There is no consensus at all on species limits in Betula, with different authors differing wildly in what species they accept, from under 30 species, to over 60. The above (incomplete) list was compiled from the references cited below. Birches will hybridise very freely, particularly in cultivation but also in the wild where conditions and species present permit. While differing chromosome number (diploid, tetraploid, etc.) may reduce interbreeding, it is not an absolute bar to it. Many botanists regard differing chromosome number as a specific discriminant, though not all do so (e.g. some include B. cordifolia and B. neoalaskana as varieties within B. papyrifera).