Multi-species forests can sequester twice as much carbon as monoculture plantations, according to a new study in the Proceedings of the Royal Society of London B.

“With increased species richness, more carbon is stored both above and below ground—in trunks, roots, deadwood, mould, and soil,” University of Zurich plant biologist and study co-author Prof. Bernhard Schmid told Carbon Brief. “You can roughly say that a diverse forest stores twice the amount of carbon as the average monoculture.”

However, “the total amount of CO2 that could be offset using afforestation remains uncertain,” the UK news outlet reports, in a post republished on Resilience.org. “This is because there are still lots of unknowns—including which areas should be targeted for afforestation and which tree species would be the best to use.”

Scientists who weren’t involved with the study also caution its small scale may have led the researchers to overstate the benefit they’d discovered.

The study focused on forest plots in China’s Gutianshan National Nature Reserve, each of them containing three to 20 tree species. It found that carbon stocks grew 6.4% for each new species added to a plantation.

In the first phase, the researchers measured changes in the weight of biomass amassed in the different plots between 2008 and 2010. “The average amount of carbon stored across all the plots was 149 tonnes per hectare, with above-ground carbon contributing 56 tonnes per hectare, ground carbon contributing 11 tonnes per hectare, and below-ground carbon contributing 81 tonnes per hectare,” Carbon Brief recounts.

Then the study team used statistical methods to estimate the share of the difference in carbon storage among plots that could be attributed to species diversity. While they concluded that diversity accounted for about one-quarter of the difference, and forest age for another quarter, they acknowledged a variety of “residual” factors they couldn’t account for.

Finally, the researchers compared carbon storage in the diverse forests against Chinese monoculture plantations, most of them made up of eucalyptus, bamboo, or Japanese cedars. On that basis, “our recommendation is that afforestation programs should switch from focusing on monocultures to mixtures,” Schmid said. “Until we know more about the contribution of each particular species, the best approach is simply to plant several species in mixtures. This will help both carbon storage and the preservation of biodiversity,” since a more diverse forest also supports a wider mix of animal species.