Notes on the enigmatic Thesium linophyllon and report on Thesium ramosum in Lithuania

Abstract Two species of the genus Thesium (T. ebracteatum and T. linophyllon) are constantly recognized as members of the flora of Lithuania. However, the study on plants currently occurring in Lithuania and usually identified as T. linophyllon revealed that in fact they belong to T. ramosum. The analysis of all available data on the occurrence of T. linophyllon in Lithuania indicated that this species has never been recorded. The report on the occurrence of T. linophyllon, published at the end of the 18th century, was based on misidentified T. ebracteatum. Thus, T. linophyllon should be excluded from the list of vascular plants of Lithuania. Occurrence of T. ramosum was reported for the first time in the country. Currently, one population of this species with two subpopulations are known in the environs of Varnikai village (Trakai distr., Trakai Historical National Park). Morphological description of T. ramosum and diagnostic features of this species were provided. The structure and habitat conditions of two subpopulations were investigated in 2017. The present population of this species was revealed to consist of 52 individuals. Occurrence of young individuals in one subpopulation indicates generative reproduction of T. ramosum. Negative relationship between the number of T. ramosum individuals and the coverage of Poaceae species, and positive relationship between the coverage of Fabaceae species were revealed. Localities of T. ramosum in Lithuania are far from the area of its native distribution, and this species is ascribed to the group of established alien species. Further spread and invasion of the species is not expected.