· Forests planted with many different tree species have substantially higher aboveground carbon stocks and greater carbon fluxes than monocultures, shows an international study led by the University of Freiburg.

· For the study, published in Global Change Biology, researchers analysed data from the world’s longest-running tropical tree diversity experiment.

· The findings highlight the benefits of mixed-species forests for mitigating climate change.

Forests with many tree species can store significantly more carbon than those with only one species: An international study led by the University of Freiburg, published in Global Change Biology, supports this finding using data from the world’s oldest tropical tree diversity experiment. Researchers found that forests planted with five tree species had substantially higher aboveground carbon stocks and greater fluxes between the carbon stores than monocultures. The results highlight the benefits of mixed-species forests for forest restoration initiatives that aim at mitigating climate change through carbon sequestration.

New data from the world’s longest-running experiment on tropical tree diversity

Growing evidence suggests that tree diversity enhances ecosystem functions like carbon sequestration. However, previous studies struggled to isolate this effect from other factors or focused on young plantations, making it uncertain whether the findings applied to older forests. To address this, the researchers analysed data from the world’s longest-running tropical tree diversity experiment, located in Panama. The Sardinilla experiment, established in 2001 on a former pasture, comprises 22 plots with one, two, three or five native tree species, which have reached a comparatively advanced stage of stand development due to the rapid growth of trees in the tropics. The team examined data related to a range of different carbon stocks and fluxes ranging from carbon in aboveground tree biomass to carbon in leaf litter and in mineral soil.

The scientists found that planted forests with five tree species had significantly higher aboveground carbon stocks and greater carbon fluxes than those with only one species. For instance, the species-rich forests captured 57% more carbon in aboveground tree biomass than monospecific forests. However, there were no differences in carbon stocks and fluxes belowground.

Diverse forests sequester more carbon – even through hurricanes and droughts

Remarkably, the positive tree diversity effect on aboveground carbon stocks strengthened over time, despite repeated climatic extreme events such as a severe El Niño-driven drought and a hurricane that hit the experiment. “This is important, because in the face of climate change, the long-term carbon balance of forests will depend largely on their stability to disturbances. Diverse forests exhibit greater ecological stability and the risk that the stored carbon is released back to the atmosphere is lower than in monocultures”, said Dr. Florian Schnabel, first author of the study, forest scientist at the University of Freiburg’s Faculty of Environment and Natural Resources, and head of the Sardinilla experiment.

More tree species, greater climate benefits

According to the researchers, these results show that tree mixtures should be preferred over monocultures in projects that plant new forests to sequester carbon. However, the team also emphasises that it is important to remain realistic about the potential of new forests to contribute towards mitigating climate change. “The average yearly net CO₂ uptake of the planted forests was 5.7 tonnes CO₂ equivalents per ha and year. It would thus need one-year tree growth on 11 ha of this type of forest to compensate for the emissions of a single one-way flight between Frankfurt and Panama City”, said Dr. Catherine Potvin, head of the Sardinilla experiment until 2024 and co-initiator of the study, from McGill University in Montréal, Canada.