The Bossoleto CO2 spring emits CO2 which has a stable carbon isotopic ration (δ13C = −8‰). We determined δ13C on leaves of several individual species growing in Bossoleto and in a nearby control site at ambient CO2. δ13C was 6% more negative in leaves of species collected from the grassland community of Bossoleto, indicating discrimination (Δ) against the heavy carbon isotope. No such changes were found in ruderal species growing in teh same spring, suggesting that photosynthetic capacity was much less affected. Δ was substantially increased under elevated CO2 in leaves of Quercus pubescens but not in Quercus ilex, which also did not show any increase in non-structural carbohydrates. Gas-exchange measurements made on Plantago lanceolata, supported the view that photosynthetic capacity is decreased in plants grown under elevated CO2 and on poor soils.