We aimed to gain knowledge on the changes in intrinsic water use efficiency (iWUE) in response to increasing atmospheric CO2 concentrations and climate change over the last century. We investigated the variation in the iWUE of mature Fagus sylvatica trees located in the higher, central and lower altitudinal forest limits (HFL, CFA and LFL) of one of the southernmost sites of beech distribution in Europe, the Montseny Mountains in Catalonia (northeast Spain), during the last century by analysing the δ13C of their tree rings. Pre- and post-maturation phases of the trees presented different trends in δ13C, Δ13C, Ci (internal CO2 concentration), iWUE and basal area increment (BAI). Moreover, these variables showed different trends and absolute values in the LFL than in the other altitudinal sites, CFA and HFL. Our results show the existence of an age effect on δ13C in the CFA and HFL (values increased by ca. 1.25‰ coinciding with the BAI suppression and release phases, previous to maturation). These age-related changes were not found in the LFL, whose beech trees arrived to maturation earlier and experienced drier conditions during the suppression phase. In the last 26 years of comparable mature trees, the increase of iWUE deduced from the Δ13C analyses was ca. 10% in LFL, ca. 6% in CFA and not significant in HFL. These results show that climate change towards more arid conditions accounted for these higher Δ13C-values and increases in the LFL more than the continuous increase in atmospheric CO2 concentrations. This increased iWUE in the LFL did not avoid a decline in growth in these lowest altitudes of this beech southern range-edge as a result of warming. Furthermore, since there was no apparent change in iWUE and growth in the beech forests growing in the more standard-adequate environments of higher altitudes in the last 26 years, the rate of sequestration of C into temperate ecosystems may not increase with increasing atmospheric CO2 concentrations as predicted by most models based on short-term small scale experiments.