The genus once had nearly 600 species in Eurasia and North America . After morphologic and molecular research on the genus during the 1990s , it was decided that the North American species are better treated in a series of other related genera . The genus once contained nearly 600 species in Eurasia and North America , but after morphologic and molecular research on the genus during the 1990s , it was decided that the North American species are better treated in a series of other related genera .