In the fight against too much carbon in the atmosphere, researchers have found that mixed forests — and specifically mixed European hardwoods — are dramatically better at storing carbon than single species forests and plantations. That’s important news for reforestation efforts that mostly plant single-species, so-called monocultural forests.
Although the benefits of diverse forest systems are well known, many countries’ restoration commitments focus on establishing monoculture plantations. Given this practice, an international team of scientists has compared carbon stocks in mixed planted forests to carbon stocks in commercial and best-performing monocultures, along with the average of monocultures.
“Diverse planted forests store more carbon than monocultures – upwards of 70%,” said Dr Emily Warner, a postdoctoral researcher in ecology and biodiversity science at the Department of Biology, University of Oxford, and first author of the study published in Frontiers in Forests and Global Change. “We also found the greatest increase in carbon storage relative to monocultures in four-species mixtures.”
The community with the greatest relative gain in aboveground carbon was a 3.5 years-old four-species mixture comprised of silver birch (Betula pendula), European beech (Fagus sylvatica), Sessile oak (Quercus petraea), and large-leaved linden (Tilia platyphyllos) in Saxony-Anhalt, Germany. These are all broadleaf trees that can be found across Europe.
Mixes with two species also had greater aboveground carbon stocks than monocultures and stored up to 35% more carbon. Forests made up of six species, however, showed no clear advantage to monocultures.
Aboveground carbon stocks in mixed forests were 70% higher than in the average monoculture. The researchers also found that mixed forests had 77% higher carbon stocks than commercial monocultures, made up of species bred to be particularly high-yielding.
Replanting trees has long been seen as vital to slow the effects of climate change, conserve biodiversity, and meet sustainable development goals. Restored forests store carbon in soil, shrubs, and trees. Mixed forests are especially effective at carbon storage, as different species with complementary traits can increase overall carbon storage.
Compared to single-species forests, mixed forests are also more resilient to pests, diseases, and climatic disturbances, which increases their long-term carbon storage potential. The delivery of other ecosystem services is also greater in mixed-species forests, and they support higher levels of biodiversity.
“As momentum for tree planting grows, our study highlights that mixed species plantations would increase carbon storage alongside other benefits of diversifying planted forests,” said Dr Susan Cook-Patton, a senior forest restoration scientist at The Nature Conservancy and collaborator on the study. The results are particularly relevant to forest managers, showing that there is a productivity incentive for diversifying newly planted forests, the researchers pointed out.
Researchers cautioned that their study is not without limitations, including the overall limited availability of studies addressing mixed vs. monoculture forests, particularly studies from older forests and with higher levels of tree diversity.
“This study demonstrates the potential of diversification of planted forests and the need for long-term experimental data to explore the mechanisms behind our results,” Warner said. “There is an urgent need to explore further how the carbon storage benefits of diversification change depending on factors such as location, species used, and forest age.”
Access the original research article, “Young mixed planted forests store more carbon than monocultures—a meta-analysis.”
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