Cotoneaster horizontalis Decaisne, Fl. Ser. Jard. L’Europe 22: 168, 1879.
Synonym: C. acuminata Lindley var. prostrata J.D. Hooker ex Wenzig
Section Adpressi, series Horizontales
Origin: China (Sichuan, Gansu).
Presence in western Europe: Naturalized in France (Farille & al. 2010), Germany (John & Frank 2008; Dickoré & Kasperek 2010), Great Britain (Stace 2010). Furthermore known from Austria, Czech Republic, Luxembourg, the Netherlands, Poland, Switserland (Dickoré & Kasperek l.c.), Italy (Celesti-Grapow & al. 2009), Sweden (Hylmö 1993), etc. Doubtlessly present and relatively common in many parts of Europe.
Cultivation in Belgium and the Netherlands: “common” (De Koning & van den Broek 2009).
Comparative taxonomy: Cotoneaster horizontalis is here accepted in a narrow sense, following Fryer & Hylmö (2009). Dickoré & Kasperek (2010) and Flora of China (Lingdi & Brach 2003) accept this species in a (much) wider sense. Both subsume many species of Fryer & Hylmö’s (l.c.) series Horizontales under Cotoneaster horizontalis.
Illustrations: Hylmö (1993), Grevtsova (1999), Roloff & Bärtels (2006), John & Frank (2008), De Koning & van den Broek (2009), Fryer & Hylmö (2009), Stace (2010). See also: http://www.british-wild-flowers.co.uk/C-Flowers/Cotoneaster,%20Wall.htm.
The naturalisation history of Cotoneaster horizontalis in Belgium is poorly documented. It was first collected in the wild in 1982 on top of a calcareous slope in Comblain-au-Pont in the valley of river Meuse (Verloove 2006) but its escape from cultivation might well have started (much) earlier. In the 1980’s it was further documented from calcareous rocks in the valley of river Ourthe near Werpin. In the past decades Cotoneaster horizontalis has been increasingly recorded in large parts of Belgium, although it seems less frequent on acidic soils (see up-to-date distribution map in Piqueray & al. 2008). It is absent in large parts of the Kempen and in eastern Belgium. On the contrary, an important incidence was observed in large urban entities, especially around Brussel and Gent. It is necessary to stress that part of these records surely corresponds with similar, related species, mostly Cotoneaster hjelmqvistii and, to some extent, also C. ascendens (and possibly still other, as yet, unidentified species).
Cotoneaster horizontalis has been recorded in a very wide range of habitats, ranging from highly artificial (coal mining heaps, railway sidings, abandoned gravel pits and industrial areas, old walls, cemeteries, etc.) to natural ones (mainly calcareous grassland and scrub in coastal dunes). It is usually confined to dry, sun-exposed sites, mostly on calcareous substrates. Typical invaded habitats include Mosan Xerobromion (Piqueray & al. 2008) and coal mining heaps. In such suitable habitats it is able to naturalise and proliferate (see below). In coastal dunes Cotoneaster horizontalis is not rare (especially between De Panne and Nieuwpoort) but, at least for the time being, it never builds dense stands. Less frequently (and perhaps often merely ephemeral) Cotoneaster horizontalis also occurs in (half-) shade and on rather acidic soils. It has also been observed as an epiphyte in pollard willows.
The degree of naturalisation of Cotoneaster horizontalis in Belgium is fairly variable. It is one of the commonest species in cultivation and is easily dispersed by birds. Hence, it is a frequent xenophyte but often occurs as single individuals or in relatively small numbers. In parts of Germany, for instance, it is a very rare and often ephemeral species (compare with Henker & Kiesewetter 2006). However, in suitable habitats (see above) Cotoneaster horizontalis may quickly develop dense and large thickets. As such, Cotoneaster horizontalis locally has become an invasive species in Belgium and appears on the black list (http://ias.biodiversity.be/species/show/49). Piqueray & al. (2008) investigated its impact in biodiversity hotspots in Belgium. Cotoneaster horizontalis mostly occurred in complexes of xeric grasslands and calcareous rocks and was often accompanied by rare and vulnerable native species. Its naturalisation process in such habitats proved to be very effective. Three-year-old individuals already reproduced successfully and berries are subsequently dispersed by birds, which allows long distance dispersal towards uninvaded grasslands. Piqueray & al. (2008) recommend that Cotoneaster horizontalis should no longer be commercialized (not very probable) and that intensive (early) management in high-value habitats should be undertaken. Public awareness should also be raised. Eradication of Cotoneaster horizontalis in affected grasslands is not straightforward: uprooting seriously damages neighbouring vegetation as a result of its much outspread root and rhizome system. Cutting (at least every three years) prevents Cotoneaster horizontalis from flowering and fruiting and thus limits its further development but does not allow its eradication. Its biological and phenological traits should be studied further before coherent and efficient management strategies can be developed (Piqueray & al. 2008).
