New forest is hailed by many as a solution to the increasing greenhouse effect. US President Donald Trump supports the initiative to plant 1,000 billion trees in 10 years to reduce carbon dioxide emissions. The British Labor Party wants to plant the double. A large number of countries have joined the Bonn initiative, where 350 million hectares (almost eight times the area of ​​Sweden) will be reforested, including India and Brazil.

A noted study from Science magazine last year said that two-thirds of greenhouse gases emitted since the industrial revolution could be tied if the world's forested areas increased by 25 percent. But the study was based on satellite photos and model calculations and has been questioned by other researchers.

Forest plantation on carbonaceous soils leak CO2

Now measurements of the carbon content of the soil on 619 forest plantations in northern China with different tree species, soil and climate show that in almost half of the cases more carbon leaked to the atmosphere than the new forest band of carbon dioxide. This applied to soils that already had high carbon content and that the researchers do not recommend for forest plantation. The researchers measured the carbon in the soil because the carbon content of the soil is much greater than that which is bound by the wood of the trees. The study also checked 163 comparison soils where there was no reforestation.

"Forest planting is complex and cannot solve all of our climate problems," says researcher Anping Chen at Colorado State University who led the study.

The results are also important for Sweden

- This is an important and well-made study because measured in reality and it has relevance for us in Sweden as Northern China's climate is similar to ours. The results show the risk of exaggerating forest planting as a climate solution, says sustainability researcher Wim Carton at Lund University.

Contributions to tree planting can reduce biodiversity

Another study looked at one of the world's longest and most influential forest contribution schemes to private landowners, the one in Chile in 1974-2012, and copied by many countries and aid projects. The result was more plantations and less natural forest, which led to fewer species and that less carbon dioxide disappeared from the atmosphere.

"If the policy of stimulating tree planting is poorly designed, there is a high risk of not only wasting tax money but also emitting more carbon dioxide and losing biodiversity," said researcher Eric Lambin at Stanford.