Data from: 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.