Response of photosynthesis, growth and water relations of a savannah-adapted tree and grass grown across high to low CO₂

Background and Aims: By the year 2100, atmospheric CO₂ concentration ([CO₂]_a) could reach 800 ppm, having risen from ∼200 ppm since the Neogene, beginning ∼24 Myr ago. Changing [CO₂]_a affects plant carbon-water balance, with implications for growth, drought tolerance and vegetation shifts. The evolution of C₄ photosynthesis improved plant hydraulic function under low [CO₂]_a and preluded the establishment of savannahs, characterized by rapid transitions between open C₄-dominated grassland with scattered trees and closed forest. Understanding directional vegetation trends in response to environmental change will require modelling. But models are often parameterized with characteristics observed in plants under current climatic conditions, necessitating experimental quantification of the mechanistic underpinnings of plant acclimation to [CO₂]_a. Methods: We measured growth, photosynthesis and plant-water relations, within wetting-drying cycles, of a C₃ tree (Vachellia karroo, an acacia) and a C₄ grass (Eragrostis curvula) grown at 200, 400 or 800 ppm [CO₂]_a. We investigated the mechanistic linkages between trait responses to [CO₂]_a under moderate soil drying, and photosynthetic characteristics. Key results: For V. karroo, higher [CO₂]_a increased assimilation, foliar carbon:nitrogen, biomass and leaf starch, but decreased stomatal conductance and root starch. For Eragrostis, higher [CO₂]_a decreased C:N, did not affect assimilation, biomass or starch, and markedly decreased stomatal conductance. Together, this meant that C₄ advantages in efficient water-use over the tree were maintained with rising [CO₂]_a. Conclusions: Acacia and Eragrostis acclimated differently to [CO₂]_a, with implications for their respective responses to water limitation and environmental change. Our findings question the carbon-centric focus on factors limiting assimilation with changing [CO₂]_a, how they are predicted and their role in determining productivity.