The reestablishment of forest cover either naturally or artificially. Given enough time, natural regeneration will usually occur in areas where temperatures and rainfall are adequate and when grazing and wildfires are not too frequent. For example, Canada was basically treeless 16,000 years ago (during the last glacial period). During that time, spruce trees probably dominated much of present Illinois and Missouri. Due to global warming, spruce forests gradually spread north and now cover much of Canada. Within 16,000 years, more than 1 billion acres (400 million hectares) of forests in Canada were regenerated naturally.