Effects of increasing atmospheric CO 2 on leaf water δ 18 O values are small and are attenuated in grasses and amplified in dicotyledonous herbs and legumes when transferred to cellulose δ 18 O values.

The oxygen isotope composition of cellulose (δ 18 O values) has been suggested to contain information on stomatal conductance (g s ) responses to rising pCO 2 . The extent by which pCO 2 affects leaf water and cellulose δ 18 O values (δ 18 O LW and δ 18 O C ) and the isotope processes that determine pCO 2 effects on δ 18 O LW and δ 18 O C are, however, unknown. We tested the effects of pCO 2 on g s , δ 18 O LW and δ 18 O C in a glasshouse experiment, where six plant species were grown under pCO 2 ranging from 200 to 500 ppm. Increasing pCO 2 caused a decline in g s and an increase in δ 18 O LW , as expected. Importantly, the effects of pCO 2 on g s and δ 18 O LW were small and pCO 2 effects on δ 18 O LW were not directly transferred to δ 18 O C but were attenuated in grasses and amplified in dicotyledonous herbs and legumes. This is likely because of functional group-specific pCO 2 effects on the model parameter p x p ex . Our study highlights important uncertainties when using δ 18 O C as a proxy for g s . Specifically, pCO 2 -triggered g s effects on δ 18 O LW and δ 18 O C are possibly too small to be detected in natural settings and a pCO 2 effect on p x p ex may render the commonly assumed negative linkage between δ 18 O C and g s to be incorrect, potentially confounding δ 18 O C based g s reconstructions.