Seasonal water relations of three broadleaved species (Fagus sylvatica L., Quercus petraea (Mattuschka) Liebl. and Quercus pyrenaica Willd.) in a mixed stand in the centre of the Iberian Peninsula.
Water relations were monitored in the Hayedo de Montejo, Spain, in a 30-yr-old natural mixed stand of beech (Fagus sylvatica), sessile oak (Quercus petraea) and melojo oak (Quercus pyrenaica). The location represents the southern limit of beech in the Iberian Peninsula, and the sessile oaks here are one of the few, small populations of this species found at this altitude (1250 to 1500 m). Soil moisture 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 (early June to end September). Leaf samples of the three species were periodically taken at dawn and pressure-volume curves were performed with the transpirational method. Osmotic potentials, relative water content at turgor loss point, maximum bulk modulus of tissue elasticity and the dry weight/turgor weight ratio were obtained for the curves. Differences for osmotic potential at full turgor and at turgor loss point were significant between dates, considering all species altogether, with minimum values on 6 September, matching minimum soil moisture. Osmotic potential at full turgor decreased as soil drought increased, the extreme mean values (-1.32 MPa in spring and -2.31 MPa in summer) being reached by melojo oak. Osmotic potential at turgor loss point followed a similar pattern of variation to the one at full turgor. Melojo oak and beech also showed significant differences for the whole period. No apparent pattern was associated with relative water content at the turgor loss point and maximum bulk modulus of tissue elasticity, although there were significant differences for the growing season as a whole. The three species ranked according to decreasing values of the relative water content at the turgor loss point, in parallel to their increasing xerophytic character throughout the growing season. Beech had the highest value (87.22%) and melojo oak the lowest (83.52%), sessile oak having an intermediate value (85.94%). The capacity of adaptation for living together may explain why the expected differences in the parameters between species and dates were not always significant. However, the data highlight the higher xerophytic condition of melojo oak and the more mesophytic character of beech despite the relict condition of the stand.