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Subgenus Betulenta - Wintergreen oil birches
Bark on twigs rich in methyl salicylate (oil of wintergreen). Female catkins erect.
Subgenus Betulaster - Large-leaf birches
Bark on twigs contains some methyl salicylate. Female catkins pendulous.
Subgenus Neurobetula - Costate birches
Bark on twigs without methyl salicylate. Female catkins erect.
Subgenus Betula - Typical birches
Bark on twigs without methyl salicylate. Female catkins pendulous.
Subgenus Chamaebetula - Dwarf birches
Small shrubs with small rounded leaves. Female catkins pendulous.
Notes
There is no concensus 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).
References
Bean, W. J. 1976, 1988. Trees & Shrubs hardy in the British Isles. Eighth edition, revised, vol. 1 (1976) & Supplement (1988); editor D. L. Clarke.
Hunt, D. 1993. Betula. Proceedings of the IDS Betula Symposium 2-4 October 1992. International Dendrology Society.
Rushforth, K. D. 1999. Trees of Britain & Europe. Collins. (Useful details on chromosome numbers of many European & Asian birches).
Flora of North America online - Betula.