Drought x CO2 interactions in trees: a test of the low-intercellular CO2 concentration (Ci) mechanism

Models of tree responses to climate typically project that elevated atmospheric CO2 concentration (eC(a)) will reduce drought impacts on forests. We tested one of the mechanisms underlying this interaction, the 'low C-i effect', in which stomatal closure in drought conditions reduces the intercellular CO2 concentration (C-i), resulting in a larger relative enhancement of photosynthesis with eC(a), and, consequently, a larger relative biomass response. We grew two Eucalyptus species of contrasting drought tolerance at ambient and elevated C-a for 6-9 months in large pots maintained at 50% (drought) and 100% field capacity. Droughted plants did not have significantly lower C-i than well-watered plants, which we attributed to long-term changes in leaf area. Hence, there should not have been an interaction between eC(a) and water availability on biomass, and we did not detect one. The xeric species did have higher C-i than the mesic species, indicating lower water-use efficiency, but both species exhibited similar responses of photosynthesis and biomass to eC(a), owing to compensatory differences in the photosynthetic response to C-i. Our results demonstrate that long-term acclimation to drought, and coordination among species traits may be important for predicting plant responses to eC(a) under low water availability.