A full 3-D model was developed to simulate the elasto-mechanical behaviour of trees subjected to wind and gravitational forces with the aim of estimating the stress and strain distribution at the surface of the stem. The model was adapted to geometry and material properties of a 64-year old Douglas fir tree. The results are comparable, on the whole, with those of a finite element model of this tree. Original stem and crown data, as well as the applied forces, were modified manifold in order to study their importance on the change in fibre stress and thus, on the safety reserve against stem breakage.