Effects of thinning in a Pinus sylvestris L. stand on foliar water relations of Fagus sylvatica L. seedlings, planted within the pinewood

Planting of beech in old Scots pine plantations, could facilitate the exten sion of adjoining small natural populations of beech in the centre of the I berian Peninsula. To be successful the survival and growth of seedlings aft er outplanting must be compatible with microclimatic conditions within thin ned pine woods. The present paper deals with water relations in beech seedl ings following the variation of available radiation and water as a result o f thinning. The seedlings were established under an old Scots pine plantati on. After pine felling, four situations were generated in terms of radiatio n. Hemispherical photographs were taken, and global relative irradiance was calculated for every treatment. During two growing seasons predawn water p otential (Psi (pd)) was measured and leaf water parameters were derived fro m P-V curves. The Psi (pd) diminished through the season in all four situat ions, and the highest values, were found when pine density was lowest. Osmo tic potential at turgor lost (Psi pi (0)) was higher in early summer. A neg ative relationship was-found between osmotic potential at full turgor (Psi (pi full)) and symplast solute content on a dry weight basis (N-s). Under t he lowest irradiance (unthinned pine trees), Psi (pi full) and Psi (pi0) we re the highest and the bulk modulus of elasticity (epsilon (max)) the lowes t; epsilon (max) decreased in response to the lowest Psi (pd) measured at t he end of summer 1998. It was followed by a reduction in the symplastic rel ative water content and N-s, irrespective of the irradiance. Osmotic adjust ment, as a drought tolerance mechanism, was limited under shade conditions (unthinned pine trees), restricting the acclimation of beech seedlings to d rought. Therefore, the presence of overstory, necessary for a successful im plantation, should not be extended for too many years because of the risk o f negative effects on growth and survival, particularly after frequent dry summers.