On the Vegetation of former River Beds in the Netherlands

This study deals with the vegetation of about 125 former beds of the larger rivers in the Netherlands. It includes all communities of higher plants except the carrs, which are dealt with in a separate paper by Kop (1961). The investigation of the communities aimed at a knowledge of their floristic composition as well as at a definition of their habitat. The description and the classification of the units was carried out according to the concepts and methods of the Braun-Blanquet school (Braun-Blanquet, 1932, 1951; Becking, 1957). Moreover, among the former river beds types were recognized, characterized by a special set of communities and by correlated abiotical properties. A number of vegetation-units are described here for the first time, viz. The Polygoneto-Nymphoidetum (alliance Potamion) with the subass. typicum and the subass. potametosum pectinati. According to descriptions of vegetations found in the literature the subass. typicum is also present in former river beds of the Rhine in Germany about up to Bingen (Lauterbron, 1917); more to the south it is replaced by the Trapo-Nymphoidetum (Oberdorfer, 1957). The Sparganieto-Glycerietum fluitantis polygonetosum (alliance Glycerieto-Sparganion). The main difference with the habitat of the other subassociations (see Maas, 1959), where the water is moving either permanently (brooks) or at least now and then (ditches), is that the vegetation is influenced by the current only during the shortlasting annual floods. The Cicuteto-Caricetum pseudocyperus (alliance ; Phragmition) is to be divided into two subassociations, viz. the subass. typicum and the subass. comaretosum. The main difference between the habitats of the two subassociations appears to be that the first is eutrophic and the second more mesotrophic. The ' Scirpetum triquetri et maritimi typhetosum (alliance Phragmition). In contrast with the other subassociations (see Zonneveld, 1960), this one occurs only in oligoto mesohalinic, stagnant water. The * Caricetum elatae (alliance Magnocaricion) is revised. Carex hudsonii is the only characteristic species found throughout the area in which the association occurs. The community everywhere participates in the hydrosere on sand or peat. The following subdivision was made; Subass. typicum; the community is eutraphentous; according to the literature it is found in Switzerland (Koch, 1926), S. Germany (Oberdorfer, 1957) and Belgium (Lebrun c.s., 1949; Vanden Berghen, 1952 a). Subass. comaretosum: more mesotraphentous than the subass. typicum; . „ , found in N. Germany (Tüxen, 1937; Passarge, 1955 b) and the Netherlands. Of the Valerianeto-Filipenduletum (alliance Filipendulo-Petasition) two new subassocaitions are established, viz.: Subass. juncetosum; it is the replacing-community of a mesotraphentous variant of the Alnetum glutinosae. Subass. senecietosum; represented in the river forelands outside the tidal area; it replaces there an eutraphentous •Wiaon-community, and may be natural if the development of trees is prevented by ice-drift. Eight types of former river beds were distinguished. Two of these could be subdivided into some subtypes. Their classification according to their communities and their abiotical properties is summarized in table 26. Descriptions of habitats which more or less resemble one of these types of former river beds, are known from other parts of the Netherlands and from the adjoining parts of Germany and Belgium. However, as far as we know, of the types described by us, viz. those represented in the river forelands along the upper courses of the rivers, seem to differ from all habitats that have been described so far.