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

Increasing atmospheric CO₂ is changing the dynamics of tropical savanna vegetation. C₃ trees and grasses are known to experience CO₂ fertilization, whereas responses to CO₂ by C₄ grasses are more ambiguous. Here, we sample stable carbon isotope trends in herbarium collections of South African C₄ and C₃ grasses to reconstruct ¹³C discrimination. We found that C₃ grasses showed no trends in ¹³C discrimination over the past century but that C₄ grasses increased their ¹³C 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. Combined with previously published evidence that grass biomass has increased in C₄-dominated savannas, these trends suggest that increasing water-use efficiency due to CO₂ fertilization may be changing C₄ plant-water relations. CO₂ fertilization of C₄ grasses may thus be a neglected pathway for anthropogenic global change in tropical savanna ecosystems.