New Study Reveals the Carbon Storage Benefits of Savanna Grasses Over Trees

Planting trees in African savannas may undermine biodiversity without delivering the expected gains in carbon storage, a groundbreaking new study has revealed. The research, led by the University of Cape Town (UCT) and Conservation South Africa, discovered that open grasslands are highly efficient carbon sinks, with grasses—not trees—responsible for more than 90 per cent of the carbon locked away in sandy savanna soils.

Published in the journal Functional Ecology, the findings challenge the widely held global assumption that increasing tree cover is an infallible weapon against climate change, warning that well-intentioned tree-planting initiatives could do more environmental harm than good.

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Using specialised open-top growth chambers to simulate future environmental shifts, researchers grew five species of savanna trees alongside the grass species Themeda triandra under both current and elevated atmospheric carbon dioxide levels. Because trees use a photosynthetic pathway that is typically less efficient at capturing carbon dioxide than that of grasses, scientists initially expected rising emissions to give trees a competitive advantage.

Surprisingly, elevated carbon dioxide levels failed to alter the balance between the plants or increase overall soil carbon storage. Instead, the team discovered that soils containing grasses stored about 10 per cent more carbon than soils with trees alone, with almost all of that carbon traced back to underground grass roots rather than aboveground biomass.

Crucially, the study found that most of this extra carbon accumulates as particulate organic matter, a highly active form of soil carbon derived directly from plant material. While this organic matter builds up rapidly, it remains exceptionally vulnerable to degradation.

Activities such as ploughing, erosion, or the mass planting of non-indigenous trees can disrupt the fragile soil ecosystem, inadvertently releasing this vast underground carbon store back into the atmosphere. Many African savannas are naturally open landscapes where wildlife, livestock, and local communities have relied on the co-evolution of grasses and scattered trees for thousands of years. Transforming these ancient ecosystems into dense forests risks destroying unique habitats and failing to achieve long-term climate goals.

Global climate strategies must abandon their "tunnel vision" focus on forests and recognise healthy grasslands as productive ecosystems providing vital climate services rather than empty spaces waiting to be wooded.

While reforestation remains vital where forests occurred naturally in the past, conserving the grass layer in savannas offers a dual benefit by protecting critical biodiversity whilst simultaneously securing an undervalued underground carbon shield.

Present global climate models suffer from an important gap by consistently overlooking grassy ecosystems, and policymakers must urgently adjust their targets to ensure that the drive to plant trees does not uproot the very habitats keeping the planet's carbon in check.