Drought by CO2 interactions in trees: a test of the water savings mechanism

Elevated atmospheric CO2 (eC(a)) may benefit plants during drought by reducing stomatal conductance (g(s)) but any 'water savings effect' could be neutralized by concurrent stimulation of leaf area. We investigated whether eC(a) enhanced water savings, thereby ameliorating the impact of drought on carbon and water relations in trees. We report leaf-level gas exchange and whole-plant and soil water relations during a short-term dry-down in two Eucalyptus species with contrasting drought tolerance. Plants had previously been established for 9 to 11 months in steady-state conditions of ambient atmospheric CO2 (aC(a)) and eC(a), with half of each treatment group exposed to sustained drought for 5 to 7 months. The lower stomatal conductance under eC(a) did not lead to soil moisture savings during the dry-down due to the counteractive effect of increased whole-plant leaf area. Nonetheless, eC(a)-grown plants maintained higher photosynthetic rates and leaf water potentials, making them less stressed during the dry-down, despite being larger. These effects were more pronounced in the xeric species than the mesic species, and in previously water-stressed plants. Our findings indicate that eC(a) may enhance plant performance during drought despite a lack of soil water savings, especially in species with more conservative growth and water-use strategies.