Rising atmospheric CO₂ is boosting growth of C₄ grasses in African savannas (especially under dry conditions), according to a 2026 University of Sheffield-led study published in Nature.

The paper (“Increasing CO₂ levels fertilize C₄ grass production”) combines a meta-analysis of 70 CO₂-enrichment experiments on wild C₄ grasses, a 32-year observational record of grass production from 533 sites in South Africa’s Kruger National Park (1989–2021), and modeling with the Community Land Model.

Experimental results: Elevated CO₂ increases aboveground biomass and photosynthesis more strongly under water limitation than under well-watered conditions. Grasses reduce stomatal conductance (limiting water loss) while maintaining or increasing carbon gain, which alleviates drought stress and improves leaf water potential. Root biomass responses were more variable.

Field observations: Grass production in Kruger rose by about 28% over the period (a CO₂-linked increase of roughly 75 g m⁻², or 0.37 tons C ha⁻¹ of annual production). The relative increase was largest in the driest areas. Other factors (rainfall, temperature, grazing, fire, nitrogen deposition, and species shifts) did not fully explain the trend; the pattern aligned best with rising atmospheric CO₂.

Modeling: Simulations indicate CO₂-driven gains in aboveground C₄ grass productivity could continue through the 21st century, though higher temperatures and reduced rainfall may weaken (but not eliminate) the effect.

This challenges the long-standing view that C₄ grasses (which dominate many tropical/subtropical savannas and already use an efficient photosynthetic pathway adapted to hot, high-light conditions) respond little to rising CO₂. The benefit appears largely indirect—via improved water-use efficiency under drought—rather than a strong direct photosynthetic boost under ample water.

Savannas cover ~20% of Earth’s land surface (and about half of Africa) and account for a large share of global plant productivity.

Potential consequences include:

• Greater fuel loads and altered fire regimes.
• Changes in forage for wildlife and livestock, affecting animal distributions and habitats.
• Shifts in plant competition (including with trees) and community composition (taller, more productive grass species became more dominant in Kruger).
• Uncertain effects on long-term carbon storage, because much of the extra grass biomass may be consumed by herbivores or burned, releasing carbon rather than sequestering it.

Lead author Dr. Kimberley Simpson (University of Sheffield) noted that the results overturn assumptions about C₄ unresponsiveness under dry conditions.

Co-authors include researchers from Princeton and other institutions; Professor Colin Osborne (Sheffield) highlighted the consistency between experiments and long-term field data.

In short, rising CO₂ is already “supercharging” grass growth in water-limited African savannas via water conservation, with ongoing effects likely as concentrations continue to climb.

This adds nuance to understanding of savanna dynamics, woody encroachment, fire, and the global carbon cycle.

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C4 grasses have inherently high water use efficiency (WUE) compared to C3 plants, primarily due to their CO₂-concentrating mechanism (CCM), and elevated atmospheric CO₂ further enhances this advantage—especially under water-limited conditions.

This is central to their dominance in warm, seasonally dry environments like African savannas and helps explain findings from the recent University of Sheffield-led Nature study on rising CO₂ boosting C4 grass growth.

Water Use Efficiency (WUE) measures how effectively a plant converts water into biomass or fixed carbon.

Common metrics include:

• Intrinsic WUE (iWUE): Net CO₂ assimilation rate (A) divided by stomatal conductance (gₛ), reflecting leaf-level efficiency (µmol CO₂ mol⁻¹ H₂O).
• Instantaneous or photosynthetic WUE (PWUE): A divided by transpiration rate (E).
• Whole-plant or biomass WUE: Biomass produced per unit of water transpired or used (often g dry mass kg⁻¹ H₂O).

Higher WUE means more carbon fixed (or biomass grown) per unit of water lost.

C4 photosynthesis spatially separates initial CO₂ fixation (via PEP carboxylase in mesophyll cells) from the Calvin cycle (in bundle-sheath cells). This CCM elevates CO₂ around Rubisco, nearly eliminating photorespiration.

Key consequences for water use:

• C4 leaves saturate photosynthesis at lower intercellular CO₂ concentrations (Cᵢ) than C3 leaves.
• They achieve high A at lower gₛ, reducing water vapor loss through stomata.
• Typical Cᵢ/Cₐ ratios are ~0.3–0.4 in C4 vs. ~0.6–0.7 in C3 plants, roughly doubling theoretical WUE under similar conditions.

Empirical differences:

• Leaf-level iWUE or PWUE is commonly 1.5–3× (or more) higher in C4 than C3 grasses under comparable conditions.
• Whole-plant WUE is often ~2× higher.
• C4 grasses maintain higher A while operating at lower gₛ and transpiration, especially in hot, high-vapor-pressure-deficit environments.

This efficiency, combined with high temperature optima for photosynthesis, allows C4 grasses to thrive where water is scarce during the growing season (e.g., tropical/subtropical savannas with summer rainfall). Subtypes (NADP-ME, NAD-ME, PCK) show some variation: NAD-ME types often exhibit particularly high WUE under drought.

C4 plants also tend to have hydraulic traits (e.g., higher leaf hydraulic conductivity relative to gₛ) that support efficient water transport while maintaining the low-conductance strategy.

Although the C4 CCM already saturates Rubisco at relatively low Cᵢ, rising atmospheric CO₂ still improves WUE mainly through stomatal responses:

• Elevated CO₂ reduces gₛ (partial stomatal closure), lowering transpiration while A is maintained or only modestly increased under well-watered conditions.
• Under drought or water limitation, the reduction in water loss improves plant water status (higher leaf water potential), alleviates stress, and allows continued photosynthesis and growth. This indirect fertilization effect is stronger in dry conditions.

