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About the Fagales[This presentation is based upon that given in the Introduction to World Checklist and Bibliography of Fagales (1998) by R. Govaerts and D. G. Frodin but has been somewhat modified to take into account research since the end of 1997.]
The Fagales as covered by this option are an order of flowering plants comprising the families Betulaceae (birches, alders), Corylaceae (hazels, filberts and hornbeams), Fagaceae (beeches, southern beeches, chestnuts, chinquapins and oaks) and Ticodendraceae. They are circumscribed here as in Vascular plant families and genera by R. K. Brummitt (1992, Kew: Royal Botanic Gardens) with the inclusion of the Central American family Ticodendraceae which research in 1991-94 (cited under that family) showed as being more closely associated with Fagales (and more particularly with Betulaceae; see Kato et al. 1998) than the Urticales with which it was initially grouped. The same limits (without of course Ticodendraceae) were adopted in Flowering plants of the world edited by V. H. Heywood (1978, Oxford: Oxford University Press) with the addition of the Austronesian family Balanopaceae, now not considered to be so closely related. With additional data emerging from genome research, however, suggestions have been advanced that the Fagales should be viewed as a less exclusive group, requiring the inclusion of Casuarinaceae, Juglandaceae, Myricaceae and Rhoipteleaceae so as more accurately to reflect historical phylogeny (cf. An ordinal classification for the families of flowering plants, Angiosperm Phylogeny Group, 1998, in Annals of the Missouri Botanical Garden 85: 531-553). At the same time, there is apparently not now a sufficient case for separation of Corylaceae (though such was first proposed as long ago as 1815), while an increasingly perceived antiquity for Nothofagus has led to a widespread adoption for it of family rank (as first proposed by the Russian palynologist Lyudmila Kuprainova in 1962 and further advocated in the 1980s). These changes are likely to come into effect when the Fagales options are next revised.. The order as a whole is a prominent constituent of the Holarctic floristic "kingdom" originally postulated by Engler in the latter part of the nineteenth century. It is well represented in most of its forests south of the conifer belt – with the birches additionally penetrating throughout that zone and into the tundra as well as arctalpine vegetation. The inclusion of Nothofagus gives it a further strong representation in the southern temperate or Antarctic "kingdom". In the Betulaceae, Alnus makes a great leap southwards in the Americas along the Andes, while in Fagaceae Castanopsis, Lithocarpus, Quercus and Trigonobalamus collectively carry the order into northwestern South America, Malesia, New Guinea and New Britain but not Africa south of the Sahara or Australia; in New Guinea and New Britain they meet and sometimes mix with Nothofagus. All, or nearly all, the species are trees or shrubs and are by themselves or collectively often dominant in temperate forests. This prominence, along with their useful wood, bark and other parts, has ensured their effective place in northern temperate and tropical highland civilisations as well as their absorption and exploitation by European migrants to the southern hemisphere. Numerous species are also cultivated for ornament and shade. Within Fagales sensu stricto, numerous issues in classification remain with respect to developing an understanding of the interrelationships of supraspecific taxa as well as family, generic and specific limits. Even the monophyly of the order has been challenged as investigations of the phylogeny of what were called the "higher" hamamelids have proceeded. The separation of Corylaceae from Betulacaeae acknowledges the thoughts of Charles Fran�ois Mirbel, Alphonse de Candolle, Ivan Palibin and Armen Takhtajan in contrast to those of Adolf Engler and Karl Prantl as well as Hubert Winkler and several twentieth-century authors including Ernst Abbe and Arthur Cronquist. In the opinion of some a distinct Corylaceae furnishes a slightly better framework for assessment of relationships among suprageneric taxa. In the latter connexion, also worthy of note is the opinion of Kuprianova (1963, see Corylaceae, Special) recommending the further segregation of Carpinaceae (for Carpinus, Ostrya and Ostryopsis) from Corylaceae. Although rarely accepted as a family, Kuprainova's association of these three genera (and, by extension, the Palaeocene fossil Cranea, first described in 1998) remains that adopted as a subdivision