Plant-Migration Studies near the former Island of Urk in the Netherlands

1. In the neighbourhood of the former island ofUrk within the North-Eastern polder an area of about 800 ha, called Land of Urk, is distinguished from the remainder of the reclamation district by very peculiar soil conditions. Compared to plant establishment elsewhere in the polder also the development of a natural vegetation showed great differences. From 1941 until 1951, the development of natural vegetation proceeded almost undisturbedly. 2. Based upon differences in soil conditions in the Land of Urk six habitats were distinguished. Differences in soil conditions were strikingly parallelled by variation in vegetation. 3. The Land of Urk was extremely well suited to studies in the field of dispersalbiology. In our investigations use was made of dispersal types distinguished by Westhoff (1947), based upon the degree of interaction between the propagules and the various dispersal agents (Table 1). Only dispersal of fruits and seeds has been of importance. 4. From the euchorous dispersal types represented in the Land of Urk the highest percentage was taken in by species propagules of which can be wind-borne over many km (type A). However, in the dispersal spectra of the closed or almost closed vegetations of the adjacent islands ofUrk and Schokland, type A was represented in a considerably smaller degree (Table 2). 5. In 1941 among the representatives of type A in the Land of Urk the major part did not occur on the island, whereas the other types for the most part were found on Urk. In all probability the most important source of propagules of type A has been the island of Schokland and/or the mainland between Lemmer and Kampen (Fig. 1, Tables 3, 4 and 5). 6. A comparison of the dispersal spectra of the six types of habitat distinguished demonstrated that, excluding saline soils, in open vegetations type A was the most dominant euchorous dispersal type. The accessibility of a habitat appears to be of great influence upon the composition of the dispersal spectrum. 7. The authors propose the thesis that at least in Western Europe type A is the dispersal type par excellence for open vegetations and the characteristics of this type are important features of pioneers, pioneer vegetations usually being open.