Scientists Find Grasslands Important as Carbon ‘Sinks’

An overlook in the Cascade Siskiyou National Monument affords a glimpse of the rolling scrub oak grassland toward Ashland, Oregon

(CN) — With five of California’s most destructive wildfire seasons happening since 2006, that state should include grasslands and not just forests as promising carbon sinks, according to researchers at the University of California, Davis.

The environmental scientists found that California grasslands are better at storing carbon from the atmosphere than fire-prone trees and forests, which have transitioned from carbon sinks (reserves) to carbon generators.

Forests have been a major way to store atmospheric carbon, but when they burn they become carbon generators, and years of wildfire suppression and drought have increased wildfire risks.

Grasslands have the capacity to be more drought- and fire-resilient than forests, and should be considered in California’s carbon cap-and-trade market, which was established in 2012, according to the study published Tuesday in the journal Environmental Research Letters.

California established its carbon trade program to reduce the state’s greenhouse gas emissions to 40 percent below 1990 levels by 2020. Industrial polluters are allowed carbon offsets of up to 8 percent of their compliance obligation but the offsets are restricted to projects in forestry, urban forestry, dairy digesters, ozone destroyers and mine methane capture.

The new study recommends that grasslands be a larger part of California’s climate program.

Researchers said their discovery could help shape similar carbon offset programs around the world, particularly those in semi-arid environments, which cover about 40 percent of the planet.

“Our model simulations show that grasslands store more carbon than forests because they are impacted less by droughts and wildfires,” said lead author Pawlok Dass, a postdoctoral scholar at UC Davis.

“This doesn’t even include the potential benefits of good land management to help boost soil health and increase carbon stocks in rangelands.”

Grasslands sequester most of their carbon underground, while forests store it mostly above ground in wood and leaves. When wildfires burn trees, the carbon they stored is released into the atmosphere, but when fire burns grasslands, the carbon stored underground tends to stay in the roots and soil, the study showed.

This makes grasslands more adaptive to climate change, the authors said.

“In a stable climate, trees store more carbon than grasslands,” said co-author 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.”

Houlton said California is on the “front lines of the extreme weather changes that are beginning to occur all over the world.”

“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.”

Researchers said the only way California’s trees could be more reliable than grasslands as carbon sinks is if carbon emitters make even more aggressive efforts to curtail global greenhouse gas emissions than those provided in the Paris Climate Agreement.

But on the current path of global carbon emissions, grasslands are the only viable net carbon dioxide sink through the next century, the study found. Grasslands also continue to store carbon even during extreme drought simulations.

The authors say the results of their work can provide options for maintaining carbon sinks in natural and working lands in California.

Ranchers are beginning to use innovative management approaches to improve carbon storage, so for cap-and-trade and carbon offsets, conserving grasslands and promoting rangeland practices that sequester carbon could help California meet the state’s emission-reduction goals, according to the study.

The authors said they did not consider forest-management strategies such as forest fuels reduction in the study, but said that as long as trees are part of the state’s cap-and-trade portfolio, reducing severe wildfire through prescribed burns, strategic thinning and replanting would reduce carbon losses.

Five of California’s most destructive fire seasons have happened since 2006 — virtually every other year — and this fire season is off to an early and horrific start.

“Trees and forests in California are a national treasure and an ecological necessity,” Houlton said. “But when you assume they’re carbon sinks and trade them for pollution credits while they’re not behaving as carbon sinks, emissions may not decrease as much as we hope.”

A study this year from the University of Wyoming found that grasslands have unrealized economic value in commercial carbon programs.

“Biodiversity conservation may help increase carbon storage, but the value of this influence has been difficult to assess,” the researchers wrote. “Our work helps move beyond mere speculation about the economic importance of biodiversity and lends an economic argument to biodiversity preservation for climate protection.”

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