Forests are one of the world’s largest carbon sinks, absorbing around 7.6 Gt CO2eq annually (Harris et al, 2021), the equivalent of the combined emissions of the European Union and the United States in 2022. At the same time, forests harbor most of the earth’s terrestrial biodiversity, hosting 60,000 tree species, 80 % of global amphibian species, 75% of bird species, and 68% of the world’s mammal species (UNEP, 2020).
Despite those impressive numbers, forests are also a threatened ecosystem. Between 2001 and 2023, we lost 488 million hectares of forest cover (roughly the equivalent of the European Union surface and representing 12% of the forest cover of 2000) with 16% of it being pristine tropical. This loss of forest represents 207 Gt CO2eq (Global Forest Watch) emitted over the same period.
It is in this challenging context that Afforestation, Reforestation, and Regeneration1 (ARR) projects come as an opportunity to restore both the carbon and biodiversity-related services, provided by these precious ecosystems. Like always, the devil is in the details, and if restoring vegetation cover seems like a good initiative, an aspect that determines the sustainability of this action lies in what is being planted.
Biodiversity encompasses the variety of life in all its forms—genes, species, and ecosystems—within a specific region or the entire planet. It includes the richness of species (the number of different species), genetic diversity (variations within species), and ecosystem diversity (the variety of ecosystems).
One important indicator of biodiversity in an ARR project refers to tree species that are being planted or supported through the activities.
Recently, hummingbirds’ team has carried out an analysis of all the ARR projects registered in the public registries of the largest carbon standards (see link). We found out that ARR projects use around 590 tree species. This number highlights the wide range of trees used and the impact that ARR projects could have on biodiversity. The most common are Teak (Tectona grandis), Eucalyptus (Eucalyptus grandis), Pinus (Pinus patula), Guava (Psidium guajava), and Silky Oak (Grevillea robusta).
However, among the diversity of project typologies, some projects plant more species than others. For instance, commercial plantations use on average four species. This difference between project typology raises important questions: why do forestry projects plant so few species and what could be the benefits of implementing more biodiverse plantations?
Count of projects along the number of species in their technical itinerary depending on their typology
Outside the context of the Voluntary Carbon Market, many studies have identified that most of the world plantations are monocultures, consisting of a small number of common tree genera (found as well in our analysis), such as Eucalyptus, Pinus, Acacia, Tectona, Picea, Pseudotsuga, Swietenia and Gmelina (Kelty, 2006; Piotto, 2008; Alem et al., 2015).
The advantages of monocultures are well understood and documented. Many different timber and other forest products can be grown in this kind of large-scale plantation system). Fast-growing, exotic, and low-density wood species, such as Eucalyptus, Pinus and Acacia are largely used for timber, paper pulp, charcoal, and fuel, because they have short rotation period and have advantages in competing for light, nutrients, and water resources over native plants (Li et al., 2014).
Thus, foresters implement monoculture systems, streamline planting, maintenance, and harvesting processes, which reduces labor and management costs. Uniform growth rates and plant structures simplify operations, enabling the use of specialized equipment and standardized practices. This leads to low development and maintenance costs, high productivity, and reduced costs, especially in large-scale operations. The growth of fast-growing species, such as pine or eucalyptus, allows for quicker returns on investment, making monocultures economically attractive.
Single-species plantations are exactly what they are meant to be: very productive ecosystems under predicted and steady conditions. However, monoculture forests are highly susceptible to pests and diseases due to the genetic similarity of trees. If a particular pest or pathogen is adapted to the species planted, it can spread rapidly through the entire plantation as all trees are identical and closely spread (Field, 2024). For example, in France, Contarinia sp. causes significant damage to monoculture Douglas plantations (OFF, 2023). Without the natural barriers and resistance provided by diverse species, entire forests can be wiped out in a short time, leading to substantial economic losses.
