In Poland, there are large discrepancies between the area of forest lands recorded in land−use registers of local authorities and the actual state of forest area in the field. These discrepancies were estimated to 800,000 hectares in 2016, which is 2.5% of the land area of Poland. The divergences result primarily from different definitions of a forest in forest regulations and legislation concerning land−use registry and the insufficient update of land registry databases. The aim of the paper is to define the major causes of reclassification of non−forest lands into forest lands and to determine the origin of forests on those lands. The study is based on a mail questionnaire survey carried out in all local public authorities all over the country (altogether 314 rural and 66 municipal counties). The questions focused on manners lands were used in a county, including the area of artificial and natural afforestation and the area of lands reclassified into forest lands over the period 2009−2013, as well as causes of such reclassification. Altogether 232 responses were collected (61.1%), 122 of which contained data of sufficiently good quality. The results show that afforestation of non−forest lands were mostly carried out in an artificial way. Reclassification into forest lands was conducted almost exclusively within rural counties. The reclassified lands were largely regenerated by natural succession. The key factor of land reclassification were works related to forest management planning in non−state owned forests. Afforestation carried out within the Rural Development Programme, co−financed from EU funding, was the second most important factor. In case of afforestation, land reclassification is obligatory no later than in the fifth year after a new forest was planted. The paper concludes with suggestions that there are no effective legal regulations that would make land owners to reclassify their afforested lands into forest lands, except cases of agricultural lands afforestation within the Rural Development Programme. Therefore, it is recommended to seek to cover all non−state owned forests with forest management plans and to obtain the compliance of land registries with the real situation in the field.