Getting the mix right will be key, according to experts who stress that reforestation and afforestation efforts should not supplant work to protect existing forests. Although trees suck up carbon more rapidly in their early years, Lewis and Wheeler note that mature, natural forests are 40 times better than plantations at storing carbon and six times better than forestry (where crops and useful trees are grown together).

This is important because tree planting attempts can have unintended consequences. A study of four developing countries that switched from net deforestation to net reforestation between 1961 and 2007 found that most ended up importing more wood and agricultural products from abroad—potentially leading to forest loss or degradation elsewhere.

In China, ambitious national afforestation plans have succeeded in vastly increasing the number of trees. But native forests have effectively been displaced by tree plantations, according to research published in the journal Biological Conservation, and the addition of non-native species could have long-term impacts on the nation’s water resources.

“Number one priority is to protect what we have,” says Marie Noëlle Keijzer, co-founder and CEO of the Belgium-based not-for-profit WeForest. “The number two priority is to restore; the trees take 10 years to become significant and then 30 years to really have absorbed all the carbon they can absorb, so you don’t want to compare a new tree with an existing tree or an existing forest with all the biodiversity and everything there.”

Nor should reforestation divert attention from the restoration of less glamorous habitats such as grassland, wetland, peatland, and bog, experts say. The authors of an article on nature-based climate solutions published in June in Nature Climate Change warn that uncontrolled afforestation could threaten some of this treeless terrain, which they found “particularly troubling given that the original habitat can often provide greater and more-resilient carbon storage benefits.”

As well as taking into account how these schemes are implemented, experts argue that reforestation efforts might be best targeted at particular areas of the world. Trees grow and take up carbon more quickly near the equator, for example, where it is warm and humid, and land is relatively cheap and available. A study published earlier this year in Science Advances found that more than 3.3 million square miles of lost tropical rainforest across Africa, Southeast Asia, and the Americas could be restored.

Keijzer calls tropical reforestation the “low-hanging fruit” for two reasons: “First, because you do create economic values for countries that need it most, so it’s an opportunity to lift millions of people out of extreme poverty.” Secondly, she points out that “if you wanted to plant a tree in Belgium, for example, you would probably spend more than 10 euros per tree if not 15,” but in tropical regions “you can plant it for half a dollar.”

How exactly forest land is restored depends on two key factors: what it currently looks like and what the ultimate aim of reforestation is.

But tropical reforestation can require significant research so projects need to be realistic about what they can achieve. “There are very few parts of the tropics where there’s enough expertise and knowledge to bring back native forests on a large scale,” says Andrew Marshall, head of the not-for-profit ecological restoration organization Reforest Africa.

He compares the U.K., where there are fewer than 20 tree species native to the whole country, to Tanzania, which has the same amount of diversity in a single acre. “You’re talking hundreds of species that you need to get methods for and/or work out a key few that grow well and the others come back,” he says. “You can’t work with everything.”

The land might already host a degraded forest, with less tree cover, fewer species, and poorer soil. It may have been deforested, where many trees have been cut down and the land is primarily used for another purpose such as farming or infrastructure. It might be dominated by an invasive species such as lianas—the big woody vines that Tarzan swings from which can quickly take over tropical land—or molinia—a grass that spreads across the Welsh uplands after fields stop being grazed.

In the most extreme cases, the land may even have become incapable of hosting life, but Keijzer says she has never come across a place that can’t be restored.

In theory, reforestation in many places could be achieved through natural regeneration, where land is left to return to forest with minimal human intervention. “The safest way to do it is to find places that will recover naturally and areas that are already near other areas of forests, areas that have just very recently been cut down,” says Marshall. “Because you would expect that there would still be some seedstock in the soil and the birds and wildlife will be dispersing seeds.”

This option also has the benefit of being cheap, but letting nature take its course is not always feasible for a mixture of practical, social, and economic reasons, and a helping hand is often needed. Across the Sahel Desert in northern Africa, farmers are successfully using a managed natural regeneration technique in which they carefully nurture the remnants of old tree roots under the ground to bring trees back to life.