Drought × CO₂ interactions in trees: a test of the low-intercellular CO₂ concentration (C_i) 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.