Where trees should grow
Reforestation is in vogue as a climate cure. Governments, businesses, and philanthropists are pledging billions to put seedlings in the ground. Yet planting trees in the wrong places can damage ecosystems, undermine livelihoods, and squander scarce resources. A study recently published in Nature Communications offers a sobering correction, reports Marina Martinez.
Led by scientists from The Nature Conservancy (TNC) and partner institutions, the team reviewed 89 maps of reforestation potential published since 2011, many of which had fueled grandiose claims. Earlier work, most notably by researchers at ETH Zürich in 2019, had suggested that 900m hectares could be covered in trees, an expanse larger than Brazil. Such estimates inspired initiatives like the Trillion Trees movement. But they also drew fire for including grassy biomes and fire-prone savannas where forest does not naturally occur.
The new analysis, by contrast, uses stricter definitions, high-resolution land-cover data, and a suite of safeguards to avoid perverse outcomes. After excluding croplands, wetlands, peatlands, and areas with frequent fires, the authors identified just 195m hectares worldwide suitable for reforestation. That is a reduction of 71-92% compared with previous maps.
Still, the potential remains significant. Restoring these lands could sequester 2.2bn tonnes of carbon dioxide each year, about 5% of annual global emissions from fossil fuels and land-use change.
“Reforestation is a readily-available, scalable, cost-effective carbon removal solution, but we have neither time nor resources to restore trees everywhere” says TNC’s Susan Cook-Patton, Ph.D.
Most promising are areas adjacent to existing forests, where natural regeneration is more likely and biodiversity gains are greater. Yet only 90m hectares also meet social safeguards such as secure land tenure and minimal conflict with rural livelihoods. The research highlights the difficulty of optimizing for carbon, biodiversity, water, and human rights at once. Few places satisfy all objectives.
For policymakers, the message is clear: reforestation is not a silver bullet. Protecting standing forests delivers greater climate benefit, and tree planting must be carefully targeted to succeed.
As Kurt Fesenmyer, the study’s lead author, notes, “The biggest climate benefits come from protecting the carbon within existing forests, including both young and old.”