Beech seedlings were established in the understory of a Pinus sylvestris plantation close to one of the southernmost populations of beech in Europe, the beech-oak forest of Montejo de la Sierra. Four years later, the overstory was partially reduced by removing pine trees. Solar radiation in the understory was evaluated by hemispherical canopy photographic technique and the effects of relative irradiance increment on the leaf anatomy of beech seedlings were analyzed during the two years after opening the stand. The increase in specific leaf mass (SLM) in seedlings during both years runs in parallel with the increase in relative irradiance estimated by the global light factor (GLF) which expresses the proportion of global radiation relative to that in the open. There were significant relationships between the light index as a surrogate of light environment and the morphological and anatomical characteristics of the leaves. In the first year, SLM increase was more related to total blade thickness. In the second year, thickness of palisade parenchyma (PP) appears more relevant than that of spongy tissue (SP) as indicated by the absence of significance in the relationship between SP and SLM. Moreover, stomatal density was also higher according to increasing relative irradiance. The shift response of beech seedlings to the overstory opening makes evident their capability of acclimatization to light increase through changes in leaf anatomy.