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Tropical savannas have a hidden weapon against global warming 

In a new study led by Young Zhou from the Quinney College of Natural Resources at Utah State University, researchers have explored the climate mitigation potential of tropical savannas. These are unique ecosystems where trees and grasses coexist. 

With the ever-increasing threat of global warming, efforts are being made worldwide to capture carbon dioxide and other heat-trapping gases from the atmosphere. 

Some of the most commonly proposed nature-based solutions for carbon capture are  planting trees and allowing forests to regrow.

Focus of the study

The research was focused on afforestation, planting trees with the prime goal of capturing atmospheric carbon dioxide. As these trees grow, carbon is stored both in their woody structure and the surrounding soils. 

Although the carbon storage potential of trees has long been recognized, the role of grasses and their surrounding soil has not been well-studied.

Tropical savannas 

The experts set out to gain a better understanding of the mechanics of carbon storage in the soils of tropical savannas. The findings have been published in the journal Nature Geoscience.

The analysis was based on a case study in South Africa’s Kruger National Park, as well as data from tropical savannas worldwide. 

What the researchers discovered 

The experts discovered that soils enriched with carbon from grasses had notably higher carbon concentrations. Impressively, grasses were responsible for more than half of the soil carbon content in these ecosystems, even beneath tree canopies. 

This revelation highlights the substantial role of grasses in the carbon sequestration mechanism associated with tropical savannas.

The role of tree cover

“This underscores the nuanced nature of increasing tree cover on the dynamics of carbon in savanna soils,” said Zhou.

“On average, the increase in soil carbon storage resulting from the expansion of tree cover across tropical savannas is negligible.” 

This observation aligns with the team’s earlier findings that pinpointed the effect of increased tree cover due to fire suppression: it led to heightened carbon storage in trees but left soil carbon storage largely unchanged.

Study implications 

“Our findings challenge the commonly held assumption that afforestation uniformly boosts soil carbon storage,” Zhou said. 

“However, we have yet to pinpoint the precise factors responsible for the substantial variation observed in the soil carbon storage response to increased tree cover across tropical savannas.”

Carbon reservoirs 

While forests primarily store carbon in their above-ground parts, grass-dominated habitats, like savannas and grasslands, primarily tuck away their carbon underground. 

These subterranean carbon reservoirs, found in extensive grass root systems and decaying organic matter, offer a robust form of long-term storage. 

Crucial roles of savannas 

In a world that faces increased warming, and with threats of droughts and wildfires becoming more frequent, soil carbon storage emerges as a resilient solution.

“It makes even more clear that savannas play crucial roles in the global carbon cycle in their unique ways, underscoring the importance of preserving and protecting these ecosystems in an equitable manner,” said Zhou.

The research team included experts from Yale University, Lawrence Berkeley National Laboratory, University of Cape Town, Texas A&M, Kruger National Park, Harvard University, and the University of Oregon

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