Tree Planting Is Booming. Here’s How That Could Help, or Harm, the Planet.

A tree planted for every T-shirt purchased. For every bottle of wine. For every swipe of a credit card. Trees planted by countries to meet global pledges and by companies to bolster their sustainability records.

As the climate crisis deepens, businesses and consumers are joining nonprofit groups and governments in a global tree planting boom. Last year saw billions of trees planted in scores of countries around the world. These efforts can be a triple win, providing livelihoods, absorbing and locking away planet-warming carbon dioxide, and improving the health of ecosystems.

But when done poorly, the projects can worsen the very problems they were meant to solve. Planting the wrong trees in the wrong place can actually reduce biodiversity, speeding extinctions and making ecosystems far less resilient.

Addressing biodiversity loss, already a global crisis akin to climate change, is becoming more and more urgent. Extinction rates are surging. An estimated million species are at risk of disappearing, many within decades. And ecosystem collapse doesn’t just threaten animals and plants; it imperils the food and water supplies that humans rely on.

Amid that worsening crisis, companies and countries are increasingly investing in tree planting that carpets large areas with commercial, nonnative species in the name of fighting climate change. These trees sock away carbon but provide little support to the webs of life that once thrived in those areas.

“You’re creating basically a sterile landscape,” said Paul Smith, who runs Botanic Gardens Conservation International, an umbrella group that works to prevent plant extinctions. “If people want to plant trees, let’s also make it a positive for biodiversity.”

There’s a rule of thumb in the tree planting world: One should plant “the right tree in the right place.” Some add, “for the right reason.”

But, according to interviews with a range of players — scientists, policy experts, forestry companies and tree planting organizations — people often disagree on what “right” means. For some, it’s big tree farms for carbon storage and timber. For others, it’s providing fruit trees to small-scale farmers. For others still, it’s allowing native species to regenerate.

The best efforts try to address a range of needs, according to restoration experts, but it can be hard to reconcile competing interests.

“It’s kind of the Wild West,” said Forrest Fleischman, a professor of environmental policy at the University of Minnesota.

‘Harm in the Name of Doing Good’

There is not enough land on Earth to tackle climate change with trees alone, but if paired with drastic cuts in fossil fuels, trees can be an important natural solution. They absorb carbon dioxide through pores in their leaves and stash it away in their branches and trunks (though trees also release carbon when they burn or rot). That ability to collect CO2 is why forests are often called carbon sinks.

In Central Africa, TotalEnergies, the French oil and gas giant, has announced plans to plant trees on 40,000-hectares in the Republic of Congo. The project — on the Batéké Plateau, a rolling mosaic of grasses and wooded savanna with patches of denser forests — would sequester more than 10 million tons of carbon dioxide over 20 years, according to the company.

“Total is committing to the development of natural carbon sinks in Africa,” said Nicolas Terraz, who was then Total’s senior vice president for Africa, exploration and production, in a company news release on the project in 2021. “These activities build on the priority initiatives taken by the group to avoid and reduce emissions, in line with its ambition to get to net zero by 2050.”

To achieve net zero, companies must remove at least as much carbon from the air as they release. Many, like TotalEnergies, are turning to trees for help with that. On the Batéké Plateau, an acacia species from Australia, intended for selective logging, will cover a large area.

The project, part of a Congolese government program to expand forest cover and increase carbon storage, would create jobs, the company said, and ultimately broaden the ecosystem’s biodiversity as local species are allowed to grow in over decades.

But scientists warn that the plan may be an example of one of the worst kinds of forestation efforts: planting trees where they would not naturally occur. These projects can devastate biodiversity, threaten water supplies and even increase temperatures because, in some cases, trees absorb heat that grasslands — or, in other parts of the world, snow — would have reflected.

“We don’t want to cause harm in the name of doing good,” said Bethanie Walder, executive director of the Society for Ecological Restoration, a global nonprofit.

The Batéké Plateau is one of the least-studied ecosystems in Africa, according to Paula Nieto Quintano, an environmental scientist who has focused on the region. “Its importance for local livelihoods, its ecology and ecosystem functions are poorly understood,” Dr. Nieto said.

Those who study forest restoration emphasize that trees are not a cure-all.

