In late January, the multibillionaire Elon Musk took to Twitter and abruptly announced, “Am donating $100M towards a prize for best carbon capture technology”. This triggered a deluge of sarcasm across the platform: “You mean, like, trees?” “I planted a tree, do I win?” “Lol just plant some trees dumbass.”
Musk replied, “They are part of the solution, but require lots of fresh water & land. We may need something that’s ultra-large-scale industrial in 10 to 20 years.”
This wasn’t good enough for hundreds of Twitter users, who saturated his timeline with yet more support for trees, reinforcing a powerful narrative that’s taken hold around the world: Trees are the answer to climate breakdown and many other environmental and social ills, so we need to plant more of them — billions or even trillions more.
Recent years have seen massive tree-planting projects announced and rolled out. They’re politically popular, media-friendly, and often have stunning numbers attached: In 2019, Ethiopia claimed to have planted 350 million saplings in less than 12 hours, smashing the world record for trees planted in a day. In 2014, a province of Pakistan launched a “Billion Tree Tsunami” planting project, which was expanded into a nationwide “10-Billon Tree Tsunami” project in 2018. China is on track to plant 35 million hectares (87 million acres) of trees in its northern arid areas by 2050 to make a so-called Great Green Wall the size of Germany. Fossil fuel colossus Shell says that 700 million hectares of forest must be planted this century to keep global warming under 1.5 degrees C — an area just smaller than Brazil.
Last year, the World Economic Forum (WEF) launched its 1t.org project, which aims to mobilize commitments from corporations, governments, and NGOs to “conserve, restore, or grow” one trillion trees by 2030. One of its leading supporters is Marc Benioff, the billionaire CEO of Salesforce, who pledged funds to plant 100 million of them. During the 2020 WEF meeting in Davos, Switzerland, Benioff announced that 300 companies and numerous governments have committed to hitting 1t.org’s target. And their numbers keep growing: A company called tentree recently promised to plant a billion trees by 2030, beating Benioff’s commitment by an order of magnitude. “Nobody’s against trees,” said Benioff, adding, “This is a time of action, not words. We are at that point of urgency with our planet.”
Tree planting can distract from the greater priorities of protecting existing forest and reducing fossil fuel use, critics say.
But a growing number of scientists and environmentalists are challenging this narrative on tree-planting. They say that planting programs, especially those based on large numerical targets, can wreck natural ecosystems, dry up water supplies, damage agriculture, push people off their land — and even make global warming worse. They point to flaws in the studies that have inspired large-scale programs, and say that harmful types of tree planting are regularly conflated with beneficial natural forest restoration. Tree planting can distract from the greater priorities of protecting existing forests and reducing fossil fuel use, they say, and conserving and restoring natural open ecosystems, like grasslands, can often deliver more benefits than afforestation.
Sally Archibald, an ecologist at Wits University in South Africa, says that planting enthusiasts often overlook the fact that “people are using all this land that everyone thinks is available for planting trees. If you plant trees there, you’re losing livelihoods, you’re losing future opportunities, and you’re damaging natural biodiversity.”
Many planting proponents acknowledge that trees can have negative effects, but that putting the “right trees in the right places” can sequester carbon, rebuild forests, and enhance agricultural productivity. “If we plant the right trees in the right places and manage them in the right way, we can accomplish a lot,” says Wes Swaffar of American Forests, which leads the U.S. chapter of 1t.org. “Forests are the best nature-based solution to climate change,” he says, adding that “trees also provide many benefits to people, like improving health and creating [forestry-related] job opportunities.”
Although some large tree planting programs, like China’s, began decades ago, many observers tie the recent enthusiasm for tree-planting to the 2011 Bonn Challenge, a joint initiative of the German government and the International Union for the Conservation of Nature (IUCN) to get governments to commit to restoring 150 million hectares of “degraded and deforested” land by 2020. The initiative reached its commitment target early and now aims for 350 million hectares by 2030, and it has generated many offshoots and subsidiary programs.
