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Summary Increasing atmospheric CO₂is changing the dynamics of tropical savanna vegetation. C₃trees and grasses are known to experience CO₂fertilization, whereas responses to CO₂by C₄grasses are more ambiguous.Here, we sample stable carbon isotope trends in herbarium collections of South African C₄and C₃grasses to reconstruct¹³C discrimination.We found that C₃grasses showed no trends in¹³C discrimination over the past century but that C₄grasses increased their¹³C discrimination through time, especially since 1950. These changes were most strongly linked to changes in atmospheric CO₂rather than to trends in rainfall climatology or temperature.Combined with previously published evidence that grass biomass has increased in C₄‐dominated savannas, these trends suggest that increasing water‐use efficiency due to CO₂fertilization may be changing C₄plant–water relations. CO₂fertilization of C₄grasses may thus be a neglected pathway for anthropogenic global change in tropical savanna ecosystems.