within Betulaceae s.l. by some current students (cf. Manchester and Chen 1998 on Cranea; Chen, Manchester and Sun 1999). On the other hand, Kato, Oginuma, Gu, Hammel and Tobe (1999) follow Abbe, Hall and Furlow (and the World Checklist in associating Ostryopsis (and, presumably, also Cranea) with Corylus in Coryleae. Genome-based research and associated phylogenetic analysis, however, does not – as already noted – now favour separation. With respect to genera, a conservative approach has been adopted, reflecting the opinions of Forman (1966 – under Fagaceae, Special – and personal communication). Duschekia is thus not segregated from Alnus, Trigonobalanus is retained in its wider sense (without Colombobalanus and Formanodendron, although of the three the last may be closest to Quercus), Pasania and Cyclobalanus are subsumed into Lithocarpus, Limlia uraiana (and some related species) are included with Lithocarpus rather than Castanopsis, and Cyclobalanopsis (still often separated in East Asia) is treated as a subgenus of Quercus. Castanea is kept separate from Castanopsis although a case could be made for their merger. We also uphold Chrysolepis as distinct from Castanopsis to which its two species, both in North America, were formerly referred (and continue to be in some quarters). However, the last word surely has not been said, particularly with respect to Trigonobalanus; a strict phylogenetic view would recognise its two segregates. At species level, the genera Betula and Quercus are notorious for difficulties in drawing effective species limits due to their extensive, partially delimitable variability as well as extensive hybridisation. Indeed, the latter genus has occasioned frequently cited general discussions of species concepts (e.g. Burger 1975, Van Valen 1976; for both, see Quercus). Approaches and philosophy continue to vary, sometimes depending upon the geographical extent of the research. There is a need for new monographs of all the larger genera; the most substantial of recent date is that for Asian Quercus by Menitsky (1984; see Quercus). That Lithocarpus remains imperfectly known is shown by the considerable suite of novelties and records published for Malaysian Borneo by Julia and Soepadmo (1998); however, little else has been published in recent years. Microsystematic research, including assessment and delimitation of meaningful taxa, continues to be prominent in current writing on Quercus and Fagus and, increasingly, Nothofagus. Noteworthy in the last-named genus has been an attempt to integrate fossil as well as Recent species into an overall framework (Jordan & Hill 1999). With respect to larger relationships, however, besides the continuing interest in Nothofagus (cf. Manos et al. 1997), those within Quercus have also attracted renewed attention, initially in 1993 (Nixon 1993; see Quercus) and again since 1997 (Samuel, Bachmair, Jobst and Ehrendorfer 1998; Manos, Doyle and Nixon 1999). No single new phylogenetically based scheme has, however, found general favour and little resolution below the major groups has been essayed. Manos et al. – from a sample of 25 species – suggest that an expanded Cerris group (inclusive of the Ilex group) appears to be monophyletic though basally plesiomorphic (and moreover nearest the cycle-cup oaks of subgen. Cyclobalanopsis, with all other oaks, starting with the red oaks (Lobatae), forming a sister group), while Samuel et al. claim – on the basis of a sample of ten species – that the Cerris group, though also seen as plesiomorphic, is polyphyletic, with links to both Lobatae (red oaks) and Quercus (white oaks). Nevertheless, from the work of Manos et al. there appears to be some support for Axelrod's idea (Axelrod 1983; see Quercus) that the spread of Quercus was largely complete by the early Tertiary and that subsequent differentiation within the major groups has largely been in situ, although Manos et al. suggest that Quercus s.s. (sect. Quercus) in Eurasia is the result of early land migration from North America. Much more broadly structured but critical research, including developmental studies, is needed. ReferencesOnly references not also in the printed work are given below. Those published in World Checklist and Bibliography of Fagales may also be searched for in the database. Additional references reflective of recent (and earlier) research are given in Kew Record of Taxonomic Literature. Chen, Zhi-duan, Manchester, S. R. & Sun, H.-Y. (1999). Phylogeny and evolution of the Betulaceae as inferred from DNA sequences, morphology, and paleobotany. Amer. J. Bot. 86: 1168-1181. |
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