Taxonomy of Betula

=Subgenus "Betulenta" - Wintergreen oil birches=

Bark on twigs rich in methyl salicylate (oil of wintergreen). Female catkins erect.
*"Diploid" (2"n" = 28).
**"Betula lenta" - Sweet Birch or Cherry Birch
***"Betula lenta" subsp. "uber" - Cherry Creek Birch
*"Hexaploid" (6"n" = 84).
**"Betula allegheniensis" - Yellow Birch ("B. lutea")
*"Decaploid" (10"n" = 140).
**"Betula austrosinensis" - South China Birch
**"Betula globispica" -
**"Betula insignis" -
**"Betula medwediewii" - Caucasian Birch
*"Duodecaploid" (12"n" = 168).
**"Betula megrelica" -
*"chromosome number not reported"
**"Betula corylifolia" - Hazel-leaf Birch
**"Betula grossa" - Japanese Cherry Birch
**"Betula insignis" -

ubgenus "Betulaster" - Large-leaf birches

Bark on twigs contains some methyl salicylate. Female catkins pendulous.
**"Betula alnoides" - Alder-leaf Birch
***"Betula alnoides" subsp. "luminifera" -
**"Betula maximowiczii" - Monarch Birch

ubgenus "Neurobetula" - Costate birches

Bark on twigs without methyl salicylate. Female catkins erect.
*"Diploid" (2"n" = 28).
**"Betula calcicola" -
**"Betula chichibuenis" -
**"Betula costata" -
**"Betula nigra" - River Birch or Black Birch
**"Betula potaninii" - Potanin's Birch
*"Tetraploid" (4"n" = 56).
**"Betula albosinensis" - Chinese Red Birch
***"Betula albosinensis" var. "septentrionalis" - North Chinese Red Birch
**"Betula ermanii" - Erman's Birch
**"Betula jacquemontii" ("B. utilis" subsp. "jacquemontii") - White-barked Himalayan Birch
**"Betula utilis" - Himalayan Birch
*"Hexaploid" (6"n" = 84).
**"Betula davurica" - Dahurian Birch
**"Betula delavayi" - Delavay's Birch
**"Betula raddeana" - Radde's Birch
*"Octoploid" (8"n" = 112).
**"Betula chinensis" - Chinese Birch
*"chromosome number not reported"
**"Betula fargesii" - Farges's Birch
**"Betula schmidtii" - Schmidt's Birch

ubgenus "Betula" - Typical birches

Bark on twigs without methyl salicylate. Female catkins pendulous.
*"Diploid" (2"n" = 28).
**"Betula cordifolia" - Heart-leaf Birch or Mountain Paper Birch
**"Betula pendula" - Silver Birch
**"Betula mandschurica" - Manchurian Birch
***"Betula mandschurica" var. "japonica" - Japanese Birch
**"Betula neoalaskana" - Alaska Birch or Yukon Birch
**"Betula occidentalis" - Water Birch or Red Birch ("B. fontinalis")
**"Betula platyphylla" ("Betula pendula" var. "platyphylla") - Siberian Silver Birch
**"Betula populifolia" - Gray Birch
**"Betula szechuanica" ("Betula pendula" var. "szechuanica") - Sichuan Birch
*"Tetraploid" (4"n" = 56).
**"Betula celtiberica" - Iberian White Birch
**"Betula pubescens" - White Birch, European White Birch or Downy Birch
***"Betula pubescens" subsp. "tortuosa" - Arctic White Birch
*"Pentaploid" (5"n" = 70).
**"Betula kenaica" - Kenai Birch
*"Hexaploid" (6"n" = 84).
**"Betula papyrifera" - Paper Birch, Canoe Birch or American White Birch (sometimes tetraploid or pentaploid)

ubgenus "Chamaebetula" - Dwarf birches

Small shrubs with small rounded leaves. Female catkins pendulous.
*"Diploid" (2"n" = 28).
**"Betula glandulosa" ("B. nana" subsp. "glandulosa") - American Dwarf Birch
**"Betula nana" - Dwarf Birch
*"Tetraploid" (4"n" = 56).
**"Betula minor" - Quebec Dwarf Birch
**"Betula pumila" - Swamp Birch
*"chromosome number not reported"
**"Betula fruticosa" -
**"Betula hallii" - Cascades Dwarf Birch
**"Betula humilis" - Arctic Dwarf Birch
**"Betula michauxii" - Newfoundland Dwarf Birch
**"Betula microphylla" -
**"Betula middendorffii" -

Notes

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").

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.
*cite journal| last = Li, J.| first = Shoup, S. & Chen, Z.| title = Phylogenetic Relationships of Diploid Species of Betula (Betulaceae) Inferred from DNA Sequences of Nuclear Nitrate Reductase| journal = Systematic Botany| volume = 32| issue = 2| year = 2007| pages = 357–365| doi = 10.1600/036364407781179699
*Rushforth, K. D. 1999. "Trees of Britain & Europe". Collins. (Useful details on chromosome numbers of many European & Asian birches).
*Skvortsov, A. K. 2002. A new system of the genus Betula. "Byulleten Moskovoskogo Obshchestva Ispytatelei Prirody Otdel Biologie" 107: 73–76.
* [http://www.efloras.org/florataxon.aspx?flora_id=1&taxon_id=103887 Flora of North America online - "Betula".]