Stomatal and non-stomatal limitations in savanna trees and C4 grasses grown at low, ambient and high atmospheric CO2

By the end of the century, atmospheric CO2 concentration ([CO2](a)) could reach 800 ppm, having risen from similar to 200 ppm similar to 24 Myr ago. Carbon dioxide enters plant leaves through stomata that limit CO2 diffusion and assimilation, imposing stomatal limitation (L-S). Other factors limiting assimilation are collectively called non-stomatal limitations (L-Ns). C-4 photosynthesis concentrates CO2 around Rubisco, typically reducing L-S. C-4-dominated savanna grasslands expanded under low [CO2], and are metastable ecosystems where the response of trees and C-4 grasses to rising [CO2], will determine shifting vegetation patterns. How L-S and L-Ns differ between savanna trees and C-4 grasses under different [CO2](a), will govern the responses of CO2 fixation and plant cover to [CO2](a) - but quantitative comparisons are lacking. We measured assimilation, within soil wetting-drying cycles, of three C-3 trees and three C-4 grasses grown at 200, 400 or 800 ppm [CO2](a). Using assimilation-response curves, we resolved L-S and L-NS and show that rising [CO2] alleviated L-S, particularly for the C-3 trees, but L-NS was unaffected and remained substantially higher for the grasses across all [CO2] a treatments. Because L-NS incurs higher metabolic costs and recovery compared with L-S, our findings indicate that C-4 grasses will be comparatively disadvantaged as [CO2](a) rises.