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

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

Ecological comparison between duckweeds in Central Italy: the invasive Lemna minuta vs the native L. minor

The American duckweed Lemna minuta shows an invasive behaviour in Europe, causing weed problems in aquatic habitats there. Few studies addressed this species’ ecological requirements for a suitable establishment in a site. In this paper, L. minuta populations were analysed through feld surveys so as (1) to defne the autoecology of this duckweed as regards the main environmental factors characterizing invaded habitats, and (2) to identify possible overlaps/differences in ecological requirements between the alien L. minuta and the common native L. minor, with which it is often associated and in direct competition. The occurrence/abundance of the two species and environmental data were collected from 41 wetlands in central Italy. Currently, L. minuta is more common and abundant than L. minor in the study-area, despite its recent arrival there (2007). The two species have a partially overlapped autoecology. However, L. minuta differs from L. minor since it occurs in waters which are less alkaline, slightly less warm, and richer in nitrates. It shows tolerance for environmental conditions which are limiting for most of macrophytes, including L. minor, such as high shading and low water oxygenation. This enables L. minuta to increase its invasion potentiality and thus to enlarge its distribution area

Does the alien L. minuta show an invasive behaviour outside its original range? Evidence of antagonism with the native L. minor L. in Central Italy

The American duckweed Lemna minuta Kunth is considered an invasive species in several European countries (e.g., Belgium, Germany, Hungary, Ireland, Italy), but its invasiveness outside its original range remains poorly studied. We therefore analyse: (i) the speed with which L. minuta has spread throughout Europe; and (ii) its capability to compete with L. minor L., the most common European native duckweed. Based on literature data, the increasing number of observations of L. minuta since it was reported for the first time in France (1966) indicated the wide extent reached by this species in most of Europe, highlighting its high capacity to colonize new areas. By samplingLemna populations from 41 stations in standing waters in Central Italy, it emerged that the frequency of the two species was similar, but the alien species was more abundant than the native one, showing generally higher coverages, and predominating in mixed Lemna populations. A negative correlation between L. minuta and L. minor was observed in our study. This relationship was not explained by different ecological requirements, since the two species responded similarly to the main environmental gradient, but was linked to an antagonistic dynamism between both species. A multitemporal analysis, based on the comparison of Lemna coverage data recorded at the same stations at two different times (last vs. first record), revealed that L. minuta had appeared where it had been absent previously and that its abundance had increased to the detriment of L. minor, which in some cases disappeared within a few years. The arrival of L. minuta in a site can be considered a real threat to conservation of the native plant diversity due to its competition with the congeneric L. minor