Kimberley Simpson, University of Sheffield

Plants use carbon dioxide (CO₂) in photosynthesis to make the carbohydrates they need to grow. As the CO₂ in the atmosphere rises, mainly because humans are burning fossil fuels, many plants can photosynthesise more or use water more efficiently. This is known as “CO₂ fertilisation”.

Scientists have carried out numerous studies into CO₂ fertilisation in forests, crops and temperate ecosystems. What has been much less researched is how wild grasses in tropical savannas respond.

For decades, the grasses that dominate the ground layer of these savannas were expected to respond less strongly to rising CO₂ than many trees and other plants. Most use a type of photosynthesis called C4 photosynthesis. They include common African savanna species such as red grass (Themeda triandra) and bushveld signal grass (Urochloa mosambicensis).

C4 grasses already concentrate CO₂ inside their leaves to help them make food through photosynthesis more effectively. Scientists therefore thought that the grasses already had almost as much as they could use and that extra CO₂ in the air would not make much difference.

I am a plant ecologist who studies how savannas respond to environmental change. With an international team of ecologists and ecosystem scientists, I set out to test whether wild savanna grasses grow more when there is more CO₂ in the air, and whether the effect differs between wet and dry conditions.

First, we looked at 70 published experiments that compared how wild C4 grasses responded to normal and higher CO₂ levels under plentiful and scarce water supplies.

We then analysed yearly grass growth from 533 sites in South Africa’s Kruger National Park between 1989 and 2021. The Kruger park was an ideal study site not only because C4 grasses dominate the landscape, but because it offers a rare dataset built on over three decades of continuous field monitoring.

We tracked changes as the CO₂ in the atmosphere rose. Finally, we used a computer model to explore how C4 grasses might grow during the rest of this century as CO₂ levels rise and temperature and rainfall change.

Our research found that grass production in Kruger National Park rose by 28% over three decades. This is because more CO₂ in the air helps savanna grasses hold onto water and grow more in dry conditions.

We also found that this growth boost could continue even as the climate gets hotter and drier. This matters because savannas cover about half of Africa. Any changes to how much grass the savannas produce could affect food for wildlife and livestock. It also affects the amount of fuel available for fires, and the movement of carbon through the land and atmosphere.

Rising CO₂ boosts grass growth most in the driest savannas

Our research found that when water was scarce, higher CO₂ allowed wild C4 grasses to lose less water while continuing to photosynthesise. In other words, they gained more carbon for each unit of water lost. In water-limited savannas, this meant grasses exposed to higher CO₂ produced more above-ground plant material. The effect was much weaker when water was plentiful.

We found much the same pattern in the Kruger National Park. We used the amount of grass above ground in the first year after a fire as our best estimate of how much grew each year. This increased by 28% between 1989 and 2021.

Over the same period, CO₂ levels in the air rose by about 18%. Even after accounting for changes in rainfall, fire and other environmental factors, grass production still showed an upward trend. This closely matched our experimental findings: rising CO₂ was helping grasses use limited water more efficiently.

As expected, the percentage increase was greatest in the Kruger Park’s driest savannas, where saving water was predicted to make the biggest difference.

We also found that taller, more productive grass species became increasingly prominent. This suggests that increasing CO₂ may be affecting both how much grass grows and which species dominate.

Our modelling suggested that this response could continue through the 21st century. Higher temperatures and increasing dryness weakened the CO₂ effect but did not eliminate it. C4 grass production looks set to increase as atmospheric CO₂ continues to rise.

Why this matters

Savannas cover roughly half of Africa, and grasses are central to how these ecosystems function.

More grass could provide more food for grazing animals, but quantity is not necessarily the same as quality. Higher CO₂ levels can change the balance of carbon and nitrogen in plants and potentially make them less nutritious.

Grass also fuels most savanna fires, so more growth could mean more material available to burn. It could also change how grasses and trees compete and how carbon moves between plants, soils and the atmosphere.

More grass growth means plants take up more carbon while growing. But taking up carbon is not the same as storing it over the long term.

Savanna grasses are eaten, decompose and burn, returning much of the carbon in their leaves and stems to the atmosphere. Long-term carbon storage in grasslands occurs mainly below ground, particularly in soils, and our review of experiments found no consistent increase in the amount of roots under higher CO₂ levels.

We therefore cannot conclude that the extra grass growth we observed offsets global warming. Instead, our study shows that rising CO₂ in the atmosphere – a main cause of climate change – is also directly reshaping ecosystems like grasslands that are exposed to it.

Scientists and land managers should therefore no longer predict savanna change from rainfall and temperature alone. CO₂ in the atmosphere is itself changing these ecosystems.

For managers, this means tracking how much grass grows, which grass species are present, grazing and the amount of material available to burn over the long term. Fire and grazing plans based on past patterns may need rethinking if the amount and type of grass growing under similar rainfall conditions are changing.

What needs to happen next

Next, we need to find out what happens to the extra grass.

We need long-term monitoring and experiments to explore the the fate of grass as it grows – whether it ends up as food for animals, goes up in smoke during a fire, or turns into rich soil organic matter.

That, in turn, determines effects on wildlife and livestock, fires in savannas and carbon storage.

Our findings therefore add an overlooked process to the climate-change story. Rising CO₂ changes savannas both through warming and shifts in rainfall, and by directly changing how grasses grow and use water. Understanding both effects will help us predict how African savannas will change in the coming decades.

Kimberley Simpson, Research Fellow, School of Biosciences, University of Sheffield

This article is republished from The Conversation under a Creative Commons license. Read the original article.