In the 1990s, deforestation remained the most important land-use change in the tropical regions. By contrast, in the non-tropical regions of the developed countries, deforestation has been reversed by natural reforestation of former croplands and pastures. A modelling experiment has shown that even very extensive reforestation and afforestation activities over the next 50 years would result in reduction of the atmospheric CO₂ concentration of only about 40–70 parts per million by volume by 2100. Large uncertainties in constraining potential sink capacities for atmospheric CO₂ through afforestation and reforestation arise from a lack of understanding of the response of soil organic carbon (SOC) pools in the mineral topsoil to land-use and land-management change activities. Changes of the SOC pools after afforestation depend on the former land use, type of forest established, and climate and soil texture. Afforestation with hardwood broadleaves often results in higher SOC accumulation rates in the mineral topsoil than afforestations with softwood conifers.