Scirpus radicans (Cyperaceae), a newly-discovered native species in Lithuania: population, habitats and threats

A previously unrecorded native species, Scirpus radicans (Cyperaceae), was discovered in the southern part of Lithuania in 2020. Although this species has a large distribution area in other parts of Eurasia, it is currently recognised as threatened in many European countries. Recordings of S. radicans in Lithuania had been expected, as these plants do occur or have been reported to occur in neighbouring regions. The aim of this study was to analyse the distribution of S. radicans in southern Lithuania, to determine the occupied areas and the size of populations, to study their capacity of vegetative reproduction, their habitats and associated plant communities, as well as to perform an assessment of the species' conservation status within the country. We studied S. radicans populations at two sites on the shores of Lake Pabezninkai and of Lake Netečius in the Varėna District (southern Lithuania), from August to October 2020. A total of 993 individuals of S. radicans were counted and their stands comprised 0.73 ha. The density of individuals in the studied plots ranged from 0.02 to 0.78 plants/m(2). Only 0.5% of individuals at Lake Pabezninkai and 20.7% of individuals at Lake Netečius were at the stage of generative reproduction. Individuals at the stage of vegetative reproduction comprised 56.4%, whereas generative individuals amounted to 1.7% of all studied plants. One to seven stolons developed per plant and the mean number of rosettes per stolon was 2.8 ± 1.2. Potentially, a total of ca. 2860 rosettes could be expected from all individuals of the studied plots, but the actual realised rate of vegetative reproduction is unknown. A more detailed study of the reproductive capacities of S. radicans would be required for better understanding the causes of the lately observed decrease of this species in most of the countries of Central Europe. Our analysis of the associated plant communities has enabled us to confirm the presence of a Scirpetus radicantis Nowiński 1930 association previously not recorded in Lithuania. By assessing S. radicans in accordance with the IUCN Criteria, we conclude that this species should be classified as endangered (EN) in Lithuania. Based on this assessment, we propose to include this species on the list of legally protected species of Lithuania. Conservation of shallow lakes with sandy or muddy shores and with significant natural fluctuations of the water level is the main measure for ensuring the survival of S. radicans, as well as other rare and endangered species adapted to such a type of habitat.