Morphological study of Lemna minuta Kunth, an alien species often mistaken for the native L. minor L. (Araceae)

•A certain morphological discrimination of Lemna minuta and L. minor is difficult.•Use of usual characters does not allow to distinguish 17% of collected specimens.•Other morphological characters to better discriminate the two species were identified.•Decision Trees to facilitate the determination between the species are proposed. Lemna minuta Kunth and Lemna minor L. are two small aquatic floating plants easily mistakable for similar morphology. The need to distinguish with certainty these two species is a relevant issue, not only from a floristic viewpoint, but also for establishing management plans in wetlands where L. minuta is an invasive alien species as in Europe. The vein number and frond length are considered by most authors as the main morphological characters discriminating the two species. However, in this study the use of these two characters has not been effective for the determination of 43 specimens out of 248 living specimens of Lemna collected from 15 different wetlands of the Central Italy. Statistical analyses of the dataset made it possible to better define the variability of the morphological characters of these two species and to identify additional diagnostic characters to use for improving the morphological discrimination between them. Among these characters, width, apex and shape of the frond, were the most helpful. Decision Trees were elaborated for differentiating L. minor from L. minuta with greater efficiency both in the laboratory and in the field. The increase in percentage of correct determination of Lemna specimens through measurement and parallel utilization of the emerged morphological characters will facilitate the botanists’ activities, but also will have practical implications, such as the ability to contribute better to the decision-making system in drawing up plans for plant biodiversity protection and/or eradication of the alien species.