Rising CO2 Boosts Grass Growth in African Savannas

Rising carbon dioxide levels from the burning of fossil fuels are increasing African savanna grass production by 28% over three decades, according to a study published in Nature. The research tracked continuous field monitoring data from 533 sites in South Africa’s Kruger National Park between 1989 and 2021, revealing that extra atmospheric carbon helps wild grasses use scarce water more efficiently.

The finding challenges long-standing assumptions about how tropical ecosystems respond to climate change. For decades, scientists expected that C4 grasses—which dominate the ground layer of African savannas and already concentrate carbon dioxide inside their leaves—would show little to no growth response to rising atmospheric carbon.

To test this hypothesis, an international team of ecologists analyzed 70 published experiments examining wild C4 grasses under varying water conditions. They combined those experimental results with field data gathered across Kruger National Park from 1989 to 2021, tracking plant growth as atmospheric carbon dioxide concentrations climbed by about 18%.

The investigation revealed that above-ground grass production in the park increased by 28% over the study period. This upward trend persisted even after researchers accounted for variations in rainfall, temperature, grazing, and fire.

Drier Savannas Experience the Strongest Growth Boost

The growth enhancement was most pronounced in arid environments where water scarcity limits plant activity. When water was scarce, higher carbon dioxide levels allowed wild C4 grasses to lose less moisture while maintaining photosynthesis.

This physiological shift meant grasses gained more carbon for each unit of water lost, producing more above-ground plant material in water-limited regions. Conversely, the fertilization effect was much weaker when water supplies were plentiful.

Computer models used in the study suggest this productivity boost will persist through the 21st century. While higher temperatures and increasing dryness are projected to weaken the carbon fertilization effect, they will not eliminate it entirely.

Wildfires and Wildlife Face New Ecosystem Pressures

Savannas cover approximately half of Africa, making ground-layer grasses central to regional ecosystem function. The accelerating plant growth carries significant consequences for both fauna and fire regimes.

Lead author Kimberley Simpson, an ecologist at the University of Sheffield, noted the complex outcomes of this increased biomass. The wider impact depends on where the extra grass ends up. If grazing animals consume the additional forage, animal populations could rise, though higher carbon dioxide levels may alter plant chemistry and reduce nutritional quality.

Alternatively, if the surplus grass is left untouched, it provides heavier fuel loads that can fuel larger wildfires. These fires can rapidly release the newly sequestered carbon back into the atmosphere, altering regional carbon cycles.