China’s forestation on marginal land was less efficient in carbon sequestration compared with non-marginal land

Forests are the major carbon sink in China, but it remains unclear how forestation activities have shifted in space, thus affecting tree growth and carbon sequestration. Using data from 1,928 planted forests across China, we found that forestation has increasingly occurred on marginal land from 1980 to the 2010s. Due to less favorable climatic and soil conditions, marginal sites exhibited lower tree carbon accumulation (−847.7 g C m−2) and allocated more carbon to roots for accessing nutrients and water (+6.3% root:shoot ratio) compared with non-marginal sites. These results indicate that the land suitability affects the forest biomass carbon sink, and an increasing reliance on marginal land may reduce the effectiveness of forestation of contributing to China’s ambitious goal of carbon neutrality by 2060.