Stomatal and non-stomatal limitations in savanna trees and C grasses grown at low, ambient and high atmospheric CO.

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