Yerba maté (Ilex paraguariensis, Aquifoliaceae) is a subtropical, evergreen, dioecious, South American tree. In one preliminary study, it was observed that the functional strategy of yerba mate females, aiming to finish reproductive process, was increased transpiration relative to photosynthetic rates compared with males, on self-shaded leaves. We hypothesised that the long-term gas exchange response of males and females can evolve independently of phenological stage and cultivation type. In this spirit, the primary aim of the study was to analyse the physiological sexual dimorphism of this species, evaluating fluctuations of gas exchanges related to microclimate and phenological stages. A field study was conducted on adult plants of yerba maté cultivated in monoculture (MO) and in forest understorey (FUS), and measurements carried out in situ on microclimate and leaf gas exchange parameters. The photosynthetic photon flux density that was attained at leaf level in FUS was reduced roughly 10-fold compared with that in MO. Various leaf age populations were observed during a 2-year period at 2-month intervals and grouped into four categories: young, young-fully-expanded, fully-expanded and old. Young and young-fully-expanded leaves were the most active in photosynthesis. Leaves of female plants showed greater photosynthetic rate than those of male plants, which was expressed on all leaf age categories in MO, but only during vegetative stages previous to flowering and fruit ripening. The photosynthesis of young-fully-expanded leaves of females grown in FUS was superior to males but only during winter growth pause. The stomatal conductance differed in relation to cultivation type and leaf age but did not show the sexual differentiation. Physiological sexual dimorphism in yerba mate is shown to be plastic, responding to environmental conditions. The cost associated to the reproduction of yerba maté could be most easily met showing physiological differentiation of both sexes. A higher reproductive investment of females might be compensated for by exhibiting greater leaf photosynthesis than males that occurs in vegetative stages that precede flowering and fruit ripening.