Planting of beech in old Scots pine plantations could facilitate the extension of adjoining small natural populations of beech in the centre of the Iberian Peninsula. To be successful the survival and growth of seedlings after outplanting must be compatible with microclimatic conditions within thinned pine woods. The present paper deals with water relations in beech seedlings following the variation of available radiation and water as a result of thinning. The seedlings were established under an old Scots pine plantation. After pine felling, four situations were generated in terms of radiation. Hemispherical photographs were taken, and global relative irradiance was calculated for every treatment. During two growing seasons predawn water potential (&#79pd) was measured and leaf water parameters were derived from P-V curves. The &#79pd diminished through the season in all four situations, and the highest values were found when pine density was lowest. Osmotic potential at turgor lost (&#79&#63o) was higher in early summer. A negative relationship was found between osmotic potential at full turgor (&#79&#63full) and symplast solute content on a dry weight basis (Ns). Under the lowest irradiance (unthinned pine trees), &#79&#63full and &#79&#63o were the highest and the bulk modulus of elasticity (&#41max) the lowest; &#41max decreased in response to the lowest &#79pd measured at the end of summer 1998. It was followed by a reduction in the symplastic relative water content and Ns, irrespective of the irradiance. Osmotic adjustment, as a drought tolerance mechanism, was limited under shade conditions (unthinned pine trees), restricting the acclimation of beech seedlings to drought. Therefore, the presence of overstory, necessary for a successful implantation, should not be extended for too many years because of the risk of negative effects on growth and survival, particularly after frequent dry summers.