CO2 enrichment does not alter the relative physiological performance of seedlings of two close pine species under water stress

Abstract Rising atmospheric CO 2 ([CO 2 ]) may partly alleviate the drought stress that threatens forests worldwide. Plasticity of functional traits in response to drought and [CO 2 ] may influence future plant performance. We analyzed the performance of seedlings of two Mediterranean pine species with close ecological niches, but different drought tolerance ( Pinus pinaster is less drought-tolerant than Pinus pinea ), under two [CO 2 ] (380 ppm vs 800 ppm) and two watering regimes (well-watered vs water stressed; 75% vs 40% field capacity) for 161 days. We quantified biomass allocation patterns, net photosynthetic rates (A n ), stomatal conductance to water vapor (g wv ), and water-use efficiency at plant (WUE) and leaf levels (intrinsic, WUEi). In both species, elevated [CO 2 ] increased total biomass, A n , WUE and WUEi, but reduced g wv . Most functional traits were similar between species with ample water but differed under water stress. Under water stress, P. pinaster had lower shoot:root ratios, A n , g wv and WUEi than P. pinea , but higher biomass when well-watered. Species’ WUE and WUEi responses to water stress differed. The effects of water stress on these traits were more negative under low [CO 2 ]. Under enriched [CO 2 ], functional leaf adjustments benefited the carbon budget of both species by stimulating higher A n despite lower g wv , particularly in P. pinea . Overall, P. pinea performed better under water stress than P. pinaster . This relative performance between species was not modified by elevated [CO 2 ]. Thus, under future more arid climate and higher [CO 2 ], P. pinea seedlings would be expected to maintain their relative competitive advantage over P. pinaster .