Fraxinus L.
Fraxinus is a genus of ca. 65 species (all trees or shrubs) or, based on more recent molecular data, probably less than 50 (Wallander 2012). The genus is monophyletic (Wallander & Albert 2000). All species are native in the northern hemisphere. The largest species diversity occurs in North America and China. One species, Fraxinus excelsior L. is a common native tree in Belgium (Lambinon & Verloove 2012). About half of the known species of the genus are cultivated as ornamentals or for timber in Europe (e.g. Green 1997).
A peculiar form of our native ash with leaves reduced to a single leaflet, var. diversifolia Ait (syn.: var. monophylla (Desf.) Gren. & Godr.), is sometimes planted and has been recorded repeatedly as an escape. It is uncertain whether or not this taxon occurs in the wild in Belgium. According to most present-day authors (e.g. Tison & de Foucault 2014) it is a simple mutant of Fraxinus excelsior.
Fraxinus is a surprisingly complex genus. The value of many diagnostic characters is still insufficiently understood, despite the presence of numerous more or less recent taxonomic treatments (see for instance Scheller 1977). Moreover, diacritic features of juvenile plants remarkably differ from those of adult plants. Shape, dimensions, indumentum and number of leaflets, color of leaf-buds, etc. are strikingly different in young and adult specimens. Likewise, young growth of adult plants (for instance after cutting) is always atypical. Analogously, saplings cannot be identified with confidence in the absence of (presumable) parental trees. Therefore, the present treatment focusses on vegetative as well as generative characters.
Additional information on the genus Fraxinus is available at the Oleaceae Information site at: http://www.oleaceae.info/.
1 Leaves reduced to one terminal leaflet, occasionally with two additional leaflets. Leaf-buds black (native?) === Fraxinus excelsior var. diversifolia
Leaves with at least five leaflets. Leaf buds brown, grey or black === 2
2 Corolla and calyx present. Leaf-buds grey or brown. Leaflets stalked === Fraxinus ornus
Corolla absent, calyx absent or present. Leaf-buds black or brown. Leaflets sessile or stalked === 3
3 Calyx present, campanulate, ca. 1 mm long. Leaf-buds brown. Leaflets stalked, usually 5-7 in number === 4
Calyx absent. Leaf-buds black or brown. Leaflets sessile, usually more than 7 in number (9-11) === 5
4 Lateral leaflets acuminate at apex. Leaflet stalk s often more than 2 mm long, especially in the lower ones === F. pennsylvanica
Lateral leaflets acute at apex (or abruptly acuminate). Leaflet stalks less than 2 mm long === F. latifolia
5 Leaf-buds (dark) brown. Leaflets with ca. as much serrations as lateral veins. Leaves usually at most 20 cm long. Inflorescence racemose === F. angustifolia
Leaf-buds black. Leaflets with more serrations than lateral veins. Leaves usually at least 20 cm long. Inflorescence paniculate (native) === F. excelsior var. excelsior
Literature:
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De Koning J., Van den Broek J.W., Van de Laar H.J. & Fortgens G. (2000) Nederlandse dendrologie (13e druk). H. Veenman & zonen, Ede: 585 p.
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