The water caltrop (Trapa natans L.) in Ukraine: new areas of expansion in the north of the country and contradictions in concepts of its protection and population management

This paper reviews data on the distribution of the water caltrop Trapa natans L. in Ukraine. This relict species (listed in the Red Data Book of Ukraine) had strongly declined in the early 1979s, but has now locally recovered and even excessively spread in the upper reaches of the Kaniv and Kyiv reservoirs on the Dnipro River. Therefore, both protection and regulation of its local abundance are on the agenda today in the country. Work is currently underway to monitor the population of the species and search for new habitats. Previously the presence of the water caltrop in Sumy Oblast has been documented only in floodplain waterbodies of the Desna River. On 18 June 2017, we found four new record localities of Trapa natans in the Psel River within the city boundaries of Sumy (north-eastern Ukraine). All of them were located along the right bank of the river. The four groups of plants were comprised of 8, 70, 2, and 100 individual plants, respectively, growing at a depth of 0.5–3.2 m and about 5–15 m far from the riverbank. A second visit to the location on 15 August 2017 revealed a total of 591 rosettes. Three years later (visited on 28 August 2021), the number of individual caltrop plants in this location increased as many as eight times (total of 4766). The plant spread 3 km further downstream and its mats (variable in size) could be found at both banks of the river. The transformation of the Psel River from one of the fastest tributaries of the Dnipro into a cascade of numerous channel reservoirs led to a slowdown in water flow and the continuous siltation of the bottom created favourable conditions for the water caltrop. Most likely, seeds of this plant were artificially introduced into the river (possibly by aquaculture enthusiasts) a few years prior to the first finding and the plant spread downstream. This was facilitated by the spring flood, which carried the fruit down the river. In summer, the current carries plants that were uprooted by vacationers. Leaves of the water caltrop were seen consumed by the mallard Anas platyrhynchos L., whereas seeds of the plant were eaten by the European water vole Arvicola amphibius. The newly discovered occurrence spot of the water caltrop is currently the only one on the Psel River and it complements the information on the current distribution of this rare plant species. It is located about 145 km south-east from the closest population on the Desna River, which described the presence of this species in Sumy Oblas