A growing global push to plant over a trillion trees to combat climate change and biodiversity loss may fall short unless strategies shift from quantity to quality, new research suggests. Scientists from the Smithsonian Environmental Research Center warn that simply planting trees especially single-species plantations could create fragile ecosystems that fail to deliver long-term environmental benefits.
The findings highlight results from a decade-long experiment known as BiodiversiTREE, which compared monoculture plantations with mixed-species forests. The study reveals that forests with diverse tree species grow faster, support richer biodiversity, and are more resilient to climate stress.
Globally, large-scale tree-planting campaigns have been promoted under initiatives such as the United Nations Decade on Ecosystem Restoration and the Bonn Challenge. However, many of these efforts rely heavily on monoculture plantations vast stretches of land planted with a single tree species primarily for timber production. While efficient in the short term, these plantations are highly vulnerable to drought, pests, and disease.
Researchers point to past failures as warning signs. In one case, nearly 90 per cent of millions of saplings planted in Turkey died within months due to drought and poor maintenance. Such outcomes underscore the risks of prioritising scale over ecological design.
In contrast, the BiodiversiTREE experiment demonstrates that diversity matters. Since 2013, scientists and volunteers have planted nearly 18,000 trees across multiple plots, using a mix of 16 native species. Some plots contained only one species, while others mimicked natural forests with a mix of four to twelve species.
The results are striking. Trees grown in mixed-species environments were found to be up to 80 per cent larger than those in monoculture plots. These diverse forests developed denser canopies, which helped regulate temperature, retain moisture, and create favourable conditions for other organisms. As a result, mixed forests supported up to 50 per cent more insects, birds, and other wildlife.
Scientists attribute this success to ecological interactions. Different species complement each other by using resources more efficiently, reducing disease spread, and even producing natural chemical defenses that deter herbivores. These factors collectively enhance growth and resilience.
Despite the clear advantages, monocultures remain dominant in commercial forestry due to their simplicity, lower initial costs, and ease of management. However, researchers argue that this approach overlooks long-term risks and missed opportunities for ecosystem restoration.
To bridge the gap between science and practice, researchers have launched a new initiative called “Functional Forests.” This project aims to design tree combinations that deliver multiple benefits, including timber production, wildlife habitat, food resources, and climate resilience. By testing different species combinations, scientists hope to develop practical models that balance ecological health with economic viability.
The study also draws on findings from TreeDivNet, a global network of forest experiments involving more than 1.2 million trees. Across regions and climates, the conclusion remains consistent: diverse forests outperform monocultures in growth, carbon storage, and resistance to environmental stress.
Experts say the implications are significant as governments and organisations invest billions in restoration projects. Poorly designed plantations risk becoming “biological deserts” that lack resilience and fail to support ecosystems.
“The goal is not just to plant trees, but to create forests that can survive and thrive,” the researchers emphasise. They argue that future restoration efforts must focus on biodiversity, ecological balance, and long-term sustainability rather than simply meeting planting targets.
As climate change accelerates and biodiversity continues to decline, the study highlights a crucial shift in thinking: restoring nature requires not just more trees, but better forests.