Tree-planting initiatives have been further boosted by academic papers that claim enormous benefits from tree planting and maps that identify areas that could support more trees. One of the most widely-publicized is the Atlas of Forest and Landscape Restoration Opportunities published by the World Resources Institute (WRI), which delineates 2 billion hectares of what it terms “degraded” and “deforested” areas that it says are suitable for partial or full reforestation.
The Crowther Lab, a research group at ETH Zürich, a Swiss research university, published a similar map in a 2019 Science paper titled “The global tree restoration potential,” which identified 900 million hectares of land available globally for new tree cover. The researchers determined this area by mapping land with sufficient rainfall to support trees, but that is not currently occupied by forests, agriculture, or urban areas.
They calculated that planting continuous forest on this area, almost the size of the United States, would increase global forested area by a quarter and could store about a quarter of the current atmospheric carbon pool, an extraordinary amount.
The paper generated over 700 media stories and further elevated the profile of the lab’s head, Thomas Crowther, a young British ecologist. His lab has about 30 staff and an in-house public relations unit, and Crowther himself speaks on high-profile stages like TED and has access to wealthy businesspeople and top politicians. He is credited as an inspiration for the WEF’s 1t.org project and sits on its advisory council.
The 2019 tree paper earned withering criticism from many ecologists. Science published six critiques of the paper, criticizing its methods and results, and the Crowther Lab was forced to issue corrections.
Ecologists say tree promoters don’t do enough to highlight the negative effects of planting trees in areas that are naturally open.
African ecologists noticed that the Crowther map, like the WRI map, showed high-profile conservation areas — such as the Serengeti plains and the Kruger National Park — as suitable for afforestation, as well as millions of acres of grasslands and savannas that have been used by herders for millennia. This, to many, smacked of colonialism: By not excluding conservation areas and traditional rangelands, they say, these maps promote the idea that Africa’s natural heritage can be turned into industrial tree plantations to offset the rich world’s carbon emissions.
Many ecologists say that influential tree promoters don’t do nearly enough to highlight the negative effects of planting ecologically inappropriate trees or afforesting areas that are naturally open. “It’s dangerous to conflate restoration with adding trees,” says Archibald, of Wits University, because “often restoration means taking trees away.”
In South Africa, for example, many naturally open habitats, including grasslands and heathlands, have been invaded by introduced trees like eucalyptus and acacia species from Australia and pines from the Northern Hemisphere. Research shows that these non-native trees can consume significantly more water than native plants and can dry up rivers and wetlands. They also crowd out native species and increase the fuel available to wildfires, making these more dangerous. South Africa has recently faced critical water shortages affecting major cities that have been exacerbated by invasive trees, and the government spends millions of dollars annually on a program called Working for Water to remove them from key watersheds and conservation areas.
Non-native eucalyptus and pine species are commonly used in tree planting projects worldwide because they grow fast in a range of climates, but the fact that so-called ecological restoration projects are planting billions of these saplings makes no sense to ecologists who have witnessed their negative impacts.
Tree-planting critics have pointed out that in some countries there is an inherent conflict between using land for trees and using it for agriculture to feed growing populations.
Usman Ashraf, now a doctoral researcher at the University of Helsinki, documented how Pakistan’s mega tree-planting drive destroyed the traditional livelihoods of the nomadic Gujjar people in Khyber Pakhtunkhwa province. Most of the land in Ashraf’s research area is owned by members of the Pashtun ethnic group, who have rented winter pastures to Gujjar goat-herders for centuries. As part of the province’s “Billion Tree Tsunami,” which began in 2014, the government gave free seedlings to landowners and paid them annually for every tree that survived. After five years, they could cut the trees for timber.
Many Pashtuns grew alien eucalyptus on their pastures because it paid more than the traditional rentals. Gujjars lost access to large grazing areas and were forced to sell their animals, abandon their traditional way of life, and seek low-paying jobs in distant cities. And a Pakistani government investigation found that at least $3 million in Tree Tsunami funds were stolen, mostly by government officials and landowners involved in the program.
Although a leading justification for tree planting is carbon sequestration, a growing number of ecologists say many projects will not create reliable carbon stores and may actually make global warming worse.