“I fear that many corporations and governments are seeing this as an easy way out,” said Robin Chazdon, a professor of tropical forest restoration at the University of the Sunshine Coast in Australia. “They don’t necessarily have to work as hard to reduce their emissions because they can just say, ‘Oh, we’re offsetting that by planting trees’.”

‘There Have Been Bad Actors’

All trees store carbon, but their other benefits vary widely depending on the species and where it’s planted.

Eucalyptus, for instance, grows fast and straight, making it a lucrative lumber product. Native to Australia and a few islands to the north, its leaves feed koalas, which evolved to tolerate a potent poison they contain. But in Africa and South America — where the trees are widely grown for timber, fuel and, increasingly, carbon storage — they provide far less value to wildlife. They arealso blamed for depleting water and worsening wildfires.

Experts acknowledge that forest restoration and carbon sequestration are complex, and that commercial species have a role to play. People need timber, a renewable product with a lower carbon footprint than concrete or steel. They need paper and fuel for cooking.

Planting fast-growing species for harvest can sometimes help preserve surrounding native forests. And, by strategically adding native species, tree farms can help biodiversity by creating wildlife corridors to link disconnected habitat areas.

“This restoration movement can’t happen without the private sector,” said Michael Becker, head of communications at 1t.org, a group created by the World Economic Forum to push for the conservation and growth of one trillion trees with help from private investment. “Historically, there have been bad actors, but we need to bring them into the fold and doing the right thing.”

A challenge is that helping biodiversity doesn’t offer the financial return of carbon storage or timber markets.

Many governments have set standards for reforestation efforts, but they often provide broad leeway.

In Wales, one of the most deforested countries in Europe, the government is offering incentives for tree planting. But growers need only include 25 percent native species to qualify for government subsidies. In Kenya and Brazil, rows of eucalyptus grow on land that was once ecologically rich forest and savanna. In Peru, a company called Reforesta Perú is planting trees on degraded Amazonian land, but it’s increasingly using cloned eucalyptus and teak, intended for export.

Investors prefer them because they bring better prices, said Enrique Toledo, general manager of Reforesta Perú. “They are well-known species internationally and there is an unsatisfied demand for wood.”

When researchers from University College London and the University of Edinburgh evaluated national commitments toward reforestation and restoration, they found that 45 percent involved “planting vast monocultures of trees as profitable enterprises.”

‘The Same Species All Over the World’

When businesses promise to plant a tree for every purchase of a given product, they typically do so via nonprofit groups that work with communities around the world. The support may reforest after wildfires or provide fruit and nut trees to farmers. But even these projects can compromise biodiversity.

The planet is home to nearly 60,000 tree species. A third are threatened with extinction, mainly from agriculture, grazing and exploitation. But globally, only a tiny fraction of species are widely planted, according to tree planting groups and scientists.

“They’re planting the same species all over the world,” said Meredith Martin, an assistant professor of forestry at North Carolina State University who found that nonprofit tree planting efforts in the tropics tend to prioritize the livelihood needs of people over biodiversity or carbon storage. Over time, she said, these efforts risk reducing biodiversity in forests.

Nonprofit tree planting groups often say they plant nonnative species because local communities ask for them. But deeper engagement can yield a different story, said Susan Chomba, who oversees forest restoration and conservation in Africa for the World Resources Institute, a global research nonprofit group. When given the chance to consider what they want to accomplish on their land, farmers will recall, for instance, that when they had more trees, they also had streams, she said. They want the water back.

“Then you say, ‘In your traditional, local knowledge, what kind of tree species are suitable for returning water into the ecosystem?’” Dr. Chomba continued. “They will give you a whole range of indigenous tree species.”

A major hurdle is lack of supply at local seed banks, which tend to be dominated by popular commercial species. Some groups overcome this problem by paying people to collect seeds from nearby forests.

Another solution, experts say, is to let forests come back on their own. If the area is only lightly degraded or sits near existing forest, a method called natural regeneration can be cheaper and more effective. Simply fencing off certain areas from grazing will often allow trees to return, with both carbon sequestration and biodiversity built in.

“Nature knows much more than we do,” Dr. Chazdon said.