Comparative transpiration of irrigated juvenile eucalypts in the Negev desert

The summer water use patterns of five species of Eucalyptus were examined in the Negev desert in Israel where they received trickle irrigation once every 10 days during the period July-August 1986. The species transpired between 8% and 16% of the irrigation input on I₁ (where I_n is the number of days after irrigation), and between 54% to 64% at the end of the 10-day period. Evaporation from the soil surface was calculated to account for < 30% of the total evapotranspiration. Analysis of variance with repeated measurements indicated significant interspecific differences in the mean hourly transpiration rates through the 10-day period after irrigation. Eucalyptus grossa, E. salubris, E. torquata and E. woodwardii had a pattern of initially high transpiration rates ranging from 126 to 70 mg cm^-2 d^-1 on I₁, and gradually decreasing rates that stabilized at near constant rates ranging from 38 to 17 mg cm^-2 d^-1 after I₃. Intraspecihc analyses of variance indicated that the transpiration rates of these four species were significantly different on the successive days after irrigation, while the transpiration rates of E. socialis were much more constant ranging only from 53 to 23 mg cm^-2 d^-1 over the 10-day period. The rapid depletion of irrigation inputs by E. grossa, E. salubris, E. torquata and E. woodwardii may be explained as an adaptation to the summer climatic conditions in their respective provenances in southwestern Western Australia. Under natural conditions, high transpiration rates are a means of exposing competitors to more severe soil water deficits. E. socialis, whose seed provenance was in Central Australia where summer rainfall is greater, did not exhibit the same water-spending strategy. The persistence of water use patterns that correspond to the climatic conditions in the provenances of drought-adapted ucalypts is an important consideration when selecting suitable species for trickle-irrigated afforestation projects in dryland environments.