Monoculture forests are also more vulnerable to climate change. Since the entire forest consists of one species, it is not adaptable to changing conditions such as temperature fluctuations, altered precipitation patterns, or extreme weather events. A monoculture plantation may suffer from widespread tree mortality if the chosen species is ill-suited to future climatic conditions.
Coming back to the initial vision of forest as a carbon sink quite recently researchers have shown that mixed forests are 70% more efficient than monocultures at storing carbon (Warner 2023). This supports the theory that mixed-species forests generally outperform monoculture plantations in terms of carbon sequestration. The diversity of tree species in mixed forests often leads to more efficient use of light, water, and nutrients, allowing them to store more carbon in both biomass and soil. Different species have varied growth rates, rooting depths, and carbon uptake strategies, enhancing overall productivity and resilience.
At the same time while monoculture systems are sensitive, mixed species forests are more resistant to pest disease and windthrow (Verena, 2011). The presence of different species creates natural barriers against pests and diseases. Many pests and pathogens are specialized in attacking specific tree species, so diversity limits their ability to spread rapidly. Additionally, varied root systems and canopy structures improve overall forest health, making it harder for any one factor—such as wind, disease, or pests—to impact the entire ecosystem. This diversity also fosters greater nutrient cycling and resource efficiency, promoting healthier, more resilient trees.
Following with the carbon perspective it makes the carbon stock more resistant to a changing climate, which drives more intense natural hazards (storms, flooding, droughts…) and new pathogens or pests to attack forests. Like in an investment portfolio, the more diverse the carbon stock the less vulnerable to risks.
As said earlier forests are not only important for carbon management but are essential for biodiversity conservation as well. In this context mixed-species forests support greater biodiversity than monocultures due to their complex structure and diverse plant life. Different tree species provide a variety of habitats, food sources, and microclimates, supporting a wider range of animals including insects (Wang, 2019) and birds (Castaño-Villa ,2019). Varied canopy heights, leaf types, and seasonal patterns create niche environments, allowing different species to thrive.
Planting different species can also benefit local population, as the use of trees producing non timber goods such as fruits, nuts, medicinal plants, bark and resins, rubber can be a way to multiply streams of income and to enhance local livelihood if entrance in the plantation and collection is allowed. The presence of diverse species, the absence of clearcutting and the dead wood presence can also enhance wild mushroom production (Tomao, 2017). This species diversity supports traditional practices, sustains local economies, and reduces dependence on a single commodity. Additionally, these forests improve soil health and water retention, which can boost agricultural productivity in surrounding areas, directly enhancing food security.
Transitioning from monoculture to mixed-species, clear-cutting-free silvicultural systems is key to enhancing ecosystem resilience, promoting biodiversity, and boosting long-term carbon sequestration. Diverse forests not only store more carbon but also create healthier ecosystems that support richer wildlife and offer sustainable livelihoods to local communities. Such forests are more resilient to environmental stressors like pests, diseases, and climate change.
However, this transition comes with challenges, notably higher costs associated with managing more complex ecosystems. These costs, reflected in higher prices for timber and carbon credits, can make it economically difficult for some stakeholders to adopt mixed-species plantations.
One practical way to initiate this transition is by integrating biodiverse hedgerows or buffer zones between monoculture plots. These features improve landscape connectivity, allowing species to move between ecosystems, and serve as barriers that reduce the spread of pests and diseases. They also contribute to biodiversity by providing habitats for wildlife and contributing to carbon sequestration.
Another approach is to increase genetic diversification within tree species. By planting genetically diverse individuals, forest managers can enhance the resilience of trees to environmental stresses, diseases, and pests. This enhances the forest’s ability to adapt to climate change while maintaining productivity.
Incorporating these strategies offers a balanced path forward, allowing forest managers to gradually reduce dependence on monoculture systems while fostering ecological, social, and economic benefits. Transitioning to mixed-species forestry is not only an environmental necessity but a long-term investment in the health of our planet and the people who rely on it.
