Two-year-old Fagus sylvatica L. saplings were planted under the cover of a Pinus sylvestris L. stand in the Massif Central (France). The stand was thinned to different degrees to obtain a gradient of transmitted PAR (0-0.35). Eighteen Fagus saplings were sampled in this gradient and their growth (basal stem diameter increment) were recorded during six years. In the same time morphological parameters (leaf area, number and arrangement in space) were monitored using 3D-digitizing. Photosynthesis parameters (leaf nitrogen content, Vcmax, Jmax, Rd) were estimated with a portable gas-exchange analyser. Sapling photosynthesis was mainly linked to light availability while their morphology was more driven by sapling size. Annual sapling stem diameter increment was related to the amount of light-intercepting foliage (silhouette to total leaf area ratio (STAR) * total sapling leaf area (LA)) and light availability above the saplings (PARt). However, the light use efficiency, i.e. the slope of the relationship between STAR * LA * PARt and stem diameter increment, decreased over time. This decrease is due to a size effect, i.e. a relative decrease of the proportion of photosynthetic tissues in comparison with the total biomass of the sapling.