The literature treating the systematics of the birches is quite extensive. However, most of it is dedicated to problems of species size, species delineation, or descriptions of new species and interspecific hybridization studies. The author of this article has been tackling the same range of problems for many years mostly on the material collected in northern Asia. These, however, represent only a part of the genus, that is, undoubtedly, its youngest part.

While studying material for the new Flora of China (1999), the authors noticed deficiency and inadequacy in the infrageneric systematics of the genus Betula. Being time restricted by the project deadlines, the co-authors (Li Peiqiong and the author of this article) decided to refrain from delineating any infrageneric groups. J. J. Furlow, the author of Betulaceae treatment in the Flora of North America (1997), did not provide any infrageneric grouping, either—and probably for the same reason. Here I am trying to fill out this gap while relying on my previous work.

Here the Fruticosae/Nanae parts look particularly inadequate. Also, the section Costatae appears quite superficial, since the author dumped there all exotic species that were poorly known in Europe at the time. Regel outlined the sect. Lentae basing on the fact that B. lenta retains its pistillate aments intact into the winter season, which he considered somewhat reminiscent of Alnus.

However, while proposing his new system for the genus, Schneider at the same time confessed: "Eine wirklich zeitgemäße Bearbeitung der Gattung Betula fehlt": "There is no adequate contemporary treatment of the genus Betula" (1915: 312), which enables us to conclude that he was not really satisfied with his own system. Indeed, his subsection Grossae consisted of three or four components that were not closely related, while the obvious close relation between B. grossa and the American B. lenta was not depicted at all. Subsect. Chinenses also appeared heterogeneous; a close relationship between B. dahurica and B. nigra was not noticed; the peculiarity of B. corylifolia was underestimated as it was considered only in the rank of the subsection, and so on.

T. Nakai (1915) and V. N. Vasilyev (1969) proposed a few subgenera, but those were essentially rank-changing proposals, once again based only on material from northern Asia. These proposals did not constitute much progress.

Finally, a recent treatment by Z. D. Chen (1994) has been a kind of step back from Schneider's system. Chen proposed five sections (no subgenera, neither subsections). Of these five, three remained the same as those proposed by Schneider and other authors (Betulaster=Acuminatae, Betula, and Humiles); however, the majority of birch species are confined in two extremely heterogeneous sections: Costatae (B. medwediewi, B. raddeana, B. albo-sinensis, B. austro-sinensis, B. ermanii, B. costata, B. corylifolia, B. nigra, B. alleghaniensis, B. lenta) and Chinenses (B. insignis, B. delavayi, B. potaninii, B. calcicola, B. globispica, B. schmidtii, B. chinensis).

Therefore, Schneider's statement of 1915 about the absence of a truly contemporary system for the genus Betula remained valid as late as the end of the 20th century. Here I dare to bring to my colleagues' attention a system of mine. It is an honor for me to publish it in the same journal where E. L. Regel once published his fundamental works on the systematics of the birches.

I have been studying herbarium material in the following depositories: Moscow (MW, MHA), St. Petersburg (LE), Tomsk (TK), Vladivostok (VLA), Baku (BAK), Batumi (BAT), Yerevan (ERE), Tbilisi (TBI), Cambridge (A, GH), St. Louis (MO), New York (NY), Beijing (PE), Kunming (KUN), Dehra Dun (DD), Vienna (W), Kórnik (KOR). In 1960's we received a representative selection of Japanese samples from the Makino Herbarium (MAK) by exchange. I had opportunities to observe nearly all of the birch species native on the former USSR territory (except B. maximowicziana) as well as species of the Indian Himalaya and North America in their natural habitats. Observations of cultivated trees proved to be rather useful, particularly, those made at the Main Botanic Garden (GBS) in Moscow, the Central Botanic Garden in Kiev, the Arnold Arboretum in Boston, and the Dendrogarden in Pereslavl-Zalessky.

Type species: Betula alba L. (syn. B. pubescens Ehrh.)

The type specimen of the species B. alba was cited in the literature many times. Recently it has been officially designated. This is # 1109–1 of the Linnaeus Herbarium (LINN) (Holub 1989: 40; more about B. alba typification in: Vasilyev 1964).

I propose to distinguish four subgenera in the genus, two of which, monotypic, are being first described here.

1. Betula subg. Sinobetula A. Skvortsov subg. nov.

Typus (designated here): B. gynoterminalis Hsu et C. J. Wang 1983 in Acta Bot.Yunnan. 5(4): 331, cum fig.

