Carbon isotope trends across a century of herbarium specimens suggest CO2 fertilization of C4 grasses

center dot Increasing atmospheric CO2 is changing the dynamics of tropical savanna vegetation. C-3 trees and grasses are known to experience CO2 fertilization, whereas responses to CO2 by C-4 grasses are more ambiguous. center dot Here, we sample stable carbon isotope trends in herbarium collections of South African C-4 and C-3 grasses to reconstruct C-13 discrimination. center dot We found that C-3 grasses showed no trends in C-13 discrimination over the past century but that C-4 grasses increased their C-13 discrimination through time, especially since 1950. These changes were most strongly linked to changes in atmospheric CO2 rather than to trends in rainfall climatology or temperature. center dot Combined with previously published evidence that grass biomass has increased in C-4-dominated savannas, these trends suggest that increasing water-use efficiency due to CO2 fertilization may be changing C-4 plant-water relations. CO2 fertilization of C-4 grasses may thus be a neglected pathway for anthropogenic global change in tropical savanna ecosystems.