A new University of Cape Town (UCT)-led study has found that planting trees in African savannas may undermine biodiversity without delivering the expected gain in carbon storage.
The study, led by Dr Heidi-Jayne Hawkins of UCT’s Department of Biological Sciences and Conservation South Africa, found that grasses, not trees, are responsible for most of the carbon stored in sandy African savanna soil. The findings challenge the common belief that increasing tree cover will always lead to more carbon being locked away underground.
The researchers grew plants in specialised growth chambers – so-called Open-Top Chambers – to explore how rising carbon dioxide levels affect competition between savanna trees and grasses, and how these changes influence carbon storage in the soil. This was part of a larger study led by Rhodes University and the University of Sheffield.
In the soil-focused study, they grew five species of C3 leguminous savanna trees with or without the C4 grass species Themeda triandra under current (400ppm) compared to elevated atmospheric carbon dioxide levels (550ppm). Because C3 plants are less efficient at capturing carbon dioxide during photosynthesis, they expected that elevated carbon dioxide levels would favour tree over grass growth.
This is a real concern in a world where trees are already becoming too dense in savannas – and where much of the soil carbon derives from deep-rooted grasses. After growing plants under these conditions, they harvested them and collected soil to see whether grass-tree competition and increased carbon dioxide influenced long-term carbon storage.
The team found that soils containing grasses stored about 10% more carbon than soils with trees alone. Most of that carbon came from belowground – that is the grass roots rather than leaves or stems above ground. They also found that more than 90% of the soil carbon in the experiment could be traced back to grasses.
Dr Hawkins, an Honorary Associate Professor on plant-soil-microbe interactions at UCT, says the findings show that grasses play a much bigger role in fighting climate change than they are often given credit for.
“People often think trees are the main solution for storing carbon, but our study shows that grasses are doing much of the work below ground in African savannas,” she says. “Not only are grassy open ecosystems like savannas storing carbon, but they provide habitat for certain plants, wildlife and livestock – and these link to people’s livelihoods.”
The researchers discovered that most of the extra carbon was stored in particulate organic matter – a form of soil carbon that comes directly from plant material. While this type of carbon can build up quickly, it is also vulnerable to being lost if grasslands are disturbed through activities such as tree planting, ploughing or erosion. Dr Hawkins says that the accumulation of this form of carbon in response to treatments was unexpected, highlighting how little we know about carbon dynamics in savannas.
The findings come as governments and organisations around the world invest heavily in tree-planting programmes to remove carbon dioxide from the atmosphere. Dr Hawkins notes that many of these efforts continue to advocate planting non-indigenous trees and planting trees where they did not occur naturally.
Many African savannas are naturally open landscapes where grasses and scattered trees have evolved together over thousands of years. Converting these ecosystems into forests could reduce biodiversity while failing to increase long-term soil carbon storage.
Dr Hawkins says climate solutions should not have “tunnel vision” on carbon at the expense of biodiversity, and the animals and people who rely on it.
“Tree planting plays an important role in restoring forests where they occurred naturally in the past,” she says. “But in savannas, conserving the grass layer can deliver important climate benefits while also protecting biodiversity.”
The team also tested whether higher levels of atmospheric carbon dioxide – expected as the climate warms – would help trees outcompete grasses or increase soil carbon storage. They found little evidence that rising carbon dioxide levels altered the balance between trees and grasses or increased carbon storage in the soil.
“Our work highlights an important gap in global climate models which often focus on forests while overlooking the role of grassy ecosystems like savannas in storing carbon,” says Dr Hawkins, adding that conserving grasslands and savannas should be recognised as an important climate strategy alongside protecting forests.
According to Dr Hawkins, the findings add to the growing body of evidence showing that healthy grasslands are not empty spaces waiting to become forests; they are productive ecosystems that already provide valuable climate services.