Acer L.
Acer is a genus with 114 species most of which are dispersed in the northern hemisphere with a few additional species in tropical mountains (Mabberley 2008, Tison & de Foucault 2014). Three species are considered native in Belgium: A. campestre L., A. platanoides L. and A. pseudoplatanus L. (Lambinon & Verloove 2012). Their residence status, however, is uncertain since these three species are also extensively planted as ornamental trees and very easily escape. In many parts of Belgium natural populations are probably much less frequent than naturalized ones. Also, cultivars and infraspecific taxa of these three species are commonly grown and these also escape. For instance, a more or less glabrous-fruited form of A. campestre (var. campestre; syn.: var. leiocarpum (Opiz) Wallr.) appears to be the form often found in cultivation and as an escape. It is a more southern ‘race’ and unknown whether or not it naturally occurs in our country. More or less intermediate forms have also been encountered (introgression?). Similar plants with larger leaves may belong to non-native lookalikes such as A. miyabei Maxim. (see also Stace 2010). A purple-leaved cultivar of A. pseudoplatanus (f. purpurascens Pax, cv Atropurpureum) is also increasingly recorded in the wild and seems to naturalize locally in riparian habitats (Verloove 2011).
The genus Acer is extremely popular in the horticultural trade. More than 60 species and hybrids are cultivated as ornamentals in European gardens (de Jong 1997). There is a vast literature on the genus in cultivation and only a selection of references is presented here under. A particularly helpful and thoroughly illustrated account for the genus in cultivation in Belgium is provided by De Langhe & Crowley (2017). Virtually all of the cultivated species that produce viable seed are able to escape since fruits are easily wind-dispersed and readily germinate. In addition to those presented in this account, several more species are likely to occur. Wilmore (2000) additionally cited the following species from Yorkshire, British Isles: A. capillipes Max., A. davidii Franch., A. griseum (Franch.) Pax, A. nikoense Max. and A. triflorum Komar.
1 Leaves compound, ternate to pinnate === 2
Leaves not compound, merely lobed === 3
2 Leaves 3-foliolate, all leaflets distinctly petiolate. Second year shoot predominantly brown and fully brown after two years, hairy === Acer cissifolium
Leaves (3-)5(-9)-foliolate, leaflets almost sessile. Second year shoot green (sometimes purplish) and usually so on older branch-wood, glabrous === A. negundo (subsp. negundo)
3 Leaves white on lower surface === A. saccharinum
Leaves green on lower surface, although often slightly paler than on upper surface === 4
4 Petiole lactiferous (containing milky sap) === 5
Petiole not lactiferous === 7
5 Twigs green, not woody === A. cappadocicum
Twigs woody from second year onwards === 6
6 Leaves 3-5 lobed, all lobe apices obtuse (native) === A. campestre
Leaves 5-7 lobed, lobe apices acuminate (native) === A. platanoides
7 Leaves either unlobed or with up to 3 main lobes === 8
Leaves 5 or more lobed === 11
8 Leaves 3-lobed, apices obtuse, margins entire. Petiole very long, longer than lamina === A. monspessulanum
Leaves unlobed or with 3 lobes, always with serrate margins. Petiole shorter than lamina === 9
9 Young shoot not whitish-blue bloomed === 10
Young shoot clearly covered with +/-whitish-blue bloom === A. rufinerve
10 Bud covered by several imbricate scales. Leaves unlobed or with 3 lobes, up to 8 cm wide. Bark not distinctly striped === A. tataricum
Bud covered by 2 valvate scales. Leaves always conspicuously 3-lobed, at least 12 cm wide at maturity. Bark distinctly striped === A. pensylvanicum
11 Well-developed shoot at base with >4 pairs of bud scale scars === 12
Well-developed shoot at base with 2-4 pairs of bud scale scars === 13
12 Flowers red. Buds purplish. Leaves rarely wider than 10 cm === A. rubrum
Flowers greenish yellow. Buds green. Leaves often wider (native) === A. pseudoplatanus
13 Leaves predominantly 9-13-lobed, usually lobed for less than ½ of their length === A. japonicum
Leaves 5-7-lobed, lobed for much more than ½ of their length === A. palmatum
Literature:
De Langhe J. (2007) Determinatiesleutel voor de soorten en enkele hybriden van het geslacht Acer (Sapindaceae). Belgische Dendrologie 2007: 59-82. [online version avaliable at: https://www.henriettes-herb.com/files/images/jdl/acer-jdl-key-17aug2011....
De Langhe J. & Crowley D. (2017) Acer and Dipteronia: vegetative key to species in cultivation. Ghent University Botanical Garden: 222 p. [available online at : http://www.arboretumwespelaar.be/userfiles/file/pdf/170827_VEGETATIVE%20... and http://www.arboretumwespelaar.be/UserFiles/file/SleutelsPDF/Key_Acer_JDL...
