Inundation and the distribution of plant communities on Dutch river dikes

The relationship between the distribution of plant communities occurring on river dikes and the inundation by river water is described. The distribution of many species is restricted to the non-flooded upper part of the dike slope, while their lowest distribution limit coincides with the highest water-level for the growing season for the year preceding the recording of the vegetation. The phytosociological elements show the same distribution pattern in all five transects studied and can be clearly ordered by means of detrended correspondence analysis as well as by their relative inundation values: —hardly flooded; Koelerio-Corynephoretea, Festuco-Brometea and Mesobromion. —slightly flooded: Arrhenatheretum and Trifolion medii. —flooded: Arrhenatherion, Molinio- Arrhenatherelea, Artemisietea, Lolio-Cynosuretum and arable weeds. —considerably flooded: Ranunculo-Alopecuretum, Bidentetalia, Molinietalia, Phragmitetea and Lolio-Plantaginetum. Typical ‘fluviatile’ plant communities are strongly declining on river dikes. (Re) Introduction of these communities by means of special construction measures (e.g. the application of suitable soil) and special management practices is only possible above 2-2-30 m below ‘normative water level’.