Two slow-growing plant species (Chamaerops humilis, L. and Cycas revoluta Thunb.) were exposed to elevated CO2 conditions over a 20-month period in order to study the CO2 effect on growth, photosynthetic capacity and leaf carbon (C) management. The ambient isotopic 13C/12C composition (δ13C) of the greenhouse module corresponding to elevated CO2 (800 μmol mol−1 CO2) conditions was changed from δ13C ca. −12.8±0.3‰ to ca. −19.2±0.2‰. Exposure of these plants to elevated CO2 enhanced dry mass (DM) by 82% and 152% in Chamerops and Cycas, respectively, mainly as a consequence of increases in plant level photosynthetic rates. However, analyses of A–Ci curve parameters revealed that elevated CO2 diminished leaf photosynthetic rates of Chamaerops whereas in Cycas, no photosynthetic acclimation was detected. The fact that Chamaerops plants had a lower DM increase, together with a longer leaf C residence time and a diminished capacity to respire recently fixed C, suggests that this species was unable to increase C sink strength. Furthermore, the consequent C source/sink imbalance in Chamaerops might have induced the downregulation of Rubisco. Cycas plants were capable of avoiding photosynthetic downregulation due to a greater ability to increase C sink strength, as was confirmed by DM values, and 12C-enriched CO2 labeling data. Cycas developed the ability to respire a larger proportion of recently fixed C and to reallocate the recently fixed C away from leaves to other plant tissues. These findings suggest that leaf C management is a key factor in the responsiveness of slow-growing plants to future CO2 scenarios.