Africa’s savannas are changing as carbon dioxide levels climb
Rising carbon dioxide is boosting grass growth across Africa’s savannas, with consequences for wildlife, fire, and carbon storage.
Savannas are wide, grassy areas with trees spread far apart.
They cover about one-fifth of Earth’s land and support wildlife and livestock while playing an important role in fire and the movement of carbon through nature.
Carbon dioxide is known for warming the planet, but plants also use it to make food and grow.
New findings suggest that rising carbon dioxide levels are helping wild grasses grow more in dry savannas, which was not expected before.
Dr. Kimberley Simpson, a research fellow at the University of Sheffield, led the team studying C4 grasses, which use carbon dioxide very efficiently during photosynthesis.
This helps the grasses grow well in hot, sunny conditions.
“The grasses that dominate savannas have often been considered largely unresponsive to rising carbon dioxide because they already use a highly efficient form of photosynthesis under hot and high-light conditions,” noted Dr. Simpson.
“Our findings challenge that assumption. Under dry conditions, higher carbon dioxide helps these grasses conserve water, reduce drought stress and continue growing.”
Dry conditions change responses
Leaves take in carbon dioxide through tiny pores called stomata, which also allow water vapor to escape.
When carbon dioxide is more abundant, a plant can gain carbon while losing less water, helping its leaves continue photosynthesis as the soil dries.
The team combined results from 70 experimental studies covering 82 wild C4 grass species.
This type of combined analysis looks across many earlier experiments to find a pattern that may be difficult to see in any single test.
When water was scarce, higher carbon dioxide helped the grasses grow more above the ground. It also helped them continue photosynthesis while reducing water stress in their leaves.
The roots did not consistently grow more, and the overall plant response differed between experiments. The clearest effect was more growth in shoots and leaves when water was limited.
Savanna grass growth got a boost
The team then examined 32 years of records from 533 monitoring plots in Kruger National Park in <a href="https://absafricatv.com/south-africa-sentence-undocumented-foreign-nationals-to-prison-arrest-odas-for-criminal-offence/” title=”South Africa sentence undocumented foreign nationals to prison, arrest odas for criminal offence”>South Africa.
Park workers measured grass near the end of each growing season, allowing the researchers to follow changes from 1989 through 2021.
After accounting for rainfall, temperature, fire history, soil type, and other influences, annual grass production had risen by 28 percent.
Rising carbon dioxide fits the trend
Atmospheric carbon dioxide rose by 65 parts per million during the same period. Grass growth increased the most in the driest parts of the park.
Rainfall, heat, grazing, and nitrogen pollution could not fully explain why the grass increased.
However, the observations cannot prove that rising carbon dioxide was the only cause.
Professor Colin Osborne is a plant biologist at the University of Sheffield.
“Finding the same pattern in controlled experiments and across more than three decades of observations in a natural savanna provides strong evidence that rising carbon dioxide has been changing these ecosystems for some time,” noted Professor Osborne.
Implications for savannas
Professor Carla Staver, an ecologist at Princeton University, co-led the work and considered what a thicker grass layer could mean beyond plant growth.
“This increase could have important impacts on fire activity and on wildlife, as well as on the carbon cycle, in this sensitive ecosystem and others like it,” Professor Staver said.
More grass can provide extra food for grazing animals and may change where wildlife gathers.
It can also leave more dry material to burn, but the evidence for more fires in Kruger was too weak to be certain.
The computer models showed mixed results because fires depend on how much grass is available and how dry it becomes. The findings do not mean that rising carbon dioxide will increase fires in every savanna.
Extra grass may release carbon
Aboveground grass does not keep carbon out of the atmosphere permanently. Animals can eat it, fires can burn it, and dead material can break down, sending much of that carbon back into the air.
“More grass does not automatically mean more carbon will be stored. The wider impact depends on where the extra grass ends up,” Dr. Simpson said.
“If it is eaten by animals or burned in fires, much of the carbon is quickly released back into the atmosphere.”
“Because savannas cover such large areas, even modest changes could have major consequences for wildlife, livestock, fire and the global carbon cycle.”
Grass growth may keep rising
The team used a computer model to show how plants, water, climate, and carbon may change over time.
It compared a drier part of Kruger with a wetter area under lower and higher warming scenarios through 2100.
The computer models suggested that C4 grass growth could increase by six to 21 percent from 2015 levels. Higher temperatures and less rainfall reduced this effect, but did not completely remove it.
These predictions should be treated carefully because the model showed more root growth than the experiments did.
It may also overestimate underground carbon storage and did not fully match how leaves respond to drought.
Taller grasses are becoming common
Taller grasses that produce more growth became more common in Kruger over time. This explained some, but not all, of the increase in grass growth.
Professor Osborne said the next question concerns which grasses gain the greatest advantage as conditions change.
“Our next step is to understand whether rising carbon dioxide is changing not only how much grass grows, but also which species thrive,” Professor Osborne said.
“Changes in competition between grasses could have important consequences for how savannas function in the future.”
The findings show that rising carbon dioxide can affect dry savannas, but the final impact also depends on water, heat, fires, animals, soils, and the types of grass growing there.
The study is published in the journal Nature.
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