So far there is the only one species known, which has been collected only once: Yunnan, Gongshan Xian, altitude 2,600 m, Sept. 26, 1956. P. Y. Mao N 00521. KUN(!)

This amazing, truly unique birch would fully deserve a rank of a separate genus in case there were anything unusual about its aments. But they appear to be just normal birch aments, though their location is very peculiar for a birch (which is depicted in the species name). Therefore, I find it optimal to assign a rank of the subgenus. The description is based on the single specimen so far available, which is preserved at the Botanic Institute in Kunming, where I've had an opportunity to examine it.

2. Betula subg. Nipponobetula A. Skv. subg. nov.
Syn. sect. Costatae subsect. Coryfoliae C. K. Schneider 1916 in Pl. Wilson. 2: 463.

Typus (designated here, the only species so far known): B. corylifolia Regel et Maximowicz 1865 in Bull. Soc. Natur. Mosc. 38(2): 417 et tab. 8 fig. 1–3.

This birch is endemic for Honshu, though it is not so rare there (I've seen about 30 herbarium specimens total.) Though it had always been treated as a distinct species, that was Schneider who made a step ahead and promoted it to the rank of the subsection. Peculiar leaf shape and venation ^[3] different from the rest of the birches and—still more important—the fruit structure featuring rounded flattened nutlet with a pronounced style embraced by wings—these are the characters confirming the treatment of the species as a separate evolutionary line that has never developed any further.

3. Betula subg. Asperae Nakai 1915 in Fl. sylv. Koreana 2: 25 corrected (expanded) A. Skvortsov.

Typus (designated here): B. schmidtii Regel

While Nakai assigned just a single species B. schmidtii to this subgenus, I place the following three sections here: Asperae (Nakai) Kuzeneva (which corresponds to Nakai's subgenus); Chinenses (Nakai) Chen, and Lentae Regel emend. A. Skvortsov. The name Chinenses Nakai would be also appropriate for this subgenus, but I am choosing Asperae not only due to priority considerations, but also because the subgenus contains species other than Chinese. The subgenus differs from the subg. Betula (in the meaning proposed here) in two most important characters: lack of true-birch, layered bark as well as the structure of the fruit. We are going to discuss the latter in detail now.

Traditionally birch fruits have been depicted as nutlets with wings. It is only by the wing-to-nut width ratio that they differ from one another. Meanwhile, it is the very nature of wings that varies. The difference is particularly obvious when one compares a fruit of a birch belonging to the sect. Lentae and a fruit of one from sect. Betula or Acuminatae (Fig. 1). The former has its entire ovary compressed; its wings constitute the part of the ovary that is not filled by the nutlet. The wings in the latter are special outgrowths on both sides of the ovary. In the former case, the wings proceed onto the styles; in the latter, they do not reach the styles. This is a very minute, but constant and meaningful difference. In addition to that, the wings in Lentae are nearly concolorous to the nut and not transparent; usually they are rather concave, so that one can sometimes distinguish their adaxial and abaxial side. In the subg. Betula fruits are usually flat, so that one cannot identify adaxial and abaxial sides.

Unfortunately, the authors of morpho-anatomic treatments of the birch fruit, I. A. Korchagina (1974) and G. Natho (1976) did not pay attention to the details described here above.

Typus: B. alba L.

Critical characters of the subgenus: presence of "the birch bark", which is most pronounced in the species belonging to sect. Costatae and Betula; pistillate aments on slender (about 1 mm), usually glabrous leafy stalks; axils of stalk leaves usually bear buds, which prolongs the life span of the fruiting branch. Upon seed ripening, the aments tend to completely disintegrate (though occasionally bracts may persist after seed ripening); fruits bear wings: outgrowths of the lateral ovary parts; these outgrouths don't reach to the proximal parts of styles; bract lateral lobes usually differ from the central part of the bract, often reclining sidewards.

Relationships of the subgenera and sections might be approximately depicted as in Fig. 2. The subgenus Betula is by all means the youngest. If we try to derive it from another subgenus, then that other one should be Asperae. Or else it might have had some different ancestors that haven't survived.

The author is deeply grateful to all the colleagues from the institutions named in the text. A special thank-you to my long-time assistant M. V. Kostina for typing of the manuscript.

This article was completed with financial support of RFFI (Grant # 99–04–48202).

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Translation I. Kadis
19 Dec 2004