- Afforestation and reforestation (AR) have emerged as key climate change mitigation strategies.
- Forestation can be a benefit for biodiversity, but poorly planned projects can do more harm than good.
- A recent study offers a new way of gauging the potential of AR to achieve both carbon sequestration and biodiversity conservation, across different biomes.
- The study finds that some biomes have higher potential than others for AR, but considerable variation exists within biomes. The researchers caution that careful planning is needed.
Establishing forests can capture carbon and boost biodiversity — but some biomes are a better bet than others, a recent study finds. Forest restoration has emerged as a top nature-based solution to mitigate climate change, with numerous high-profile initiatives launched over the past few decades. And while there’s enthusiasm for replanting degraded forest areas, or reforestation, there’s also a growing unease that establishing forests in ecosystems that historically had little of them, or afforestation, could harm biodiversity.
The question is, how can we pinpoint the best places for afforestation and reforestation, or AR, on a global scale? A number of studies have tackled this challenge. For example, a 2025 Nature Communications study found that 195 million hectares (482 million acres) of land is suitable for reforestation when climate goals, nature, and people were taken into account. Though this represents an area the size of Mexico, it’s far smaller than previous estimates.
Now, a recent study in Environmental Research Letters describes a different way of gauging the potential of AR across 13 biomes. The study finds that overall, many areas within the tropical and subtropical moist broadleaf forest biome offer the highest compatibility with biodiversity conservation and carbon sequestration goals. However, there’s significant variation between areas, even within the same biome. In contrast, the study finds that all grasslands, shrublands and savanna biomes are poorly suited to AR.
Areas with high carbon sequestration potential are attractive for climate change mitigation projects, but could be risky for biodiversity, says Pavithra Rangani Wijenayake, a research associate at the National Institute for Environmental Studies in Japan and the study’s first author.
“[W]e need to detect these kinds of locations, especially on this global scale, to avoid massive plantation projects,” Wijenayake says.
For their study, the researchers define afforestation as the establishment of trees in areas that were unforested for at least 30 years, while reforestation refers to the establishing trees on more recently cleared or degraded forested land. The analysis also assumed native species were used in afforestation and reforestation, something that isn’t always the case on the ground.
To figure out how changes in forestation would affect biodiversity, the researchers used a global biodiversity model called AIM-BIO. The model includes 8,428 species across five taxonomic groups (mammals, birds, replies, amphibians and vascular plants), and looks at the relationship between a species’ occurrence and the environment.
First, the scientists selected species in the various biomes based on how sensitive they were to changes in forest land use. Then they looked at the distribution of each species under current and projected climate conditions, accounting for things like conservation status. Next, they created habitat suitability maps and combined these with data on carbon sequestration potential. That allowed them to see where habitat suitability, a proxy for biodiversity, and carbon sequestration potential aligned — or diverged.
The researchers found that within tropical and subtropical biomes, moist broadleaf forests offered the biggest win-wins for climate and biodiversity. There were also areas within coniferous forest that were suitable. However, establishing trees in tropical and subtropical savannas, shrublands and grasslands could displace existing biodiversity, while also disrupting existing pathways of carbon sequestration, the authors note.
In temperate biomes, coniferous forests had the highest habitat suitability index, and relatively high carbon sequestration rates. However this forest type covers a relatively small area. Temperate grasslands, savannas and shrublands were least suitable for AR from a biodiversity standpoint, though they offered moderate carbon sequestration potential.
The scientists also found considerable variation within biomes. For example, while 38% of subtropical dry broadleaf forest had a relatively high habitat suitability index, in other parts of the biome, they found that AR would put biodiversity at risk. Any AR initiatives would need to carry out more localized assessments and plan carefully, the authors note.
“These results align well with our previous understanding that natural climate solutions can only be successful in the long term if they are tailored to supporting native biodiversity,” writes Thomas Crowther, professor at the King Abdullah University of Science and Technology in Saudi Arabia, by email.
