Transpiration and Water Use Efficiency of Mediterranean Eucalyptus Genotypes Under Contrasting Irrigation Regimes

Water scarcity is a key constraint for commercial <i>Eucalyptus</i> plantations, particularly given the increasing frequency of droughts driven by climate change. This study assessed annual transpiration (Tr) and water use efficiency (WUE) across eight genotypes subjected to contrasting irrigation regimes (WR). A split-plot design was implemented, comprising two irrigation levels: high (maintained above 75% of field capacity) and low (approximately 25% above the permanent wilting point). The genotypes included <i>Eucalyptus globulus</i> (EgH, EgL), <i>E. nitens × globulus</i> (EngH, EngL), <i>E. nitens</i> (En), <i>E. camaldulensis × globulus</i> (Ecg), <i>E. badjensis</i> (Eb), and <i>E. smithii</i> (Es). Between stand ages of 7 and 9 years (2020-2023), we measured current annual increment (CAI), leaf area index (LAI), Tr, and WUE. Under high WR, CAI ranged from 8 to 36 m³ ha^-1 yr^-1, Tr from 520 to 910 mm yr^-1, and WUE from 0.7 to 2.9 kg m^-3. Low irrigation reduced CAI by 5-25% and Tr by 10-35%, while WUE responses varied across genotypes, ranging from a 12% decrease to a 48% increase. Based on their functional responses, genotypes were grouped as follows: (i) stable performers (Es, Ecg, Eb) exhibited high WUE and consistent Tr under both WR; (ii) partially plastic genotypes (EgH, EngH) combined moderate reductions in Tr with improved WUE; and (iii) water-sensitive genotypes (EgL, EngL, En) showed substantial declines in Tr alongside variable WUE gains. These findings underscore the importance of selecting genotypes with adaptive water-use traits to improve the resilience and long-term sustainability of <i>Eucalyptus</i> plantations in Mediterranean environments.