An international study has found that mixed forests store more carbon than single tree species, with a mix of four tree species being the most effective. Not only are mixed forests more resilient to environmental threats, they also provide more ecosystem services and biodiversity. Despite these benefits, many countries still focus on the restoration of single-species forests.
Forests are excellent at absorbing and storing carbon and could play a role in achieving global net-zero carbon emissions targets. As more and more countries work to create forests, but mostly of single species, an international team of researchers has compared the carbon stocks of mixed and single-species forests. They found that mixed forests stored more carbon, and of the forests assessed, those with four tree species had the highest carbon storage than those with a single species.
To mitigate the effects of climate change, protect biodiversity and achieve the Sustainable Development Goals, reforestation is vital. Restored forests store carbon in the forest's soil, shrubs and trees. Mixed forests are particularly effective at carbon storage because different tree species with complementary characteristics can increase overall carbon storage. Mixed forests are also more resilient to pests, diseases, and climate disturbances than single-species forests, increasing their long-term carbon storage potential. Mixed forests also provide more other ecosystem services and support higher levels of biodiversity.
Research shows that mixed forests, especially those with four species, store more carbon and provide greater ecological benefits than single-species forests. Despite the advantages of species diversity, most current forest restoration efforts favor single-species forests.
Current forest restoration practices
Although the benefits of diverse forest systems are well known, restoration commitments in many countries have focused on establishing monoculture plantations. In light of this approach, an international team of scientists compared the carbon stocks of mixed plantations, the carbon stocks of commercial and top-performing monocultures, and the average carbon stocks of monocultures.
Dr. Emily Warner, a postdoctoral researcher in ecology and biodiversity sciences at the Department of Biology at the University of Oxford and first author of the study published in Frontiers in Forests and Global Change, said: "Mixed plantations store more carbon than monoculture forests - up to 70% more. We also found that mixed forests of four tree species had the largest increase in carbon storage relative to a single tree species."
The researchers analyzed studies published since 1975 that directly compared carbon storage in mixed and single-species forests and combined them with previously unpublished data from the Global Tree Diversity Experimental Network. "We wanted to bring together and evaluate the existing evidence to determine whether forest diversification can provide carbon storage benefits," Warner explained.
The species richness of the mixed plantations evaluated in the study ranged from two to six species. In the data set used by the scientists, a mixture of four species was the most efficient carbon sink. One of the mixtures consists of different broadleaf trees from across Europe. Compared with a single tree species, a mixed forest composed of two tree species has greater above-ground carbon storage, and can increase carbon storage by up to 35%. However, a forest composed of six tree species has no clear advantage over a single tree species. As a result, the researchers were able to demonstrate that forest diversification improves carbon storage. Overall, aboveground carbon storage in mixed forests is 70% higher than the average single tree species. The researchers also found that mixed forests held 77% more carbon than commercial monocultures, which are composed of particularly productive species.
Implications for future forest management
Dr. Susan Cook-Patton, senior forest restoration scientist at The Nature Conservancy, is a co-author of the study. The researchers note that these results are particularly relevant to forest managers, suggesting that new plantation diversification has a productivity incentive effect.
While the potential for mixed forests to store more carbon is improved, the researchers caution that their study is not without limitations, including limited research on mixed versus monoculture forests overall, and particularly on older forests with higher levels of tree diversity.
"This study demonstrates the potential for plantation diversification and also demonstrates the need for long-term experimental data to explore the mechanisms behind our findings," Warner said. "There is an urgent need to further explore how diverse carbon storage benefits vary with factors such as location, tree species used and stand age."