Bromus inermis Leyss. (syn.: Bromopsis inermis (Leyss.) Holub) (Euras.) – A slightly increasing, locally naturalised alien. Known at least since 1823 when it was already present in scattered locations (for instance Aywaille and Malmédy). The residence status of Bromus inermis in Belgium (and elsewhere in western Europe) is controversial (see also Lambinon 1980). Some authors pretend that it might as well be native. At present known from several, very scattered locations, chiefly in Flanders, but likely to become more widespread.
The exact origin of the Belgian populations remains unclear so far. Bromus inermis is sometimes introduced with cereals but at least part of the populations might have been the result of sowing.
As elsewhere in the genus Bromus s.l. delimitation between species is often blurred. Typical plants of Bromus inermis are easily recognised by glabrous, (almost) awnless lemmas, a rhizomatous habit (a good character but usually impractical in herbaria) and wide leaves. Some variants, however, have more pubescent lemmas with short awns and tend towards Bromus pumpellianus Scribner (syn.: Bromopsis pumpelliana (Sribn.) Holub, B. inermis subsp. pumpelliana (Scribner) W.A. Weber), a close relative of B. inermis, native in North America and East Asia. This species has been recorded from several European countries (e.g. Ryves & al. 1996, van der Meijden & Holverda 1988, Krahulec & Jiřiště 1997, Verloove 2008, Hohla 2009, …). As a matter of fact, the latter is in some respects a transitional species between Bromus inermis and native B. erectus: in addition to the pubescent, awned lemmas, Bromus pumpellianus is often less rhizomatous to even caespitose (Pavlick & Anderton 2007). Van der Meijden & Holverda l.c. already stressed its intermediate position. Moreover, since the introduction of the Eurasian Bromus inermis in North America introgression took place, which largely blurred the distinction between B. inermis and B. pumpellianus (see Elliott 1949). At least part of the European collections is thought to be of hybrid origin (Ryves & al. 1996). The situation might become even more complicated because of the introduction in western Europe of Bromus riparius Rehmann (syn.: Bromopsis riparia (Rehmann) Holub), a (predominantly) Asian species recently introduced as a pasture grass in North America. It is reported as an increasing neophyte in Switserland and Austria as well (Schmid-Hollinger 2007, Hohla 2011) and might have been overlooked elsewhere. Despite its original geographic isolation, it is scarcely distinguished from Bromus pumpellianus (Pavlick & Anderton l.c.) and B. erectus. Bromus riparius is usually smaller and forms denser clumps with a cap of reticulate-fibrous leaf sheaths of dead leaves at base (Saarela 2008). The distinction between all these species is further complicated by the development of artificial hybrids (Armstrong 1982, Williams & al. 2011). So far, Bromus pumpellianus as well as B. riparius have not been recorded in Belgium but both should be looked for in artificial grasslands and recently sown roadsides.
Some Belgian collections (chiefly older collections from the area around Liège) are aberrant in having unusually long-awned lemmas (awns frequently up to 5 mm long) but otherwise do not differ from typical Bromus inermis. In the latter species awns usually do not exceed 2(-3) mm. The taxonomic value of this particular feature remains unclear. It should be called var. aristatus Schur).
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Selected literature:
Allison K.J., Ashton D.B. & Darbyshire S.J. (2001) Identification of florets of meadow brome (Bromus riparius) and smooth brome (Bromus inermis). Seed Sci. Techn. 29(1): 99-108.
Armstrong K.C. (1981) The evolution of Bromus inermis and related species of Bromus sect. Pnigma. Bot. Jahrb. 102(1-4): 427-443.
Armstrong K.C. (1982) Hybrids between the tetraploids of B. inermis and B. pumpellianus. Can. J. Bot. 60: 476-482.
Elliott F.C. (1949) Bromus inermis and B. pumpellianus in North America. Evolution 3: 142-149.
Fasseaux W. (1995) Trouvailles floristiques: Bromus inermis à Landelies et à Saint-Aubin (district Mosan, Belgique). Nat. Mosana 48(1): 19-21.
Hohla M. (2009) Bromus pumpellianus, Mimulus ringens und Poa bigelovii – neu für Österreich, sowie weitere Beiträge zur Kenntnis der Flora des Innviertels. Beitr. Naturk. Oberösterreichs 19: 151-175.
Hohla M. (2011) Cardamine corymbosa (Brassicaceae) und Bromopsis (Bromus) riparia (Poaceae) – neu für Österreich sowie weitere Beiträge zur Adventivflora von Oberösterreich, Niederösterreich und Salzburg. Neilreichia 6: 55-79.
Krahulec F. & Jiřiště L. (1997) Bromopsis pumpelliana subsp. flexuosa – nová rostlina květeny České republiky. Preslia 69: 359-362.
Lambinon J. (1980) à propos de la publication de « Flora Europaea » volume 5. Remarques chorologiques, taxonomiques et nomenclaturales concernant la flore de la Belgique et des régions voisines. Natura Mosana 34(2): 80-104.
Otfinowski R., Kenkel N.C. & Catling P.M. (2007) The biology of Canadian weeds. 134. Bromus inermis Leyss. Can. J. Plant Sc. 87: 183-198. [available online at: http://www.umanitoba.ca/science/biological_sciences/botany_lab/pubs/2007...
Pavlick L.E. & Anderton L.K. (2007) Bromus. In: Barkworth M.E. & al. (eds.), Flora of North America north of Mexico, vol. 24: 193-237. Oxford University Press, New York-Oxford.
Ryves T.B., Clement E.J. & Foster M.C. (1996) Alien grasses of the British Isles. BSBI, London: XX + 181 p.
Saarela J.M. (2008) Taxonomy of Bromus (Poaceae: Pooideae: Bromeae) sections Bromopsis, Bromus, and Genea in British Columbia, Canada. Journal of The Botanical Research Institute of Texas 2: 323-372.
Schmid-Hollinger R. (2007) Bromopsis riparia in der Schweiz. Bot. Helv. 117: 198-201.
Thirlwell I.R. (2001) Notes on Bromopsis inermis (Hungarian brome) in S. Hants. BSBI News 88: 55-57.
Van der Meijden R. & Holverda W.J. (1988) Floravervalsing door onachtzaamheid: Bromus inermis subsp. pumpellianus nieuw voor Nederland. Gorteria 14: 95-96.
Verlaque R. & Vignal C. (1993) Etude cyto-biogéographique de Bromus erectus Hudson (du S.-O. de l'Europe), comparaison histologique avec les complexes polyploides de B. riparius et B. inermis (section Pnigma Dumort.). Rev. Cytol. Biol. Veg. Bot. 16(1-2): 47-72.
Verloove F. (2002) Ingeburgerde plantensoorten in Vlaanderen. Mededeling van het Instituut voor Natuurbehoud n° 20: 227 p.
Verloove F. (2008) Enkele nieuwe neofyten in België en Noordwest-Frankrijk. Dumortiera 94: 1-8.
Williams W.M., Stewart A.V. & Williamson M.L. (2011) Bromus. In: Kole C. (ed.), Wild crop relatives: genomic and breeding resources, millets and grasses. Springer-Verlag, Berlin Heidelberg: 15-30.