Savanna tree cover may not be a reliable climate strategy

Savannas cover about a fifth of the Earth’s surface, and have been increasingly targeted for their potential to capture carbon. This is because projections have shown that the world’s savannas capture 280 tons of carbon per hectare. Despite having fewer trees, this projection is the same amount of carbon stored in dense tropical forests. 

One proposed strategy to address climate change is to increase savanna tree cover by planting new trees or by suppressing fires. This has been considered a viable method to increase the uptake of atmospheric carbon dioxide. 

A new Yale-led study of African savannas suggests that this approach is far less effective than previously estimated.

“Increasing tree cover in savannas, whether via afforestation or fire suppression, is unlikely to yield the substantial gains in ecosystem carbon storage that have been advertised,” said study senior author Carla Staver, associate professor of ecology and evolutionary biology in Yale’s Faculty of Arts and Sciences.

Tropical savannas account for more than half of the planet’s fire-related carbon emissions. This prompted researchers to question the projected benefits of afforestation (introducing new trees to an area that has previously not been forested). 

The research team used data from an experiment in South Africa, where for 68 years, scientists have studied how fire management affects the landscape. This allowed the team to evaluate whether increased tree cover from fire suppression increases carbon sequestration.

To measure the amount of carbon stored in savannas, researchers used measurements of tree and grass biomass taken from the South African experiment. This was combined with remote sensing techniques and analysis of soil samples.

The results showed that frequently burned savannas store carbon in roots and soil more than previously estimated. Storing carbon in roots and soil occurred even when burns were prescribed annually. This highlights the importance and value of natural savannas in the fight against climate change. 

However, it was found that suppressing fire to increase tree cover did not correlate with a consistent increase in carbon storage. For example, a 78 percent increase in tree cover only resulted in 35 percent more carbon overall. 

This is equal to total gains of 23 tons of carbon per hectare. While this sounds like a big number, it is less than 10 percent of previous estimates.  

“Previous models relied on sparse data to promise a very large carbon storage benefit to increasing tree cover in savannas,” said Professor Staver. “But our direct measures show that these were not good assumptions.”

To understand how this impacts global carbon storage, researchers emphasized that this study should be replicated around the world. More data is needed to determine if other savanna sites respond in the same way. 

“But the findings are substantial,” said Professor Staver. “We need to recalibrate our assessments of the role savannas play in the global carbon cycle. And we should not be relying on afforestation to save us from accelerating human-driven carbon emissions.” 

The study is published in the journal Nature. 

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By Katherine Bucko, Earth.com Staff Writer