Sorbus quernea: taxonomic confusion caused by the naturalization of an alien species, Sorbus mougeotii.

Sorbus quernea, a hybridogenous species described in 1996 as an endemic in two places in Prague (Czech Republic), is revised. A wide range of contemporary biosystematic techniques, including molecular (nuclear microsatellite markers) and karyological analyses (chromosome counts, DAPI flow cytometry) along with multivariate morphometric and elliptic Fourier analyses, were used to assess its taxonomic status. The revision revealed that S. quernea is taxonomically identical to S. mougeotii, a western-European hybridogenous species with a large distribution area, which was described from the Vosges Mts in France in 1858. Plants from localities given in the protologues of both taxa and from the German Alps, French Alps, the Massif Central and those of unknown origin planted in the Czech Republic, were shown to be taxonomically, karyologically and genetically consistent. A negligible variation in the microsatellite pattern partly combined with variation in leaf shape was recorded only at Grand Ballon Mt. (Vosges Mts). This variability is probably caused by introgressive hybridization with diploid S. aria s. str. Specimens of S. mougeotii from woodland show more deeply lobed and broader leaves compared with those from subalpine areas. This variability is ascribed to the ecologically conditioned plasticity of the species. The type material of S. mougeotii deposited in Nancy is regarded as taxonomically consistent. At the Prague localities, S. mougeotii (the populations erroneously described as S. quernea) is considered to be an escaped and recently naturalized alien species. Sorbus mougeotii is occasionally planted in the Czech Republic and also in other European countries as a roadside, street or garden tree. In Europe, several other species of Sorbus that were planted escaped and became established in (semi-)natural, often relict (rocky) vegetation, which when combined with poor knowledge of taxonomy of the planted species, may give an impression of a natural origin for an escaped population and lead to serious taxonomic misinterpretations. An overview is provided of alien and often or occasionally cultivated hybridogenous Sorbus species in European countries.