We designed a three-dimensional slope using ABAQUS© software. Based on ground truth data, root system architecture for silver fir (Abies alba Mill.) and Norway spruce (Picea abies (L.) Karst), i.e. two of the most common species in European mountain forests, were generated using DigR and Xplo softwares (http://amapstudio.cirad.fr/), and were then embedded in the soil of mid-slope. Three root system asymmetry types were examined considering a constant total biomass: skewness to upslope, axisymmetric and skewness to downslope. The likelihood of failure of the slope, as well as stresses and strains within the soil medium and roots were then computed and visualised. Root system asymmetry had a significant effect on the efficiency of the slope stabilisation. Within a root system, roots skewed upslope and downslope differed in their mechanical response and strategy of reinforcement.