We present a fast ambient occlusion approximation al- gorithm for real-time tree rendering. The main contribution of the proposed method concerns the preprocessing stage of ambient occlusion. The proposed method is a divide- and-conquer calculation framework with limited user inter- vention. The proposed approach splits the plant geometry into clusters. On each plant primitive, ambiant occlusion approximation is then defined from statistical inner cluster properties and a fast outer cluster occlusion. The imple- mentation achieves flexible ambient occlusion effects with linear complexity. Keywords: tree visualization, natural phenomena ren- dering, global illumination, ambient occlusion, accessibility value approximation.