Effects of soil water availability on water use efficiency of Eucalyptus cloeziana and Eucalyptus argophloia plants

Effects of soil water availability on transpiration efficiency (WUE_T), instantaneous water use efficiency (WUE_i) and carbon isotope composition (δ¹³C) were investigated in 7-month-old plants of humid coastal (Gympie) and dry inland (Hungry Hills) provenances of Eucalyptus cloeziana F.Muell. and in a dry inland provenance of E. argophloia Blakely (Chinchilla), supplied with 100 (W₁₀₀), 70 (W₇₀) and 50% (W₅₀) of their water requirements. At W₁₀₀, WUE_T of the three provenances were not significantly different but as available soil moisture decreased, E. argophloia produced greater biomass and demonstrated significantly higher WUE_T than either E. cloeziana provenance. Midday WUE_i was not significantly affected by watering regime within each provenance but was lowest in E. argophloia. A decrease in soil water availability caused a consistent increase in δ¹³C values in all three provenances; however, δ¹³C values of E. argophloia in all three water regimes were significantly lower than those of E. cloeziana provenances, which did not differ significantly from each other. For all three provenances, δ¹³C was not correlated with WUE_i but height and root collar diameter were negatively correlated to δ¹³C. There was little evidence of differences in δ¹³C, WUE_T and WUE_i between E. cloeziana provenances but clear differences between E. cloeziana and E. argophloia. The high WUE_T, low WUE_i and low δ¹³C for E. argophloia may have implications in the selection of Eucalyptus provenances for commercial forestry in low-rainfall regions.