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Increasing CO2 levels fertilize C4 grass production
Nature
(2026) Cite this article
Rising atmospheric CO2 concentrations are impacting the global terrestrial biosphere through indirect climate effects and direct effects on plant performance1,2,3. 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.
The potential for direct CO2 effects on plant productivity has long been recognized, motivating a network of free-air CO2 enrichment (FACE) experiments7,8. These show widespread but context-dependent direct CO2 effects, including increased photosynthesis, reduced stomatal conductance and water use, and increased biomass production9,10. However, in situ CO2 enrichment experiments have largely omitted major tropical and subtropical ecosystems7,8. In tropical forests, long-term vegetation monitoring is filling that gap, strongly implicating CO2 fertilization as a driver of enhanced tree productivity4. In tropical and subtropical savannas, long-term observations and modelling have also suggested a role for CO2 fertilization as a driver of C3 woody plant encroachment11,12. However, although appropriate monitoring data are available13, equivalent long-term analyses of CO2 responses in C4 savanna grasses are lacking. This uncertainty means that net outcomes for tree–grass dynamics remain unclear, including possible effects on fire regimes14, limiting our ability to model and forecast savanna responses to global change.
In situ experiments in temperate grasslands and chamber experiments with savanna grasses provide a basis for generating reasonable hypotheses for how tropical C4 grasses will respond to increasing levels of CO2. Because the C4 photosynthetic pathway evolved as a CO2-concentrating mechanism that suppresses photorespiration, C4 grasses are predicted to show smaller direct photosynthetic responses to rising atmospheric CO2 levels than C3 grasses. In C4 crops, which have undergone intensive selection for high yields, this CO2-concentrating mechanism eliminates CO2 fertilization under well-watered conditions15. However, photosynthesis in wild C4 grasses is less efficient, and CO2 fertilization has been observed across a diversity of wild C4 grass species16. Although this may be partially due to enhanced activity of the photosynthetic enzyme Rubisco, higher levels of CO2 also reduce stomatal aperture, alleviating water deficits to promote carbon fixation. FACE experiments with C4 crops show that this mechanism enables CO2 fertilization under water scarcity10. Field-based experiments in mixed C3–C4 North American temperate prairies have also show