TRANSPIRATION AND STOMATAL BEHAVIOR OF QUERCUS-ILEX PLANTS DURING THESUMMER IN A MEDITERRANEAN CARBON-DIOXIDE SPRING

Variations in the water relations and stomatal response of Quercus ile x were analysed under field conditions by comparing trees at two locat ions in a Mediterranean environment during two consecutive summers (19 93 and 1994), We used the heat-pulse velocity technique to estimate tr anspirational water use of trees during a 5 month period from June to November 1994, At the end of sap flow measurements, the trees were har vested, and the foliage and sapwood area measured, A distinct environm ental gradient exists between the two sites with higher atmospheric CO 2 concentrations in the proximity of a natural CO, spring. Trees at th e spring site have been growing for generations in elevated atmospheri c CO2 concentrations. At both sites, maximum leaf conductance was rela ted to predawn shoot water potential. The effects of water deficits on water relations and whole-plant transpiration during the summer droug ht were severe. Leaf conductance and water potential recovered after m ajor rainfall in September to predrought values. Sap flow leaf conduct ance and predawn water potential decreased in parallel with increases in hydraulic resistance, reaching a minimum in mid-summer. These relat ionships are in agreement with the hypothesis of the stomatal control of transpiration to prevent desiccation damage but also to avoid 'runa way embolism'. Trees at the CO2 spring underwent less reduction in hyd raulic resistance for a given value of predawn water potential. The de crease in leaf conductance caused by elevated CO2 was limited and tend ed to be less at high than at low atmospheric vapour pressure deficit. Mean land diurnal) sap flux were consistently higher in the control s ite trees than in the CO2 spring trees. The degree of reduction in wat er use between the two sites varied among the summer periods. The cont rol site trees had consistently higher sap flow at corresponding value s of either sapwood cross-sectional area or foliage area, Larger trees displayed smaller differences than smaller trees, between the control and the CO2 spring trees. A strong association between foliage area a nd sapwood cross-sectional area was found in both the control and the CO2 spring trees, the latter supporting a smaller foliage area at the corresponding sapwood stem cross-sectional area. The specific leaf are a (SLA) of the foliage was not influenced by site. The results are dis cussed in terms of the effects of elevated CO2 on plant water use at t he organ and whole-tree scale.