Trees can modify the orientation of their trunk and branches by asymmetrical production of reaction wood. Even though biomechanical models that simulate gravitropic movements are proposed in the literature, recent studies show that they cannot fit the righting-up movement of young, fast growing poplar trees at the intra annual scale. This discrepancy could be explained by the fact that cell maturation is not considered in these models. Another important point is that the knowledge of viscoelastic properties of green wood is poorly studied in the literature, while some biomechanical models show that viscoelasticity of wood can be an important parameter in the tree gravitropic response. These models only provide qualitative results and the viscoelastic properties of wood have to be characterized. The goal of this work is to study the evolution of viscoelastic properties of green wood during maturation. The effects of viscoelasticity corresponding to mechanical solicitation (stress or strain) can be studied by creep or relaxation tests. For the wood of standing tree, internal stresses are due to the maturation and growth of the tree. These constraints can thus act as mechanical stresses and cause deformation of green wood specimens. The effect of these internal stresses on specimens taken from the trunk is first studied in the present work. In the second part of the paper, a study of creep bending of clamped-free wooden boards cut within the last growth ring is presented.