The responses of predawn leaf water potential (φwp), leaf conductance to water vapour diffusion (g), CO2 assimilation rate (A) and carbon isotope competition (δ13C) to a soil drying cycle were assessed in Pinus pinaster, a drought‐avoiding species with high stomatal sensitivity to drought, and Quercus petraea, a drought‐tolerant species with lower stomatal sensitivity to drought, under present (350 μmol−1) and elevated (700 μmol−1) atmospheric CO2 concentrations ([CO2]). In P. pinaster, decreasing A in response to drought was associated with increasing plant intrinsic water use efficiency (A/g) and with decreasing calculated intercellular [CO2] (C1), suggesting a stomatal limitation of A. In contrast, in Q. petraea, A/g declined and C1 increased during the drying cycle, which suggests a non‐stomatal origin for the decrease in A. In P. pinaster, a negative relationship was observed between the gas exchange‐derived values of Ci/Ca and δ13C, which conforms to the classical two‐step carbon isotope discrimination model. In Q. petraea, the relationship between C1/Ca and δ13C was positive. Possible causes of this discrepancy are discussed. Lower g values were observed under elevated [CO2] than under present [CO2] in Q. petraea, whereas g was unaffected in P. pinaster. A stimulation of A by elevated [CO2] was found in P. pinaster but not in Q. petraea. In both species, A/g was markedly higher under elevated than under present [CO2]. Whether the differences in the g response to elevated [CO2] found here can be generalized to other drought‐avoiding and non‐avoiding species remains to be assessed.

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Summary — Seedlings of pedunculate oak (Quercus robur L) were grown for one growing season under ambient (350 μmol mol -1 ) and elevated (700 μmol mol -1 ) atmospheric CO 2 concentration ([CO 2 ]) either in well-watered or in droughted (the water supply was 40% of the well-watered plants transpiration in both [CO 2 ]) conditions. In the droughted conditions, gravimetric soil water content (SWC) was on average 4 10 -2 g g -1 lower under elevated [

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The responses of CO2 assimilation rate (A), transpiration rate (E), and leaf conductance (g) to increasing leaf to air water vapor concentration difference (ΔW) were investigated (i) using excised shoots from mature trees of Abiesalba, Abiescephalonica, Abiesmarocana, and Abiesnordmanniana and (ii) in situ on a mature tree of Abiesbornmulleriana. Gas-exchange responses to increasing soil drought were also studied in plants of A. bornmulleriana, A. cephalonica, and Cedrusatlantica. Stable carbon isotope composition measurements were carried out on annual growth rings of A. bornmulleriana to estimate the time-integrated values of the ratio of intercellular leaf (Ci) to ambient (Ca) CO2 concentration. Increasing ΔW around the shoots reduced A and g in such a way that either Ci remained constant or its decrease was not pronounced enough for the changes in A to be accounted for by changes in g only. This suggests a direct effect of ΔW on photosynthesis. The different Abies species showed clear differences in water-use efficiency. Abiescephalonica and A. marocana had lower water costs of CO2 assimilation (E/A) than A. nordmanniana and A. alba. It has also been shown that A. cephalonica and A. marocana are characterized by an optimal stomatal control of leaf gas exchange. Stomata closed very rapidly in A. bornmulleriana in response to water supply being withheld, even prior to there being any important decrease in leaf predawn water potential. The stomatal response in C. atlantica was more gradual. In A. bornmulleriana, drought adaptation appears to be linked to the ability to avoid internal water stress, whereas drought adaptation in C. atlantica involves the ability to tolerate internal water stress. The high stomatal sensitivity mA. bornmulleriana is also supported by the isotopic carbon composition data, as shown by the substantial interannual variations in the estimates of Ci/Ca, ranging from 0.48 for the dryest years to 0.61 for the rainy years.