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Summary Increasing atmospheric CO 2 is changing the dynamics of tropical savanna vegetation. C 3 trees and grasses are known to experience CO 2 fertilization, whereas responses to CO 2 by C 4 grasses are more ambiguous. Here, we sample stable carbon isotope trends in herbarium collections of South African C 4 and C 3 grasses to reconstruct 13 C discrimination. We found that C 3 grasses showed no trends in 13 C discrimination over the past century but that C 4 grasses increased their 13 C discrimination through time, especially since 1950. These changes were most strongly linked to changes in atmospheric CO 2 rather than to trends in rainfall climatology or temperature. 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 CO 2 fertilization may be changing C 4 plant–water relations. CO 2 fertilization of C 4 grasses may thus be a neglected pathway for anthropogenic global change in tropical savanna ecosystems.
Toro et al. (Wed,) studied this question.