Sheds light on role of grasses in climate change control

The rising impact of global warming has led initiatives to counteract dangerous trends, often by planting trees to trap and store carbon dioxide from the atmosphere.

New research from the Quinney College of Natural Resources and the Ecology Centre, lead by Young Zhou, indicates that, in addition to trees, modest grasses play a crucial role in carbon capture — one that is more important than previously imagined.

A recent endeavour aimed at trapping carbon in tropical savannas, an environment characterised by trees and grasses sharing space. The initiative started a tree planting effort (afforestation) to trap carbon dioxide from the air, which resulted in carbon being stored in two places, the woody biomass of growing trees and soils.

While the success of storing carbon in trees has been well established in study, the role of carbon storage in soils has not been well defined, thus Zhou and his colleagues set out to discover the role grasses played in this endeavour.

The research team found that savanna soils enriched with carbon from grasses had substantially greater carbon concentrations using a case study done in Kruger National Park, South Africa, and data synthesised from tropical savannas globally.

According to their findings, grasses accounted for more than half of the soil carbon content in tropical savannas, including soils directly beneath trees. This emphasises the important role grasses play in carbon storage in tropical savannas.

As tree cover increased across tropical savannas, their data revealed both carbon gains and losses. The greatest difference was noted in savannas with more rainfall, where tree planting is more likely to succeed, as well as in places with clay soils and savanna sites with significant contributions of carbon storage from grasses.

“This underscores the nuanced nature of increasing tree cover on the dynamics of carbon in savanna soils,” Zhou said. “On average, the increase in soil carbon storage resulting from the expansion of tree cover across tropical savannas is negligible.”

This discovery is consistent with the team's earlier Nature paper, which found that increasing tree cover due to fire suppression boosted carbon storage in woody biomass but had no effect on soil carbon storage.

"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."

In general, forests primarily store their carbon in the woody trunks and aboveground leaves. In contrast, a significant portion of carbon in grassy ecosystems, such as savannas and grasslands, is stored in the soil, primarily within the extensive root systems of the grasses as well as decaying organic matter.

In the context of long-term carbon storage, carbon retained in soils proves to be more reliable, particularly for a vulnerable future marked by warming and increased likelihood of drought and wildfires, he said.

“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,” he said.