Whar are olive trees doing at night or hydraulic redistribution in a rainfed olive grove in Alentejo

Hydraulic redistribution is a mechanism that ensures the survival of superficial roots in dry soil during late summer, as they are submitted during part of the day to a negative flux, coming from roots with access to deep water. This mechanism also enables the plant to continue exploring these upper soil layers, richer in minerals and organic matter and has been observed in many ecosystems, including Montado, in Portugal. A critical question arises: to which extent is the survival of traditional rainfed ecosystems dependent on the water table condition? In order to identify the importance and dynamic of this process in olive groves, a study was conducted in South Portugal using two different, independent methods to follow sap flow in roots. We report the first results obtained with a modified heat pulse method, in a rainfed olive grove. By following roots sap flow from wet spring, 6 months without significant rain and early autumn, we found experimental evidence of hydraulic redistribution from some roots to other