Responses to anthropogenic CO2 concentrations in the Carbon-isotope discrimination factor of South African grasses

Anthropogenic increases in atmospheric CO2 are predicted to cause lasting disturbances in tropical savanna ecosystems (Gibson & Newman, 2019; Stevens et al 2017). Specifically, increases in CO2 concentrations may be influencing the competitive dynamics between C3 photosynthetic trees and C4-grasses, facilitating a process of woody encroachment in which trees increase in biomass and outcompete their grass counterparts due to a process of carbon fertilization (Bond & Midgley, 2012; Stevens et al 2016). This threatens to change the identity and structure of savanna landscapes (Devine et al 2017). In order to understand the mechanics of woody encroachment and predict the future of savanna ecosystems, it is important that we evaluate the physiological responses of the C4 savanna grass layer to increasing atmospheric CO2. Here, we use herbarium grass specimens and stable carbon and nitrogen isotope techniques to construct an 18-decade time scale of changes in water use efficiency in savanna grasses. We show that C4 grasses have undergone increases in their water use efficiency across time, and conclude that this trend could be attributed to rising CO2 concentrations. Our findings suggest that C4 tropical savanna grasses are undergoing a carbon fertilization effect, an important finding to consider the future of savanna ecosystems under anthropogenic change.