Tilia is a genus of between 30 and 45 species of trees in the Northern Hemisphere , in Asia ( where the greatest species diversity is found ) , Europe and eastern North America ; it is absent from western North America . Tilia is a genus of about 30 species of trees native throughout most of the temperate Northern Hemisphere , in Asia ( where the greatest species diversity is found ) , Europe and eastern North America ; they are not native to western North America .