Meetings of the Royal Botanical Society of The Netherlands

The international importance of Dutch vegetation is analysed (1) by compiling the available information and (2) by presenting some new quantitative methods. The association, according to the Braun-Blanquet approach, is the level of the items to be considered. The criteria by which the international importance can be determined are discussed and a number of examples of Dutch associations of international importance are given. Two criteria seem obvious, but they are operational in a few cases only; (1) presence of endemic species and (2) surface occupied by the unit in a given country in relation to the European surface. Overall criteria used by the authors are: (1) European area and relative rarity inside it, (2) central vs marginal position, (3) presence of species on ‘red data’ lists, and (4) other data such as rate of endangerment, replaceability, rate of persistence. The plant communities of greatest importance concentrate in sea dunes, salt marshes, broads and fens, and poor sandy soils with heathland and oligotrophic pools. The authors have evaluated the associations of the Dutch, German and Danish Wadden Sea including all of the Frisian Islands. Within this area about 35 associations are considered to be of international importance, viz. 5 in the xerosere, 10 in the hydrosere, 13 in the hygrosere (wet dune slacks), and 7 in the halosere.