Argania spinosa is an example of an avoider tree growing under semi-arid conditions in Morocco. To assess what are the physiological strategies of this species, different variables were measured through an annual cycle in two populations located in the species' main distribution area. Results show the expected decrease of leaf water potential (psi) with an increase of water-use efficiency (A/gs) with the onset of the dry season. In summer, leaf conductance (g(s)) was sensitive to vapour pressure deficit (VPD), and stomatal closure occurred over 30 mbar of VPD. Surprisingly, carbon isotope discrimination (delta13C) maintained very low values over the year, with almost no relationship with any physiological or morphological variable. Hence Argania spinosa presents a complex set of mechanisms to avoid water deficit, but delta13C cannot be used as an ecological tracer of long term WUE.