Bean W.J. (1976) Acer. In: Trees and Shrubs hardy in the British Isles vol.1, p.185-240. [available online at: http://treesandshrubsonline.org/]
Bean W.J. (1988) Acer. In: Trees and Shrubs hardy in the British Isles Supplement, p.36-62.
De Jong P.C. (1976) Flowering and sex expression in Acer. A biosystematic study. Meded. Landbouwhogesschool 76(2): vii + 202 pp. [available online at: http://edepot.wur.nl/286168]
De Jong P.C. (1997) Acer. In: Cullen J. & al. (eds.), The European Garden Flora, vol. 5. Cambridge University Press, Cambridge: 132-153.
De Jong P.C. (1990) Taxonomy and distribution of Acer. Int. Dendrol. Soc. Year Book 1990: 6-10.
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.
Fang W. (1939) A Monograph of Chinese Aceraceae, 346 p.
Grimshaw J. (2009) Acer. In: New Trees in Cultivation, p. 69-115.
Harris J.G.S. (1983) An account of Maples in cultivation. Plantsman 5: 35-58.
Harris J.G.S. (1991) The history of the exploration and introduction in cultivation of Maples (Acer). Arbor. J. 15: 113-126.
Lambinon J. & Verloove F. (avec coll. Delvosalle L., Toussaint B., Geerinck D., Hoste I., Van Rossum F., Cornier B., Schumacker R., Vanderpoorten A. & Vannerom H.) (2012) Nouvelle Flore de la Belgique, du Grand-Duché de Luxembourg, du Nord de la France et des Régions voisines (Ptéridophytes et Spermatophytes). Sixième édition. Jardin botanique national de Belgique, Meise: CXXXIX + 1195 p.
Lee S. (1963) Acer. In: Forest Botany of China Supplement, p.196-224.
Mabberley D.J. (2008) Mabberley’s plant-book (3th ed.). Cambridge University Press, Cambridge: XVIII + 1021 p.
Murray E. (1967) A key to cultivated maples. Morris Arbor. Bull. 18: 43-47.
Murray E. (1970) Sections of the genus Acer. Kalmia 2: 5-8.
Murray E. (1970) A check list of the species of Acer. Kalmia 2: 22-24.
Murray E.A. (1970) A cheklist of the species of Acer. Kalmia 2: 25-45.
Murray E. (1970) Series of the genus Acer. Kalmia 2: 9-22.
Murray E. (1979) Afrasian and European Maples. Kalmia 9: 1-40.
Ogata K. (1965) A dendrological study of the Japanese Aceraceae. Bull. Tokyo Univ. Forests 60: 1-99. [available online at: https://repository.dl.itc.u-tokyo.ac.jp/?action=pages_view_main&active_a...
Ogata K. (1999) Acer. In: Flora of Japan vol. 2c, p. 60-73.
Rehder A. (1940) Acer. In: Manual of cultivated trees and shrubs hardy in North America, p. 566-586.
RHS (2014) Acer. In: The Hillier Manual of Trees & Shrubs, p.15-26.
Roloff A. & Bärtels A. (2006) Flora der Gehölze (2e Auflage). Ulmer, Stuttgart: 844 p.
Sargent C.S. (1921) Acer. In: Manual of the trees of North America, p. 681-702.
Schneider C.K. (1912) Acer. In: Illustriertes Handbuch der Laubholzkunde 2, p. 192-245.
Stace C. (2010) New flora of the British Isles, 3th ed.: XXXII + 1232 p. Cambridge University Press.
Tanai T. (1978) Taxonomical reinvestigation of the genus Acer L., based on vein architecture of leaves. J. Jap. Bot. 53: 65-83.
Tison J.-M. & de Foucault B. (coord.) 2014) Flora Gallica. Flore de France. Editions Biotope, Mèze : xx + 1196 p.
Van Gelderen D.M., de Jong P.C. & Oterdoom H.J. (1994) Maples of the World, 458 p.
Verloove F. (2011) Fraxinus pennsylvanica, Pterocarya fraxinifolia en andere opmerkelijke uitheemse rivierbegeleiders in België en Noordwest-Frankrijk. Dumortiera 99: 1-10. [available online at: http://www.plantentuinmeise.be/DUMORTIERA/DUM_99/Dum%2099_1-10_Fraxinus%...
Wiegrefe S., Angus H., Gregory P. & Otis D. (2002) International Maple Symposium 2002, 88p.
Wilmore G.T.D. (2000) Alien plants of Yorkshire. Yorkshire Naturalists' Union.
Xu T., Chen, Y., de Jong P.C., Otterdoom H.J. & Chang C.-S. (2008) Acer. In: Wu Z.Y. & Raven P.H. (eds.), Flora of China, vol. 11. Beijing: Science Press, and St. Louis: Missouri Botanical Garden Press: 517-553. [available online at: http://www.efloras.org/florataxon.aspx?flora_id=2&taxon_id=100167]