Effects of increasing atmospheric CO₂ on leaf water δ¹⁸O values are small and are attenuated in grasses and amplified in dicotyledonous herbs and legumes when transferred to cellulose δ¹⁸O values

Summary The oxygen isotope composition of cellulose (δ 18 O values) has been suggested to contain information on stomatal conductance ( g s ) responses to rising p CO 2 . The extent by which p CO 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 p CO 2 effects on δ 18 O LW and δ 18 O C are, however, unknown. We tested the effects of p CO 2 on g s , δ 18 O LW and δ 18 O C in a glasshouse experiment, where six plant species were grown under p CO 2 ranging from 200 to 500 ppm. Increasing p CO 2 caused a decline in g s and an increase in δ 18 O LW , as expected. Importantly, the effects of p CO 2 on g s and δ 18 O LW were small and p CO 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 p CO 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, p CO 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 p CO 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.