Definition: Natural colonization of non-forest land
The colonization of non-forest land with forest trees through stages of natural succession without human intervention. Natural colonization may frequently occur after other (non-forest) land has been abandoned or withdrawn from its former utilization, e.g. farming or pasturing.
Economic Commission for Europe of the United Nations (UNECE)/ Food and Agriculture Organization of the United Nations (FAO), Terms and Definitions, "Temperate and Boreal Forest Resources Assessment 2000", United Nations, July 1997