- A new study in Science indicates that reforestation projects, which restore degraded or destroyed forests, are the most effective land-based method for carbon removal and biodiversity protection.
- Meanwhile, the authors found that afforestation, in which trees are added where they didn’t exist before, and bioenergy cropping, in which carbon-removing crops are planted to make biofuels, can have negative effects on wildlife, outweighing the benefits of carbon removal.
- The research highlights the importance of identifying the best places for reforestation projects, but the authors emphasize that reforestation is not a replacement for fossil-fuel reduction.
Tree-planting has become a go-to tool for taking carbon out of the atmosphere and repairing deforested habitats. And indeed, reforestation — planting trees or fostering tree regrowth in historically forested areas — is the most beneficial option, according to a new study in Science, providing habitat for wildlife while sequestering and removing carbon from the atmosphere.
But if done incorrectly, planting trees can come with a significant cost to biodiversity. By comparing the effects of replacing lost trees (reforestation), adding trees (afforestation) and growing biofuel crops over natural lands (bioenergy cropping), a team of researchers from the New York Botanical Garden, The Nature Conservancy and Princeton University found that not all techniques are equal.
As opposed to reforestation, afforestation means planting trees in historically unforested areas, such as savannas and grasslands. Bioenergy cropping means cultivating fast-growing, carbon-storing plants for biofuels. The study, published in January, incorporates IUCN data for more than 14,000 vertebrate species to measure the impact of each method on biodiversity.
“You might assume that anything we do that helps to mitigate climate change is also going to just indirectly benefit biodiversity,” said Evelyn Beaury, a co-author and assistant curator at the New York Botanical Garden’s Center for Conservation and Restoration Ecology. “That’s really the core assumption of this paper that we’re trying to test and disentangle.”
But importantly, tree-planting isn’t one-size-fits-all.
“Right tree, right place, right way,” said Susan Cook-Patton, co-author and senior forest restoration scientist for The Nature Conservancy, repeating a tagline the organization uses to encourage sustainable reforestation. Sustainable projects should prioritize native trees in a place that historically had trees that local communities actually want to steward, she explained.
This misconception that planting trees automatically creates habitat for species has often led to environmental changes that hurt both wildlife and people, Cook-Patton said. Habitat connectivity, animals’ population sizes and changing weather patterns ultimately drive whether wildlife will make use of newly forested areas, she said.
Meanwhile, planting non-native plants or adding trees to areas that are naturally sparse, such as savannas, can increase fire risk, reduce available drinking water and interfere with human activities like cattle ranching, said Kate Parr, a community and ecosystem ecologist at the University of Liverpool who was not involved with the research.
Parr led a 2024 study published in Science that employed a Mongabay reforestation app to evaluate how restoration and reforestation are conflated across Africa, revealing that many afforestation projects from the African Forest Landscape Restoration Initiative and World Resources Institute were misinformed by inaccurate environmental assessments. The groups erroneously classified some grassland and savanna ecosystems as degraded forest, when in fact they never had much tree cover to begin with. Adding trees to such ecosystems can harm local plants and animals that are adapted to these open-canopy environments, she explained. Indeed, Parr found that very few projects focus on the ground layer of savanna ecosystems — native grasses that can store lots of soil carbon — instead planting non-native trees and crops.
Another unintended effect of afforestation is albedo: the amount of sunlight reflected off of a surface. Lighter landscapes, such as a desert or savanna, reflect more solar energy, which keeps temperatures cooler. But darker landscapes, such as lush forests, absorb solar energy, increasing temperatures.
Yet, it’s understandable that afforestation would become “an enticing land-use practice for places that don’t necessarily have a lot of forests that you could sustainably restore,” Beaury said.
As countries across the globe have organized more pointed pledges to address climate change, reforestation and other land-based strategies have become an important component of many national climate goals.
“Obviously, [these strategies] aren’t the biggest part of any single plan to reach net zero, because they aren’t necessarily as efficient as shifting from coal and natural gas to solar panels, but they have this unique capability of getting us to reverse the emissions that we’ve already put into the atmosphere,” said lead author Jeffrey Smith, an ecologist at the Princeton University High Meadows Environmental Institute.
Less clear was how those strategies affect biodiversity. Smith led two analyses to assess each method’s effectiveness for the recent study: how habitat conversion affects vertebrates and how climate mitigation affects vertebrates. What they found was that altering habitats — such as removing, replacing and adding trees — had a much bigger effect on vertebrate species than climate change itself to date.
“It’s not to say that addressing climate change isn’t good for biodiversity, but we have to be really cognizant of what it means for habitat[s],” Smith said. “The more you can do to address climate change through reforms to the energy system, through shifting away from fossil fuels, the better.”
Smith noted that other nature-based methods, such as wetland restoration and applying biochar to crops, were excluded because of data deficiencies. The researchers also lacked sufficient data for invertebrates, though these make up the bulk of species on Earth.
“This is a massive global study that gives us sort of a broad sense of regional patterns, but for any sort of given location, there’s a lot of nuance that we’re missing,” Smith said.
Smith hopes the study’s results help global leaders recognize reforestation as the most efficient land-based solution to their sustainability goals. Plus, in another study from The Nature Conservancy and Conservation International, reforestation was found to be “orders of magnitude” cheaper per ton of CO2 than other carbon-removal technologies, Cook-Patton said. Although reducing fossil fuels is key, properly restoring natural habitats with native species can go a long way for countries and communities with limited resources.
“That’s what we try to get our science to do: help accelerate finding those locations where it does work,” she said.
The number-one option, however, is protecting existing forests. It sequesters more carbon, protects more biodiversity and is 7-9 times more cost-effective than tearing them down and regrowing them later, according to a 2019 study in Nature Climate Change. Given the urgency of climate action, preserving what’s left is crucial, Cook-Patton said.
Beaury said their research will hopefully encourage funding for projects that are sustainable, responsible and prioritize the needs of local communities. It’s important to scale projects quickly, she added, but also mindfully.
“It’s so easy for the general public and big international NGOs and governments to get caught up in this tree-planting frenzy,” Parr said. “We have to be very careful about how we communicate the complexities more broadly.”
Banner image: In São Paulo, Brazil a farm with a mix of native Atlantic Forest species as well as other species for the production of wood, food, and seeds. Agroforestry systems like this one can help enrich soils, boost yields, buffer crops and livestock from climate extremes, diversify incomes and support biodiversity. Image © Felipe Fittipaldi.
Study says land restoration worldwide can be funded with tiny fraction of global GDP
Citations:
Smith, J. R., Beaury, E. M., Cook-Patton, S. C., & Levine, J. M. (2025). Variable impacts of land-based climate mitigation on habitat area for vertebrate diversity. Science, 387(6732), 420-425. doi:10.1126/science.adm9485
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
Busch, J., Bukoski, J. J., Cook-Patton, S. C., Griscom, B., Kaczan, D., Potts, M. D., … Vincent, J. R. (2024). Cost-effectiveness of natural forest regeneration and plantations for climate mitigation. Nature Climate Change, 14(9), 996-1002. doi:10.1038/s41558-024-02068-1
Busch, J., Engelmann, J., Cook-Patton, S. C., Griscom, B. W., Kroeger, T., Possingham, H., & Shyamsundar, P. (2019). Potential for low-cost carbon dioxide removal through tropical reforestation. Nature Climate Change, 9(6), 463-466. doi:10.1038/s41558-019-0485-x
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