The world’s oldest tropical tree diversity experiment has delivered compelling evidence that forests with multiple tree species capture significantly more carbon than single-species plantations, according to new research published in Global Change Biology.
The study, led by scientists from the University of Freiburg, found that forests planted with five native tree species stored 57% more carbon above ground than monocultures after 16 years of growth, even while enduring severe droughts and a hurricane during the observation period.
These findings arrive at a critical time as countries worldwide invest in forest restoration as a nature-based solution to climate change, with current global initiatives aiming to restore 350 million hectares of forest by 2030.
“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, lead author of the study and forest scientist at the University of Freiburg’s Faculty of Environment and Natural Resources.
16 Years of Tropical Forest Growth
The research team collected data from the Sardinilla experiment in Panama, where scientists planted 22 plots with different combinations of native tree species in 2001 on former pastureland. The experimental site features plots with one, two, three, or five tree species, creating a controlled environment to isolate the effects of tree diversity from other variables.
What makes this study particularly valuable is its 16-year timespan. While previous research has suggested benefits from tree diversity, most studies examined younger forests or couldn’t separate diversity effects from other environmental factors.
The researchers measured ten different carbon-related variables, tracking how carbon moved through the forest ecosystem both above and below ground. This comprehensive approach allowed them to examine not just carbon storage but also carbon flows between different forest components, including tree biomass, leaf litter, coarse woody debris, and soil.
After 16 years, the experimental forest had accumulated an average of 35.9 metric tons of carbon per hectare in trees, while simultaneously losing about 11.2 metric tons of carbon per hectare from soil, resulting in a net gain of 24.7 metric tons of carbon per hectare.
Diversity Effect Strengthens Over Time
A key finding was that five-species forest plots stored 35.7 metric tons of carbon per hectare in aboveground tree biomass, compared to just 22.8 metric tons in single-species plots – a 57% increase. The researchers also found that more diverse forests produced 64% more leaf litter carbon than monocultures.
Perhaps most importantly, these positive diversity effects persisted despite the forest experiencing climate extremes, including an El Niño-driven drought in 2015 and Hurricane Otto in 2016. The diversity benefits actually strengthened over time for aboveground tree growth.
“We observed a remarkable increase in aboveground carbon stocks driven by tree diversity,” noted Dr. Schnabel. “Our structural equation models showed that higher tree growth in mixtures enhanced leaf litter and coarse woody debris carbon fluxes to the soil, resulting in a tightly linked carbon cycle aboveground.”
Interestingly, while tree diversity significantly boosted above-ground carbon storage, the researchers found no similar benefits below ground. Soil organic carbon showed a net reduction across all diversity levels during the study period, with no significant differences between diverse plots and monocultures.
Implications for Forest Restoration
These findings have direct implications for forest restoration projects worldwide. Many current reforestation efforts focus on single-species plantations, often using non-native trees. This research suggests that mixed-species plantings using native trees would deliver greater climate benefits while potentially offering higher biodiversity value.
Despite the clear advantages of diverse forests for carbon sequestration, the researchers cautioned against overestimating the climate mitigation potential of reforestation.
“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 from McGill University in Montreal, who co-initiated the study and led the Sardinilla experiment until 2024.
Finding the Right Balance
The researchers emphasize that forest restoration initiatives must be carefully designed to avoid competing with agricultural land needs or replacing other natural ecosystems like grasslands. According to their paper, targeting degraded lands suitable for forest growth offers a viable solution.
For those planning forest restoration projects with climate goals in mind, the message is clear: mixed-species plantings outperform monocultures not only in carbon storage but also in resilience to climate extremes and biodiversity benefits.
As Dr. Schnabel noted, “Mixed planted forests as a nature-based solution may not only enhance carbon stocks and fluxes vis-à-vis monocultures, as we show here, but also decrease the restored forests’ susceptibility to stress and disturbances and, thereby, increase carbon permanence while also providing higher levels of biodiversity and a broader range of ecosystem services.”
The study is part of TreeDivNet, the world’s largest network of tree diversity experiments, and represents a significant contribution to understanding how forest composition influences climate mitigation potential. As countries and organizations invest billions in forest restoration, this research suggests that maximizing tree diversity should be a key consideration for achieving optimal climate benefits.