Wijenayake notes that the 5 degree scale used in study is relatively coarse, and more suited to analysis at a global scale. One next step would be to run a similar analysis with higher resolution, though that would entail acquiring additional data.
Still, biodiversity and carbon aren’t the whole story, Wijenayake says. Many areas with potential for AR are in developing economies, where land is also needed for farming. Her next step is to add data on agricultural impacts, to see where carbon, biodiversity and agricultural opportunities align.
Paul Smith, secretary-general at the nonprofit Botanic Gardens Conservation International (BGCI) in the U.K., says the study is “useful” and that its findings confirm what we would intuitively expect: plant trees where forests once were, but not in non-tree biomes.
In 2024, BGCI and partners launched The Global Biodiversity Standard (TGBS), an assessment scheme that certifies land management projects, including reforestation, that show a boost for biodiversity.
“We have had instances of TGBS assessments that have failed to achieve net biodiversity uplift because native tree species were planted in grassland habitat, displacing grassland species,” Smith says.
He also notes that the study assumes afforestation and reforestation use native species; whereas, in fact, many use nonnative species. This can displace biodiversity, disrupt water cycles, fail to maximize carbon sequestration, and more. Wijenayake has experienced that problem herself. As an undergraduate student, she interned and later worked on an U.N.-funded ecosystem restoration project in Sri Lanka’s hill country, planting eucalyptus, pine and acacia seedlings with members of local communities. Projects need to show results quickly, she says, so they often rely on nonnative, fast-growing species, or plant trees in any seemingly available space, without fully considering the value of existing ecosystems. At the time, she says, she didn’t realize the ecological implications.
“That’s in our image since childhood, planting trees is beneficial,” Wijenayake says, but the reality is more nuanced, and planting trees isn’t always the same as ecosystem restoration.
“I want to change the mindset of the young[er] generation,” she says.
Banner image: The Pesalat Reforestation Project in Central Kalimantan, Indonesia, restores forest within a national park degraded by fire and logging. Tropical and sub-tropical forest biomes offer some of the best opportunities for boosting biodiversity and sequestering carbon. Image by James Anderson, World Resources Institute via Flickr (CC BY-NC-SA 2.0).
Citations:
Bastin, J.-F., Finegold, Y., Garcia, C., Mollicone, D., Rezende, M., Routh, D., … Crowther, T. W. (2019). The global tree restoration potential. Science, 365(6448), 76-79. doi:10.1126/science.aax0848
Di Sacco, A., Hardwick, K. A., Blakesley, D., Brancalion, P. H., Breman, E., Cecilio Rebola, L., … & Antonelli, A. (2021). Ten golden rules for reforestation to optimize carbon sequestration, biodiversity recovery and livelihood benefits. Global Change Biology, 27(7), 1328-1348. doi:10.1111/gcb.15498
Fesenmyer, K. A., Poor, E. E., Terasaki Hart, D. E., Veldman, J. W., Fleischman, F., Choksi, P., … Cook-Patton, S. C. (2025). Addressing critiques refines global estimates of reforestation potential for climate change mitigation. Nature Communications, 16(1). doi:10.1038/s41467-025-59799-8
Lewis, S. L., Wheeler, C. E., Mitchard, E. T., & Koch, A. (2019). Restoring natural forests is the best way to remove atmospheric carbon. Nature, 568(7750), 25-28. doi:10.1038/d41586-019-01026-8
Parr, C. L., te Beest, M., & Stevens, N. (2024). Conflation of reforestation with restoration is widespread. Science, 383(6684), 698-701. doi:10.1126/science.adj0899
Wijenayake, P. R., Tsuchiya, K., Ohashi, H., Hirata, A., Hasegawa, T., Fujimori, S., … Takahashi, K. (2026). Afforestation and reforestation have varying biodiversity impacts across and within biomes. Environmental Research Letters, 21(2), 024031. doi:10.1088/1748-9326/ae34c8
Credits
