Pinsapo fir (Abies pinsapo Boiss.) is an endangered Mediterranean conifer that has raised strong conservation interest as a paradigmatic example of species characterized by small and fragmented populations. We studied an old reforestation stand composed of A. pinsapo, Pinus nigra and Pinus sylvestris established in the 1910s in central-eastern Spain (about 500 km north of the species native distribution range), with the aim of evaluating the stand's suitability as an ex situ conservation unit for the fir. To this end, we investigated the long-term performance of the stand and assessed genetic diversity of A. pinsapo. Tree-ring width (TRW) and carbon isotope discrimination (Δ13C) were used to characterise growth dynamics and intrinsic water-use efficiency (WUEi), respectively. Furthermore, 42 pinsapo firs were genotyped at five microsatellite loci to compare their genetic variation with published data on natural populations. A. pinsapo showed ca. two-fold higher radial growth than pines in the last 80 years; however, a growth decrease was observed for all species from the 1990s onwards. Indexed TRW was positively associated with Δ13C at the species level, denoting inter-annual growth dependence on water availability. Overall, Δ13C was higher for A. pinsapo compared to pines, indicating lower WUEi, but Δ13C significantly decreased over the last 50 years for all species, likely as the result of tighter stomatal regulation of water loss, resulting in WUEi increases of about 25 %. Recently, however, A. pinsapo showed reduced WUEi increase in concord with growth slowdown, suggesting a threshold response for stomatal regulation. Although genetic diversity of A. pinsapo was about half of natural populations, the old-planted stand could be important for the conservation of this endemic species considering its good long-term growth and physiology. The latest decrease in performance of A. pinsapo, however, asks for urgent management measures aimed at reducing the competition for water and promoting growth and natural regeneration. This study illustrates the potential of combining tree-ring-based long-term physiological information with genetic data to ascertain the prospects of artificial stands for conservation purposes.