Carbon isotope trends across a century of herbarium specimens suggest CO fertilization of C grasses.

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