De Flora van Wieringen

The separation by a dam, now under construction, of the so-called ,,Zuiderzee” from the remaining parts of the coastal waters behind the Islands along the North Coast of Holland, will cause a considerable change in the properties of its water with reference to its brackishness. The consequence of this will be a far-reaching change in the Flora along its Shores and on its Islands. A special Committee was appointed in 1927 by the Netherlands Botanical Society to investigate and record those changes (Secretary J. L. van Soest, 31 Frankenstraat, the Hague). The Committee has already reported on several of its activities and in the present report (no. 14) gives the results of its investigation of the Flora of the Island of Wieringen, the most important island in the Zuyder Zee and the first to undergo the changes resulting from the works now under construction. In the Introduction to the Report some data regarding Topography (Sketchmap on p. 223), Soil, Hydrography, Climate and a List of the Literature, that has been consulted are given. With respect to the Soil a comparison between the general character of the Flora and that of Vollenhove also in the region of the Zuyder Zee (see Report no. 6 of the same Committee) gives the indication that the soil is a product of the erosion of the so-called „keileem” (boulder clay) deposited during the glaciation of the country. The lack of sufficient knowledge regarding the soil of the Netherlands is discussed and the need of a systematic survey similar to the excellent geological survey of the country is pointed out (p. 233). As regards the Hydrography, the influence of the saltcontent of the surrounding sea on the subsurface water has been investigated and demonstrated. A number of samples taken from the surrounding waters and from the ditches on the Island have been tested as to their chlorine contents (tables 1 and 2; pp. 238 and 243) The localities, where those samples were taken, are shown on a sketchmap (p. 240). The effect of the existing system of ditches on the irrigation and of the infiltration of seawater in the lower parts of the Island has been investigated (page 243). The dates that Excursions were held, are given on page 246. The second chapter gives the Lists of all Plants observed or collected on the Island of Wieringen viz. Algae, Fungi, Lichens, Musci and Hepaticae, Pteridophyta and Phanerogamae (pp. 249—356). The third chapter discusses the Distribution and Origin of the last two Groups. This part of the Flora contains 410 species, 60 of which belong particularly to the regions, which are under the direct influence of the sea, whereas 34 specimens have to be considered adventitious in this locality (p. 356). Wherever the individual species demand separate discussion, references to the list in the second chapter are given, where all such data regarding Distribution, Origin, etc. have been collected (p. 357). The Composition of the Flora from the point of view of its Origin and geobotanical Distribution has been investigated and results are given on page 375, where the different groups are discussed. The influence of the atlantic Subelement (as defined by J. Braun-Blanquet) is particularly evident (40 species or 10 %). The geobotanical Relation of the Island to the other parts of the Netherlands is discussed on pp. 378 to 390. There is evidently a well defined paralellism between the Island and Vollenhove, the flora of which, however, contains more species than that of Wieringen; this probably on account of climatic influences, the influence of the sea and edaphic factors. For the same reasons as hold in the case of Vollenhove, Wieringen is considered as part of the Drenthe-subdistrict. In the local Distribution of the Flora certain contrasts have been found. Mainly two factors of edaphic nature are responsible for them. One of these is connected with the difference in saltcontent of the groundwater, the influence of which could be demonstrated clearly in the case of the distribution of the diatomeae of different degree of halophyly, and of the higher plants living in or near open water. The latter are confined to a system of ditches and their immediate surroundings that receive from the higher parts of the Island the subsurface water with a low content of salt (pp. 396—406; cf. Table 5, p. 402 andsketchmapp.240). A slight contrast between the eastern and western parts of the Island can also be observed. It is of no great importance, however. The flora of a few important groups of Habitats has been reviewed, viz. of ruderal stations, arable lands, grasslands, partitionwalls built between the former from the turf, the dikes constructed of Zostera marina L. (these two latter habitats being anthropogenic in their origin and very specific as to their nature) and the decoys („eendenkooien”), the only spots on the Island where shrubs and trees prevail, its surface being almost exclusively used as fields and grasslands (pp. 406—419). Historical changes of the Flora and those expected from the works now under construction are discussed (pp. 419—422). For data necessary to support the discussions referred to above compare Table 4 on pp. 358—373. The fourth chapter is devoted to the description of the Landscape of the Island of Wieringen from a floristical point of view and gives a brief description of the old part of the island or the island proper, of a relatively young „polder” along its south coast and the tidal lands, with particular reference to Zostera marina L., its gathering and preparation, the revenues from which form part of the income of the population of Wieringen.