Rising atmospheric CO2 concentration (Ca) has been related to tree growth enhancement accompanied by increasing intrinsic water use efficiency (iWUE). Nevertheless, the extent of rising Ca on long-term changes in iWUE and growth has remained poorly understood to date in Mediterranean treeline ecosystems. This study aimed to examine radial growth and physiological responses of P. canariensis in relation to rising Ca and increasing aridity at treeline in Tenerife, Canary Islands, Spain. We evaluated temporal changes in secondary growth (tree ring width; TRW) and tree ring stable C isotope signature for assessing iWUE from 1975 through 2011. Precipitation was the main factor controlling secondary growth. Over the last 36 years P. canariensis showed a decline in TRW at enhanced iWUE, likely caused by reduced stomatal conductance due to increasing aridity. Our results indicate that increasing aridity has overridden the potential CO2 fertilization on tree growth of P. canariensis at its upper distribution limit.