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

Abstract

By the end of the century, atmospheric CO₂ concentration ([CO₂]a) could reach 800 ppm, having risen from ~200 ppm ~24 Myr ago. Carbon dioxide enters plant leaves through stomata that limit CO₂ diffusion and assimilation, imposing stomatal limitation (LS). Other factors limiting assimilation are collectively called non-stomatal limitations (LNS). C₄ photosynthesis concentrates CO₂ around Rubisco, typically reducing LS. C₄-dominated savanna grasslands expanded under low [CO₂]a and are metastable ecosystems where the response of trees and C₄ grasses to rising [CO2]a will determine shifting vegetation patterns. How LS and LNS differ between savanna trees and C₄ 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 C₃ trees and three C₄ grasses grown at 200, 400 or 800 ppm [CO₂]a. Using assimilation–response curves, we resolved LS and LNS and show that rising [CO₂]a alleviated LS, particularly for the C₃ trees, but LNS was unaffected and remained substantially higher for the grasses across all [CO₂]a treatments. Because LNS incurs higher metabolic costs and recovery compared with LS, our findings indicate that C₄ grasses will be comparatively disadvantaged as [CO₂]a rises. Show more