This scientific report presents a real-time method to animate complex scenes of thousands of trees under a user-controllable wind load. Firstly, modal analysis is applied to extract the main modes of deformation from the mechanical model of a 3D tree. The novelty of our contribution is to precompute a new basis of the modal stress of the tree under wind load. At runtime, this basis allows to replace the modal projection of the external forces by a direct mapping for any directional wind. We show that this approach can be efficiently implemented on a graphics hardware. This modal animation can be simulated at low computation cost even for large scenes containing thousands of trees.