When planting trees is bad for the planet

For decades, planting a tree has been universally understood as a good thing for Earth. In recent years, people have upped the scale, leaning on the idea of tree-planting to help mitigate climate change. The U.S. has its own billion-dollar plan. Globally, there’s the World Economic Forum’s one trillion trees initiative. Yet, despite the growing popularity of tree-planting, in many cases simply putting saplings in the ground isn’t an ecological win. When people plant trees where trees are not supposed to grow, environmental harm–not healing–can result, as exemplified in a new analysis published February 15 in the journal Science. 

Reforestation vs. afforestration

More than 70 million hectares of savannah and other non-forest habitat across Africa–in total, an area larger than the country of France–are slated for ill-advised and erroneous afforestation, according to the study. Reforestation is the process of bringing trees back to a place they’ve been removed from. In contrast, afforestation means creating forest where it hasn’t previously, naturally grown, and it’s something that people are doing in many parts of the world–from Brazil to Kazakhstan to India. In Africa, misclassification of grassland landscapes, which can include some trees but aren’t forests, means that many countries and non-governmental organizations have committed to planting trees on what should be open habitat, the researchers claim. 

“I think it’s fantastic and really timely and important work,” Meredith Martin, an assistant forestry and environmental resources professor at North Carolina State University, says of the analysis. Martin was not involved in the new study, but has similarly researched tree-planting initiatives. “In the past several years, there’s been this huge boom and interest in tree-planting–particularly around the tropics,” she says, but from the start skeptical scientists have suggested it’s not as simple an environmental solution as it sounds. Planting trees is “actually really complicated,” Martin explains. Doing it right requires land, local ecological knowledge, access to the correct tree species, equitable planning, and long-term management. Otherwise the risks and costs can easily outweigh the rewards. “I’m really pleased to see research shining a light on that and highlighting this area where trees aren’t necessarily an appropriate solution to environmental problems,” she adds.

Though lots of previous research has highlighted the problems and potential downsides of tree-planting, the new analysis is among the first to quantify the scale of the issue across an entire continent, says Karen Holl, an ecology professor studying restoration and conservation at the University of California, Santa Cruz. Tree-planting is a global trend and she’s unsurprised that tree-planting initiatives pose a risk to African grassland habitat, but she was “a bit surprised” by the magnitude of what the study authors found. They report “quite a large number” of hectares at risk, she notes.

The researchers assessed the pledges of 35 different countries participating in the African Forest Restoration Initiative (AFR100), which aims to restore 100 million hectares of forest across Africa by 2030. They found that more than 133 hectares have now been pledged towards that target, exceeding the goal. But in 18 of the participating countries, the total area pledged to reforestation exceeds the amount of natural forest, based on a detailed map of ecoregions. More than half of all that pledged land is in non-forest ecosystems, according to the study authors. 

They also assessed the active African restoration projects listed within the Mongabay Reforestation Directory, which tracks ongoing tree-planting initiatives, and found that 52% of those tree-planting efforts are occurring within savanna or grassland, despite the fact that there is more than enough forest in need of restoration. The study notes that, even after all of the current commitments are accounted for, an estimated 112.8 million hectares of degraded, unrestored forest across the continent will remain. 

“There’s quite clearly scope for extensive restoration of forests across Africa. We know there are lots of degraded forests that are currently not covered,” says Kate Parr, lead study researcher and a professor of tropical ecology at the University of Liverpool in the United Kingdom. Simultaneously, there’s lots of grasslands that could use their own form of restoration. The unfortunate discrepancy between what would be useful and what’s been promised is due in large part to how land is categorized, Parr and her co-authors say. 

Tree-planting gone wrong 

Grasslands and savannahs have wooded pockets, but aren’t the same as natural forest ecosystems, the study notes. Yet many international bodies like the United Nations define any land with a tree canopy cover exceeding 10 percent as forest. Part of this comes down to the need to classify land at scale, and an overreliance on methods like remote sensing, which discounts local, on-the-ground expertise and ecological understanding of a landscape. Such categorization, Parr explains, misses the proverbial forest through the trees. When grasslands are planted with trees biodiversity, local people, the water cycle, and even the climate can suffer for it, she says. 

Often, tree planting projects are monoculture operations. Frequently they involve non-native species. In most cases, the trees aren’t monitored after planting and in many cases the trees die en masse because saplings require care and continued resource inputs to survive. Done carelessly, big tree-planting operations are nothing more than a colossal waste of money, says Martin. But even if a diversity of native species are used and proper care is taken, grassland systems are meant to be open. Shading the understory can cause diverse grasses and shrubs to die off–triggering a ripple effect of cascading species losses all across the food web. People may rely on those savannah habitats for foraging, hunting, or grazing livestock–all benefits that can be lost with mismanaged tree-planting projects. Trees tend to be much thirstier than the plants generally adapted for semi-arid grasslands, and planting them can pull precious water out of the system. 

Climate questions 

Planting trees is often done with the intent of mitigating climate change through carbon storage, but prior research suggests the carbon stored underground by grasslands is more resilient to fire and other habitat disturbances than forest. Trees may store a lot of carbon, but they also might have a harder time holding onto it. And because leafy trees are darker in color than most grasses, forest has a higher albedo than savannah, absorbing more light and heat and potentially contributing to global warming, according to some of Parr’s past research.

The caveats

Yet the new study does have some limitations. For one, the databases they relied on are incomplete, notes Holl. Martin agrees there’s bound to be a sizable margin of error in doing such a large-scale analysis. 

Most critically, the assessment may inadvertently be conflating the entire umbrella of restoration efforts with plans for reforestation, says Ida Djenontin, an assistant geography professor at Pennsylvania State University who studies environmental governance. Djenontin is unconvinced that all AFR100 pledges are limited only to planting trees. Instead, she believes many of the countries are simply committing to landscape restoration–which could easily include grasslands and savannah in an ecologically responsible way. “It’s a language issue we are grappling with in the scientific community,” she says. Djentonin is wary of framing AFR100 pledges in a negative light, because to her the commitments indicate a political will for environmental good. “It’s up to scientists and practitioners to transform that political will into results and to do it right.” The pledges aren’t the issue, she says, the real problem is the approach that international organizations and funders may take in fulfilling those promises. 

Growing a real solution

Djentonin firmly agrees that grasslands should not be planted as forests and hopes to see meaningful environmental restoration across the continent conducted in a sustainable way. The best way to ensure that, she says, is by enabling bottom-up conservation efforts, where the direction and knowledge comes from the local community, which has a real stake in the long-term health of the land. True restoration should be a “decolonial” process, she says.

Martin, too, sees the best solution as one of land justice. When Indigenous people are allowed to manage their own land, research indicates that both humans and habitats are healthier, she says. “It’s not a panacea, but spending more effort on securing land tenure for people [across Africa] would go a huge way towards solving some of these problems,” Martin emphasizes. “People want some sort of scientific or technical solution, but these are linked social-ecological systems, and if we continue to ignore the role of local people and history, we will continue to screw up.”