Increasing CO₂ levels fertilize C₄ grass production.

Abstract

Rising atmospheric CO₂ concentrations are impacting the global terrestrial biosphere through indirect climate effects and direct effects on plant performance^1-3. In tropical forests, long-term monitoring indicates a substantial CO₂-driven carbon sink⁴. C₄-grass-dominated tropical and subtropical savannas contribute approximately 30% of terrestrial net primary production⁵, and yet equivalent long-term analyses of CO₂ responses are lacking. Here we show a clear and consistent result across a meta-analysis of 70 CO₂-addition experiments and 32 years of in situ field observations from southern Africa: CO₂ fertilization of wild C₄ grasses is widespread in dry conditions. In experiments, grasses reduced stomatal conductance under higher levels of CO₂, limiting water loss while increasing carbon gain. In the field, improved water use efficiency translated into increased C₄ grass biomass production across three decades of observations. Finally, simulations via the Community Land Model⁶ suggest that CO₂ fertilization of C₄ grass aboveground productivity may continue to increase under future conditions. Together, these results challenge the view that C₄ grasses are unresponsive to increasing levels of CO₂, demonstrating instead that annual aboveground production of grasses in the field in southern Africa has increased by 28% over three decades (a CO₂-driven increase of 75.1 g m^-2 (95% confidence interval of 74.5-75.8 g m^-2) or 0.37 tons C ha^-1 of annual production). Although the fate of this carbon is uncertain (depending on feedbacks with fire, herbivory and woody vegetation), effects on the global carbon cycle may be profound.