Stomata of trees growing in CO₂‐enriched air show reduced sensitivity to vapour pressure deficit and drought

Stomatal conductance ( g s ) and photosynthetic rate ( A ) were measured in young beech ( Fagus sylvatica ), chestnut ( Castanea sativa ) and oak ( Quercus robur ) growing in ambient or CO 2 ‐enriched air. In oak, g s was consistently reduced in elevated CO 2 . However, in beech and chestnut, the stomata of trees growing in elevated CO 2 failed to close normally in response to increased leaf‐to‐air vapour pressure deficit (LAVPD). Consequently, while g s was reduced in elevated CO 2 on days with low LAVPD, on warm sunny days (with correspondingly high LAVPD) g s was unchanged or even slightly higher in elevated CO 2 . Furthermore, during drought, g s of beech and chestnut was unresponsive to [CO 2 ], over a wide range of ambient LAVPD, whereas in oak g s was reduced by an average of 50% in elevated CO 2 . Stimulation of A by elevated CO 2 in beech and chestnut was restricted to days with high irradiance, and was greatest in beech during drought. Hence, most of the additional carbon gain in elevated CO 2 was made at the expense of water economy, at precisely those times (drought, high evaporative demand) when water conservation was most important. Such effects could have serious consequences for drought tolerance, growth and, ultimately, survival as atmospheric [CO 2 ] increases.