RESPONSE OF EFFLUENT-IRRIGATED EUCALYPTUS-GRANDIS AND PINUS-RADIATA TO SALINITY AND VAPOR-PRESSURE DEFICITS

Effects of high vapor pressure deficit (VPD) and soil salinity on grow th and physiology of Pinus radiata D. Don and Eucalyptus grandis Hill ex Maiden were studied in a five-year-old plantation irrigated with sa lt-enhanced effluent (2.2 dS m(-1)) or freshwater (0.2 dS m(-1)) for 1 4 weeks during spring and summer. Salt was then rapidly leached by ove r-irrigation with low-salinity effluent. Soil water and salinity, tree water stress, sap flux, substrate carbon conversion efficiency, folia ge and stem growth, and foliar cations and chloride were monitored thr oughout the study. An average of 9 and 1 Mg ha(-1) of salt with an ave rage hydraulic load of 660 and 780 mm was applied to the salt and cont rol plots, respectively. Maximum soil salinity in the root zone was 5. 8 and 6.8 dS m(-1) in the eucalypt and pine plots, respectively. Preda wn water potential was more than twice as sensitive to increasing sali nity in E. grandis as in P. radiata. The salt treatment reduced rates of leaf and stem growth of the eucalypts by 60 to 70% but had no effec t on leaf and stem growth of the pines. In the eucalypts, salinity dec reased mean leaf area by 26% and increased specific leaf area by 12% c ompared with control values, indicating less biomass per unit leaf are a in the salt treatment. Salinity had no effects on these two paramete rs in pine. The salt treatment significantly increased mean foliar con centrations of Na and Cl in both species, and of K in the pines. Folia r Na concentration was 6-10 times higher in the eucalypts than in the pines. Lowered water potential and increased Na concentration in the e ucalypts in response to salinity resulted in about a 50% reduction in the efficiency of conversion of carbon into biomass; however, three we eks after leaching the salt, there was no significant difference in ef ficiency of conversion of carbon into biomass between the treatments. Salinity had no effect on water use by eucalypts, but caused a nonsign ificant decrease (7%) in water use by pines. As evaporative demand inc reased, crop factor (transpiration divided by pan evaporation) decline d by up to 50 and 60% in the pines and eucalypts, respectively. We con clude that stomatal response to high VPD, not soil salinity, accounts for most of the reduction in summertime water use.