Trees in China that were planted as part of huge reforestation projects appear to grow faster than those in natural forests, a new study finds. This is possibly because the reforestation trees are responding more strongly to the rising atmospheric carbon dioxide, scientists say.

China is quickly turning green. The country has planted 66 billion trees since 1978, with plans for 34 billion more by the middle of this century, as part of its “Great Green Wall” to slow the spread of the Gobi and Taklamakan deserts. These new forests absorb large amounts of CO2, but it is unclear exactly how they differ from natural ones, study first author Yuhang Luo, a landscape ecologist at Peking University in Shenzhen, China, told Live Science.

Luo and his colleagues set out to study how differences between natural and planted forests, including species diversity, tree density and age, might affect how the forests respond to rising CO2 and climate change.

“Planted forests are widely used in climate mitigation strategies, but most global ecosystem models do not distinguish between forest types or represent age-related dynamics adequately,” Luo said. “So we felt it was important to clarify how these factors interact — not just for scientific understanding, but also for improving the models and assumptions that underpin real-world forest policy and carbon accounting.”

Planted forests are those purposefully created by humans, such as those in the Great Green Wall. Natural forests, on the other hand, grow without human intervention. The researchers used satellite data to track leaf area index, a measure of canopy density and a key driver of carbon uptake, to determine how fast the different forest types grew, and found a striking difference: Planted forests increased their leaf area 66% faster than natural ones.

Most of that difference was due to planted forests being, on average, much younger than the natural ones — and young trees grow faster than old ones. But even when comparing forests of similar age and growing conditions, the planted ones still grew 4.6% faster, and the difference was even more pronounced in mixed and evergreen forests.

This is largely due to how planted forests are managed. They tend to feature fast-growing species like eucalyptus and poplar and are often actively managed, with people removing competing vegetation and even fertilizing them. These interventions reduce competition for light, water, and nutrients, amplifying the fertilization effect of rising atmospheric CO2.

This discrepancy peaks in planted forests when trees are around 30 to 40 years old and then declines noticeably after age 40. In contrast, natural forests grow more slowly but steadily, so they have an advantage over the long term.