Increase of alien and C4 plant species in annual river bank vegetation of the River Rhine
Recent and historic relevés of the annual river bank vegetation (Polygono brittingeri-Chenopodietum rubri, Chenopodium rubrum subassociation) of the middle and lower River Rhine (Germany) were evaluated with regard to their representation of alien and C4 plant species. The relevés evaluated covered a period of more than 50 years from the second half of the 20th to the beginning of the 21st century. There was a distinct increase in the proportion of alien and C4 plant species: The mean percentage of post-1492 aliens (neophytes) increased from 9.9% in 1964 to 27.5% in 2002. In the same period, the mean percentage of C4 plants increased from 4.4% to 11.5%. In 1998-2002 99 alien plant species were recorded in the annual sand and gravel bank vegetation of the study area. Possible reasons for the increase of alien and C4 plants may include increased trade activities and increased temperatures due to global climate change.