The recent Nature study (Simpson et al., 2026) confirmed this across 70 experiments and long-term Kruger National Park data: higher CO₂ helps wild C4 savanna grasses conserve water, reduce drought stress, and increase aboveground production (notably a 28% rise over 1989–2021 in Kruger, greatest in drier sites). Modeling suggests this can continue into the future, though warming and altered rainfall may modulate it.

Meta-analyses and chamber/FACE experiments show:

• iWUE often rises proportionally with CO₂ because gₛ declines while A holds steady or increases under stress.
• Soil moisture may be conserved in some cases, further benefiting growth.
• Benefits are more pronounced for wild (vs. highly selected crop) C4 grasses and under water scarcity.

Note that C4 photosynthesis can still be sensitive to severe drought (limited alternative electron sinks like photorespiration), but the baseline high WUE and CO₂-driven improvements provide resilience.

Ecological and Practical Implications

Savannas and grasslands: High WUE underpins C4 grass dominance in water-limited systems covering ~20% of land area. CO₂-driven WUE gains can increase grass biomass, alter fire regimes (more fuel), forage availability, competition with trees, and carbon cycling.

Crops (maize, sorghum, sugarcane, millet): C4 crops already use water more efficiently; further gains under elevated CO₂ support productivity in aridifying regions, though heat and extreme drought remain challenges. Breeding targets include optimizing stomatal dynamics (faster closure) and hydraulic traits for even higher iWUE.

Climate change context: Rising CO₂ can partially offset drying effects of warming by improving WUE and plant water status, contributing to observed greening in some drylands. However, outcomes depend on interactions with temperature, VPD, nutrients, and competition.

In summary, C4 grasses are water-efficient by design thanks to their CCM, which enables high carbon gain at low stomatal opening.

Elevated CO₂ amplifies this by further reducing water loss, particularly benefiting growth in dry conditions—as demonstrated in African savanna studies. This makes them key players in future grassy ecosystems under global change.

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Increasing CO₂ levels fertilize C₄ grass production

Increasing atmospheric CO₂ levels fertilize C₄ grass production, particularly under dry conditions, according to a 2026 Nature study led by Kimberley Simpson (University of Sheffield) with co-authors including Carla Staver (Princeton University) and Colin Osborne (University of Sheffield).

The paper, titled “Increasing CO₂ levels fertilize C₄ grass production,” combines three independent lines of evidence to challenge the long-held assumption that C₄ grasses (which dominate many tropical and subtropical savannas) respond weakly to rising CO₂ because of their efficient CO₂-concentrating mechanism.

Main Findings

Meta-analysis of 70 CO₂-enrichment experiments on wild C₄ grasses

• Elevated CO₂ significantly increased aboveground biomass and photosynthetic rates under water-limited conditions (log response ratio for aboveground biomass ≈ 0.34).
• Under well-watered conditions the response was weaker and often non-significant.
• Mechanisms: Reduced stomatal conductance limited water loss while carbon gain was maintained or increased; leaf water potential improved under drought; soil water content responses were variable.
• Root biomass responses were highly variable. Effects were consistent across C₄ biochemical subtypes, clades, and regions.

Long-term field observations

• Analysis of a unique 32-year record (1989–2021) of grass aboveground production from 533 plots in South Africa’s Kruger National Park.
• Grass production increased by 28% over the period.
• After accounting for rainfall, temperature, grazing, fire, nitrogen deposition, and species composition shifts, the trend was most consistent with the rise in atmospheric CO₂ (~60 ppm over the interval).
• The relative increase was greatest at the driest sites, matching experimental patterns.
• Part of the biomass gain was linked to a shift toward taller, more productive grass species, but CO₂ remained the dominant explanatory factor.

Modeling with the Community Land Model (CLM)

• Simulations indicate that CO₂ fertilization of C₄ grass aboveground productivity is likely to continue through the 21st century.
• Higher temperatures and reduced rainfall weaken but do not eliminate the positive effect.

The study estimates a CO₂-driven increase of approximately 75.1 g m⁻² (95% CI 74.5–75.8 g m⁻²), or 0.37 tons C ha⁻¹, of annual aboveground production in the Kruger data.

Savannas cover roughly one-fifth of Earth’s land surface and contribute ~30% of global terrestrial net primary production, so even modest fertilization effects can have large aggregate impacts.

Implications

• Ecosystem function: Greater grass biomass can increase fuel loads (altering fire regimes), change forage availability for wildlife and livestock, and shift competitive dynamics with trees.
• Carbon cycle: Extra grass production does not automatically translate into long-term carbon sequestration; much of the biomass may be consumed by herbivores or burned, returning carbon to the atmosphere.
• Future trajectories: Direct CO₂ effects must be considered alongside changes in temperature and rainfall when forecasting savanna responses. The work highlights that C₄ grasses are more responsive to rising CO₂ than previously assumed, especially via improved water-use efficiency under drought.

In short, rising CO₂ is already fertilizing wild C₄ grass production in water-limited African savannas through enhanced water conservation and sustained photosynthesis, with evidence from controlled experiments, multi-decadal field data, and modeling all converging on the same conclusion.

Published:  Nature

DOI: 10.1038/s41586-026-10935-4

Provided: University of Sheffield

Authors: Kimberley J. Simpson, 
A. Carla Staver, 
James A. King, 
William J. Bond, 
Corli Coetsee, 
Nita C. M. Pallett, 
Adam F. A. Pellegrini, 
Sarah L. Raubenheimer, 
Brad S. Ripley, 
Maria Val Martin & 
Colin P. Osborne 

Abstract

Rising atmospheric CO₂ concentrations are impacting the global terrestrial biosphere through indirect climate effects and direct effects on plant performance^1,2,3. 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.