Because of climate change, forests are increasingly vulnerable to destruction by drought, fire, insects, diseases, and storms, which releases carbon back into the atmosphere. Recent research shows that large areas of the American West may have permanently lost their forest cover. Droughts, wildfire, and insect and disease outbreaks are becoming more frequent, and forests are being replaced by grassy shrublands after these disturbances, mostly because it’s now too hot and dry for new generations of saplings to survive. A review paper published in Science last year shows that these threats, although significant and intensifying, aren’t always well understood and are difficult to compensate for. It’s hard to predict how many trees they’ll kill in the near future and how much carbon that will put into the atmosphere, but it’s likely to be considerable.
Not only might tree planting fail to reliably sequester carbon, trees can also heat the atmosphere more than many other habitat types. Kathleen Smart, a post-doctoral researcher at Rhodes University in South Africa, says that replacing surfaces like grasslands or deserts —which, being pale, reflect more solar radiation into outer space — with relatively dark-colored tree plantations can have a heating effect on a local level, and that regional-scale land-use changes have been shown to affect climate and rainfall patterns. Although it would take an enormous area of new trees to measurably heat the atmosphere at a global scale, Smart says in some cases the planetary reflectance loss from new trees may outweigh carbon sequestration gains.
Scientists recently published the “10 golden rules for restoring forests,” which prioritize conservation of existing forests over planting.
Smart and Archibald say that we should be looking less at trees and forests and more under the literal surface for carbon storage solutions, because in many parts of the world, far more carbon is held in the soil than in aboveground biomass. Depending on soil types and climate, grasses often use less water to sequester soil carbon than trees do, and can do so faster and more effectively at higher temperatures. Grasses are also less vulnerable to destruction by fire, drought, and disease.
The current focus on trees rather than grasses is partly a result of technological limitations: It’s far easier and cheaper to measure above-ground carbon held in vegetation than carbon held below-ground in soil. The repeated assessment of soil carbon stocks is currently too time-consuming and expensive to use at the scale that would be required to verify stocks for carbon credits. But excluding below-ground carbon from measurements biases the picture in favor of forests, which hold more carbon aboveground than grasslands do. Grasslands, especially waterlogged grasslands, can be shown to hold much more carbon than tree-covered landscapes if subsurface carbon is included in estimates, says Archibald.
Scientists have begun their own public relations battle against numbers-driven, large-scale tree planting. In January, scientists affiliated with the Royal Botanic Gardens at Kew in Britain published their “10 golden rules for restoring forests,” which prioritize conservation of existing natural forests over planting. “Before you do anything,” says Kate Hardwick, a Kew conservation scientist who co-authored the golden rules, “see what you can protect. Don’t use reforestation as an excuse for cutting down old-growth forests, because it’s no substitute.”
Even Thomas Crowther has backpedaled on tree planting. “1 trillion trees” really is also a symbol for ecosystem restoration,” he recently said. “If a place should naturally only have one tree per hectare, that’s the level you should be restoring. If the place should be a full forest, that’s the level you should be restoring. If there should be no trees, that’s what you should also be restoring. The tree is just the symbol of ecosystem restoration.”
But even with these caveats, Archibald says that making maps of potential tree cover like the Crowther Lab’s and WRI’s, and offering large offset payments to developing countries to plant trees, places those nations in an “unfair” position. The rich world, she says, is saying to poor countries, “We can plant trees here, we’ll give you money if you do plant trees here, but it’s up to you to decide whether it’s a good idea and whether it’s really going to fulfill your development needs.”
However, many developing countries lack the scientific expertise to thoroughly assess and plan tree-planting programs, and many local communities don’t have enough political power to determine the fate of their land, she says, so there is a real risk that politicians will push socially — and environmentally — damaging projects ahead regardless of expert concerns.
One solution, says Archibald, is better information. “So we, as scientists who disagree, are now working to identify valuable ecosystems” where planting trees would be “a bad idea.”
“It’s up to us to make our own maps,” she says.