Forest management practices in French mixed broadleaved forests aim at promoting species diversity during regeneration. In this context, conceiving silvicultural pathways requires to understand the factors affecting the various stages of the recruitment process. The response to canopy opening of seedlings is an important stage. In the beech dominated, broadleaved former coppice with standards, Graoully forest (40^004'48''N, 6^001'02''E, close to Metz in Lorraine, North-Eastern France), regeneration after openings is dominated by beech (Fagus silvatica L) and sycamore maple (Acer pseudoplatanus L.). In the understorey, these two shade-tolerant species are quite plagiotropic to maximize light interception and just after opening, height growth is stimulated so that trees become more orthotropic. Moreover, openings may be associated to mechanical disturbance by wind, or gravity (growth is stimulated so weight increase is accelerated). Therefore, the posture control by both secondary and primary growths are supposed to play a key role in this developmental stage. Trees that will be able to right their trunks, and grow fast in height with lower expense in wood material, should be ecologically more competitive, and moreover, they will have vertical and cylindrical trunks that are interesting for wood production. In this study, 15 trees of each species are selected among a sample of seedlings which growth and 3D or 2D geometry have been observed during 4 years after opening in 2005. These observations have shown a significant upward movement (based on the general lean of the main stem) in both species. Sycamore maple reiterated more often than beech. Then, different parameters - mass distribution, bending moment applied at the trunk base, young's modulus, maturation strains, radial growth asymmetry - have been measured in order to explain the variability of upward movement kinetics between trees or species. Biomechanical models have oriented the choice of parameter combinations used for statistical analysis. Moreover, they were used to discuss whether the differences between maple and beech - reiteration or righting by secondary growth from the trunk base - can be explained by a physical determinism as one hypothesis could be that because of a lower stiffness and a higher centre of mass, maple could not insure its posture control only by the secondary growth and the maturation process, so that reiteration would be a physical necessity to maintain the tree habit.