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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. 

Abstract Fire and herbivory interact to alter ecosystems and carbon cycling. In savannas, herbivores can reduce fire activity by removing grass biomass, but the size of these effects and what regulates them remain uncertain. To examine grazing effects on fuels and fire regimes across African savannas, we combined data from herbivore exclosure experiments with remotely sensed data on fire activity and herbivore density. We show that, broadly across African savannas, grazing herbivores substantially reduce both herbaceous biomass and fire activity. The size of these effects was strongly associated with grazing herbivore densities, and surprisingly, was mostly consistent across different environments. A one‐zebra increase in herbivore biomass density (~100 kg/km²of metabolic biomass) resulted in a ~53 kg/ha reduction in standing herbaceous biomass and a ~0.43 percentage point reduction in burned area. Our results indicate that fire models can be improved by incorporating grazing effects on grass biomass.