Global vegetation stores decade of human carbon emissions underground

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Tropical rainforest at Los Quetzales National Park, Cerro de la Muerte, Costa Rica

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Almost a quarter of the mass of the world’s forests, shrublands and grasslands is stored underground, according to a new global map. This previously understudied biomass contains a total amount of carbon on a par with a decade’s worth of human carbon dioxide emissions, meaning it could play an important role in efforts to combat climate change.

While the biomass and carbon locked up in plants above ground is well known from field surveys and satellite observations, how much extends below the surface is only understood at very local levels, rather than regionally or globally.

To address the gap, Constantin Zohner at ETH Zurich in Switzerland and his colleagues extracted data from 18,000 previous local studies on the weight of roots below ground, and then used machine learning to extrapolate the data into a global biomass estimate and map. Finally, they checked their findings against four existing models used to estimate biomass.

The team found that, on average, 24 per cent of plant biomass exists underground globally, holding 113 gigatonnes of carbon, roughly the equivalent of 10 years of global CO2 emissions. “It’s quite a huge number,” says Zohner. The figure is in line with previous studies, which have estimated a range of 20 to 30 per cent of biomass below ground.

The findings give researchers a better handle on the size and location of the planet’s carbon sinks, enabling them to better predict how carbon will be locked away or released as climate change accelerates, says Zohner. “This opens up the below-ground world,” he says.

Temperature and rainfall appear to play a big role in the effort plants put into growing roots. The amount of carbon below ground is greater in cold and dry areas, with the highest fractions stored underground found in the Mongolian plateau. Warmer and wetter areas had much less biomass below the surface, by comparison. On average worldwide, 22 per cent of forest biomass was underground, rising to 47 per cent for shrublands and 67 per cent for grasslands.

Journal reference: Nature Ecology & Evolution, DOI: 10.1038/s41559-021-01485-1

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