Daily sap flow, leaf morphology, and stem xylem characteristics under deficit irrigation and warming during the late winter and spring in young olive trees

Water scarcity is a major constraint for olive cultivation in hot, arid regions and is likely to be exacerbated with climate change. Thus, the objective of this study was to evaluate the combined effects of a moderate deficit irrigation (50%) and warming (3–4 °C) on whole-tree daily sap flow, leaf morphological traits, and stem xylem vessel characteristics in young olive trees (cv. Coratina). Four irrigation × temperature combinations were applied to three-year-old trees in open top chambers during the late winter and spring of 2019. The combinations included: 100% irrigation with either a near-ambient temperature control (100 Tc) or a warming treatment (100 T + ) and 50% irrigation with either a near-ambient temperature control (50 Tc) or a warming treatment (50 T + ). Trees receiving the warming treatment (T + ) had a higher daily sap flow on a leaf area basis than those at the control temperature (Tc) in each of three sap flow periods. No difference in daily sap flow between T + and Tc on a per tree basis (i.e., not adjusted for leaf area) was observed in the last two sap flow periods likely due to a lower leaf area per tree with warming. No significant reductions in sap flow were apparent for the moderate deficit irrigation under the experimental conditions. However, stem water potential and stomatal conductance were lower under the deficit irrigation, which indicates that some degree of water stress did occur. New leaves were smaller under both deficit irrigation and warming, and stomatal density decreased with warming. Stem xylem vessel numbers was greater in T + in one intermediate diameter size class compared to Tc, while deficit irrigation increased the number of very narrow vessels. These results suggest that warming increases tree water use over the short term, but that changes in leaf area due to warming over the longer-term may ameliorate such increases. Future studies with different levels of warming and greater deficit irrigation seem warranted to further assess the implications for water management.