The mechanical structure of 24 root systems was studied in 50 year old Maritime pine (Pinus pinaster Ait) trees after the 1999 storm, which resulted in the loss of >4 million hectares of forest in south-west France. Half of the study trees had been uprooted during the storm and the other study trees were left standing. Topology and geometry of the root systems were measured using a 3D digitising device. The spatial distribution of roots was then analysed. The 3D representation of each root system was used to calculate the mechanical behaviour of the structure in different compass directions, using a finite element model. Results show that resistance to windthrow can be provided by four different structural adaptations of root systems, and that root architecture may explain half of the sensitivity to uprooting. There was no significant difference in uprooting resistance between trees that were damaged by the storm, and those which remained upright. However, all root systems were found to be highly asymmetric with regard to morphology. This asymmetry influenced strongly the mechanical behaviour of anchorage, depending on the direction of loading.