Grasslands may trump forests at carbon storage in a warming world

A study finds grasslands can be more effective than forests at storing carbon in places prone to drought and wildfire.

Article by Morgan Erickson-Davis.

Forests have a reputation as a big bulwark against climate change – and for good reason. They soak up massive amounts of carbon dioxide from the air and sequester it in their trunks, roots, and branches (collectively called “biomass”). This ability to reduce atmospheric greenhouse gases has led to huge multinational efforts to protect and plant forests in the hopes that doing so will keep the world from warming past the point of no return.

But non-forest ecosystems can also store lots of carbon. New research published recently in Environmental Research Letters finds grasslands in particular can be more effective than forests at storing carbon in places prone to drought and wildfire. Its authors say their findings highlight the important role grasslands can play in mitigating global warming.

Researchers from the University of California – Davis found grasslands and rangelands (open areas used for livestock pasture) in semi-arid regions of California tended to be better at long-term storage of carbon than are forests. This is because grass stores much of its carbon underground in its root mass whereas trees store most of their carbon aboveground. When a wildfire breaks out and a forest burns, much of its carbon is released into the atmosphere. But in a grassland fire, carbon remains tucked safely underground.

The study found that grasslands don’t need to be intact to be useful for carbon storage, finding rangeland used for cattle ranching also sequestered significant amounts. However, its authors say there is room for improvement, and that boosting soil health and improving land management could make rangeland better at storing carbon.

The team modeled several climate conditions, finding the only one where trees beat out grasslands is if global carbon emissions essentially stop and warming is kept below 1.7 degrees Celsius by the end of the century. This, they write, would require “even more aggressive global greenhouse gas reductions than the Paris Climate Agreement.” And as the world veers off its path to the 2-degree Celsius threshold set by the Agreement, such a scenario is looking increasingly unlikely.

“In a stable climate, trees store more carbon than grasslands,” said study coauthor Benjamin Houlton, director of the John Muir Institute of the Environment at UC Davis. “But in a vulnerable, warming, drought-likely future, we could lose some of the most productive carbon sinks on the planet.”

While their study looked specifically at California – which has been devastated by stronger-than normal-fire seasons in recent years – Houlton and his colleagues say its findings are applicable in semi-arid regions around the world, which cover more than 40 percent of the planet. They urge that grasslands in these areas be included in carbon offset schemes like Cap and Trade programs as well as greenhouse gas mitigation targets.

“California is on the frontlines of the extreme weather changes that are beginning to occur all over the world,” Houlton said. “We really need to start thinking about the vulnerability of ecosystem carbon, and use this information to de-risk our carbon investment and conservation strategies in the 21st century.”

Citation: Dass, P., Houlton, B. Z., Wang, Y., & Warlind, D. (2018). Grasslands may be more reliable carbon sinks than forests in California. Environmental Research Letters, 13(7), 074027.

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