Cotoneaster horizontalis is a rather variable species and belongs to a critical species complex (Series Horizontales of Section Adpressi). In Belgium it has hitherto been confused with both Cotoneaster hjelmqvistii and C. ascendens. Especially the former is fairly frequent and also occurs in natural habitats although it exhibits – at least for now – a much less prominent invasive behaviour (the “usual” invader in calcareous grasslands in Belgium surely is Cotoneaster horizontalis). Identification aids for their separation from the latter are discussed under these species. Records of Cotoneaster horizontalis probably also hide other related and/or similar species. Cotoneaster adpressus Bois and related microspecies like C. nanshan M. Vilmorin ex Mottet are not rare in cultivation and possibly overlooked. The latter is even quoted “common” for Belgium and the Netherlands by de Koning & van den Broek (2009). Both are distinguished by their leaves with distinctly undulate margins and dull upper leaf surface (shiny and with more or less flat margins in Cotoneaster horizontalis). Other related species include Cotoneaster atropurpureus Flinck & B. Hylmö and C. perpusillus (C.K. Schneider) Flinck & B. Hylmö, both separated by petals with a blackish red base (the latter also more diminutive in all its parts but otherwise probably rather poorly distinguished and merely reduced to synonymy with C. horizontalis by Dickoré & Kasperek 2010). See also Klotz (1957).
Celesti-Grapow L. & al. (2009) Inventory of the non-native flora of Italy. Plant Biosystems 143(2): 386-430.
De Koning J. & van Den Broek (2009) Nederlandse Dendrologie (14th ed.). K.N.N.V.: 547 p.
Dickoré W.B. & Kasperek G. (2010) Species of Cotoneaster (Rosaceae, Maloideae) indigenous to, naturalising or commonly cultivated in Central Europe. Willdenowia 40: 13-45 [available online at: http://user.uni-frankfurt.de/~kasperek/papers/dickore_kasperek_2010.pdf].
Farille M.A., Zeller J., Jordan D., Charpin A. & Garraud L. (2010) Cotonéasters échappés de jardins en Haute-Savoie. Monde Pl. 501: 1-5.
Fryer J. & Hylmö B. (2009) Cotoneasters. A comprehensive guide to shrubs for flowers, fruit, and foliage. Timber Press, Portland-London: 344 p.
Grevtsova A.T. (1999) Atlas Cotoneasters. Cotoneaster (Medic.) Bauhin. Kiev, House Orchard, Truck-Garden: 372 p.
Henker H. & Kiesewetter H. (2006) Erstnachweise kritischer Pflanzensippen für Mecklenburg-Vorpommern. Bot. Rundbr. Mecklenburg-Vorpommern 41: 5-20.
Hylmö B. (1993) Oxbär, Cotoneaster i Sverige. Svensk Bot. Tidskr. 87: 305-330.
John H. & Frank D. (2008) Verwilderte Cotoneaster-Arten in Halle (Saale) und Umgebung. Mitt. Florist. Kart. Sachsen-Anhalt 13: 3-28 [available online at: http://bv-st.de/images/Flo-Kart_2008_003-028_John_Frank.pdf
Klotz G. (1957) Übersicht über die in Kultur befindlichen Cotoneaster-Arten. Wiss. Z. Univ. Halle, Math.-Nat. 6(6): 945-982.
Lingdi L. & Brach A.R. (2003) Cotoneaster. In: Wu Z.Y. & Raven P.H. (eds.), Flora of China, vol. 9. Science Press, Beijing & Missouri Botanical Garden Press, St. Louis: 85-108 [available online at: http://flora.huh.harvard.edu/china/PDF/PDF09/Cotoneaster.PDF].
Piqueray J., Mahy G. & Vanderhoeven S. (2008) Naturalization and impact of a horticultural species, Cotoneaster horizontalis (Rosaceae), in biodiversity hotspots in Belgium. Belg. J. Bot. 141(2): 113-124 [available online at: http://orbi.ulg.ac.be/bitstream/2268/16142/1/belgian-journal-2008.pdf].
Roloff A. & Bärtels A. (2006) Flora der Gehölze (2e Auflage). Ulmer, Stuttgart: 844 p.
Stace C. (2010) New Flora of the British Isles, 3th ed.: XXXII + 1232 p. Cambridge University Press.
Verloove F. (2006) Catalogue of neophytes in Belgium (1800-2005). Scripta Botanica Belgica 39: 89 p. [available online at: http://alienplantsbelgium.be/sites/alienplantsbelgium.be/files/tabel_2.pdf].