SEASONAL WATER RELATIONS OF 3 BROADLEAVED SPECIES (FAGUS-SYLVATICA L,QUERCUS-PETRAEA (MATTUSCHKA) LIEBL AND QUERCUS-PYRENAICA WILLD) IN A MIXED STAND IN THE CENTER OF THE IBERIAN PENINSULA

Water relations were monitored in a natural mixed stand of beech (Fagu s sylvatica L.), sessile oak (Quercus petraea (Mattuschka) Liebl.) and melojo oak (Quercus pyrenaica Willd.) during a growing season. Soil m oisture content and air temperature were recorded in an especially dry year with rainfall under 15 mm for 3 months during the middle of the growing season. Leaf samples of the three species were periodically ta ken at dawn and pressure-volume curves were performed with the transpi rational method. Osmotic potentials, relative water content at turgor loss point, maximum bulk modulus of tissue elasticity and the dry weig ht/turgor weight ratio were obtained for the curves. Differences for o smotic potential at full turgor and at turgor loss point were signific ant between dates while considering the species altogether, with minim um values on 6 September, matching with the minimum soil moisture. Osm otic potential at full turgor decreases as soil drought increases, the extreme mean values (-1.32 MPa in spring and -2.31 MPa in summer) bei ng reached by the melojo oak. Osmotic potential at turgor loss point f ollows a similar pattern of variation to the one at full turgor. Meloj o oak and beech also show significant differences for the whole period . No apparent pattern is associated with relative water content at the turgor loss point and maximum bulk modulus of tissue elasticity, alth ough there were significant differences for the growing season as a wh ole. The three species ranked according to decreasing values of the re lative water content at the turgor loss point, in parallel to their in creasing xerophytic character throughout the growing season. Beech has the highest value (87.22%) and melojo oak the lowest one (83.52%), se ssile oak having an intermediate value (85.94%). The capacity of adapt ation for living together may explain why the expected differences in the parameters between species and dates are not always significant. H owever, the data highlight the higher xerophytic condition of melojo o ak and the more mesophytic character of beech despite the relict condi tion of the stand.