Interannual dynamics of a rare vegetation on emerged river gravels with special attention to the critically endangered species Corrigiola litoralis L

Emerged gravelly bottoms of large European rivers are very dynamic ecosystems endangered by modifications of their hydrological regime and water pollution. In the present study we investigated the vegetation on an emerged gravelly bottom of an incised segment of the river Elbe in the northwestern part of the Czech Republic. Our knowledge about the vegetation response to great seasonal fluctuations of the water table in incised rivers has so far been limited. We asked the following questions: (1) How are the vegetation composition and (2) occurrence of the critically endangered annual species Corrigiola litoralis affected by their position on gravel deposit, and (3) how are these factors affected by inter-annual variability in water table dynamics? We established nine permanent transects, three at each of three existing extensive gravelly sites, stretching perpendicularly from the river bank and observed annually from 2008 to 2015. At the time when the vegetation was best developed, in the period of the lowest river flow, we recorded all species present in 755 quadrats (1 m² in size) located along the transects. The results show that the year of observation, the distance from the bank and the time for which the sites were exposed significantly influenced the vegetation. Whereas perennial plants were concentrated close to the bank in places least affected by water table fluctuations, annual plants dominated in parts of the gravel deposit more prone to inundation. The frequency of C. litoralis was low; it occurred in only 2.1% of the quadrats. The minimum period of the bottom’s emergence for the establishment of the species was 50 days. The greatest frequency was recorded in 2015, which was the driest year during the study period. The continuation of the present river geomorphology and water table dynamics seems to be sufficient to preserve the continued existence of the vegetation under study, including the populations of C. litoralis.