Abstract
Atmospheric CO 2 (C a ) has risen dramatically since preglacial times and is projected to double in the next century. As part of a 4‐year study, we examined leaf gas exchange and photosynthetic acclimation in C 3 and C 4 plants using unique chambers that maintained a continuous C a gradient from 200 to 550 µmol mol −1 in a natural grassland. Our goals were to characterize linear, nonlinear and threshold responses to increasing C a from past to future C a levels. Photosynthesis ( A ), stomatal conductance ( g s ), leaf water‐use efficiency ( A / g s ) and leaf N content were measured in three common species: Bothriochloa ischaemum , a C 4 perennial grass, Bromus japonicus , a C 3 annual grass, and Solanum dimidiatum , a C 3 perennial forb. Assimilation responses to internal CO 2 concentrations ( A /C i curves) and photosynthetically active radiation ( A /PAR curves) were also assessed, and acclimation parameters estimated from these data. Photosynthesis increased linearly with C a in all species ( P S. dimidiatum and B. ischaemum had greater carboxylation rates for Rubisco and PEP carboxylase, respectively, at subambient than superambient C a ( P A up‐regulation at subambient C a in the field. No species showed down‐regulation at superambient C a . Stomatal conductance generally showed curvilinear decreases with C a in the perennial species ( P a than superambient, suggesting that plant water relations have already changed significantly with past C a increases. Resource‐use efficiency ( A / g s and A /leaf N) in all species increased linearly with C a . As both C 3 and C 4 plants had significant responses in A , g s , A / g s and A /leaf N to C a enrichment, future C a increases in this grassland may not favour C 3 species as much as originally thought. Non‐linear responses and acclimation to low C a should be incorporated into mechanistic models to better predict the effects of past and present rising C a on grassland ecosystems.