Wälder und Waldentwicklung in alten Flussbetten in den Niederlanden

In order to arrive at a classification of the former beds of the rivers Meuse and Rhine in so far as they are found in the Netherlands, and of the Waal and IJssel a study was made of their vegetation, especially of the woods that form the final stage in the succession. So long as these former beds remain within the area that is flooded from time to time by the river, they are gradually filled up by clay, whereas organic matter is nearly entirely swept away. When they are no longer in contact with the river, further deposition of clay is impossible, and then the formation of peat becomes responsible for the filling up of the bed. When about half the bed had been filled up at the time it was shut off from the main stream by the construction of a dike, the final stage in the open part proved to be different from that in the part that had already been silted up. In the open part peat is formed, and here therefore a wood pertaining to a peaty soil will develop, whereas in the marginal zone with its clay soil a wood pertaining to a mineral soil may be found. In our classification of the various types of wood Oberdorfer (1953) was followed. The woods in the former river-beds belong partly to the Alno-Ulmion Br.-B!. et Tx. 1943 and partly to the Alnion glutinosae Male. 1929. Those belonging to the suballiance Salicion Oberd. 1953 of the Alno-Ulmion are found almost always outside the dikes, and consist almost exclusively of narrow-leaved Salix species, whereas in the suballiance Alnion glutinosae-incanae Oberd. 1953 also other tree genera are represented. In the Salicion a distinction was made between woods of the tidal zone and woods of the remaining part of the grounds outside the dikes. The “ " ‘ ' Salicion occurs, as a rule, together with the Valerianeto-Filipenduletum, of which the floristic composition resembles that of the herbaceous layer in the Salicion. Both develop out of the Caricetum gracilis-vesicariae. When the development of the trees is made impossible, e.g. because they are cut down or because they are destroyed by drifting ice, the Salicion is replaced by the Valerianeto-Filipenduletum. Behind the dikes woods belonging to the Alno-Ulmion are found only on mineral soils. On peat the woods belong to the Alnion glutinosae, _ _ _ but transitions occur. On the highest clay soils the woods belong to the suballiance Ulmion Oberd. 1953 and may show an approach to the Querceto-Carpinetum. On most of the remaining rich soils the woods belong to the suballiance Alnion glutinosae-incanae. Outside the dikes the Alnion glutinosae-incanae is found only in places where the action of the water is negligible, i.e. far downstream; within the tidal zone it is also absent. When the trees are felled, these vegetations of the Alno-Ulmion too are replaced by a form of the Valerianeto-Filipenduletum. As the open beds behind the dikes are gradually filled up by peat, the final stage in the succession is a vegetation belonging to the Alnion glutionsae. In oligotrophic parts this alliance is represented by the Betuleto-Salicetum, in less poor soils by the Alnetum glutinosae. Inside the latter more or less disturbed vegetations are met with, which show some appraoch to the Alno-Ulmion. Other differences are probably due to the persistence of species belonging to preceding stages in the succession. So the Alneta in the region with river clay deposits differ from those found on the much older river loam and on other less fertile soils. The Alnetum glutinosae develops sometimes out of a Caricetum acutiformis-paniculatae, but tree species may appear already in earlier stages of the succession. When the trees are felled, the Alnetum too is replaced by a form of the Valerianeto-Filipenduletum. In former river beds bordering on high grounds consisting of sand, the final stage in the succession may be formed by a vegetation belonging to the Betuleto- Salicetum. It usually develops out of a vegetation belonging to the Caricion fuscae. Unlike the other types of wood here mentioned, the Betuleto-Salicetum is not replaced by a Valerianeto-Filipenduletum , when felled. At one place the presence of species belonging to the Sphagnion indicated that vegetations belonging to the Betuleto-Salicetum and even an Alnetum were developing in the direction of a Sphagnum bog.