Syringa L.
Syringa is a genus of ca. 20 species, mostly confined to eastern Asia. Its center of diversity lies in China where 16 species are found in the wild (Chang & al. 1996). Two species naturally occur in Europe but none is native in Belgium. However, most of the species are cultivated as ornamentals. Green (1997) and Roloff & Bärtels (2006) cite more than 20 taxa (incl. hybrids) and numerous infraspecific taxa that are grown in Europe.
Syringa as traditionally circumscribed is paraphyletic (Li & al. 2002).
In addition to the two species included in this account, several additional ones have been recorded as escapes elsewhere in the world. Some of these act as undesirable invasive species, e.g. Syringa reticulata (Blume) H. Hara in North America.
1 Leaves elliptic to elliptic-lanceolate (much longer than wide), cunate at base and acuminate at apex, slightly glaucous (paler) beneath, not leathery. Young branches and lowerside of leaves often finely hairy === Syringa josikaea
Leaves ovate (somewhat longer than wide), subcordate at base, more or less leathery and concolorous (not paler beneath). Young branches and leaves glabrous === S. vulgaris
Literature:
Chang M.C., Qiu L.Q. & Green P.S. (1996)Syringa. In: Wu Z.Y. & Raven P.H. (eds.), Flora of China, vol. 15. Science Press. Beijing and Missouri Botanical Garden Press. St. Louis: 280-286. [available online at: http://flora.huh.harvard.edu/china/PDF/PDF15/syringa.pdf]
Chen J.-Y. (2008) A taxonomic revision of Syringa L. (Oleaceae). Cathaya 17-18: VII + 170 p.
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.
Fiala J.L. (1988) Lilacs, the genus Syringa. Timber Press, Portland: 266 p.
Green P.S. (1997) Syringa. In: Cullen J. & al. (eds.), The European Garden Flora, vol. 5. Cambridge University Press, Cambridge: 580-584.
Kim K.J. & Jansen R.K. (1998) A chloroplast DNA phylogeny of lilacs (Syringa, Oleaceae): plastome groups show a strong correlation with crossing groups. American Journal of Botany 85(9): 1338–1351. [available online at: http://www.amjbot.org/content/85/9/1338.full.pdf+html]
Li J., Alexander J.H. & Zhang D. (2002) Paraphyletic Syringa (Oleaceae): Evidence from Sequences of Nuclear Ribosomal DNA ITS and ETS Regions. Systematic Botany 27(3): 592-597. [available online at: https://www.researchgate.net/publication/232673870_Paraphyletic_Syringa_...
Li J., Goldman-Huertas B., DeYoung J. & Alexander III J. (2012) Phylogenetics and Diversification of Syringa Inferred from Nuclear and Plastid DNA Sequences. Castanea 77(1): 82-88.
McKelvey S.D. (1928) The lilac, a monograph. The MacMillan Company, New York: 581 p.
Pringle J.S. (1997) An updated summary of classification in Syringa at the ranks of species, subspecies and variety. Lilacs 26(1): 19-26.
Roloff A. & Bärtels A. (2006) Flora der Gehölze (2e Auflage). Ulmer, Stuttgart: 844 p.