Jan P. Janssen - Böcker

A. Background and scope of the study ...3 1. General ..." ,-, ...11 B. Abandoned and marginal agricultural grasslands ...5 2. Some equilibrium models explaining species di- 1. Definition ...5 versity ., ..., ,-..., ...11 2. Inland areas in western Europe ...6 3. Change of the steady state .. , .. , ...14 3. Ca. stal areas in western Europe ...8 4. Dispersal of plant species ..., ,- 14 C. Nature mc.nagement practices ...10 5. 'Ecotypic' diversity .. , ...15 D. Theoretical considerations on nature management 6. Patterns in plant communities ...15 with special reference to diversity ...7...,. 11 Implications for management 16 LA. Background and scope of the study (Klapp, 1965), Belgium (Van Hecke, Impens & Behaeghe, 1981) and the Netherlands (Oomes & The degradation of flora and vegetation in natural Mooi, 1981; Elberse, Van Den Bergh & Dirven, and semi-natural landscapes has become a matter 1983; Willems, 1983A).

Number of pools (%) o 10 20 40 A moorland pool (Dutch: ven, fies, fiaas; Ger- co man: Heidetlimpel; French: etang de bruyere) is :S 2- pools present in the Pleistocene areas of The (f) Netherlands, particularly in the northern province Fig. 2. Size distribution of 346 moorland pools in Noord- of Drenthe and the southern province of Noord- Brabant (after A. W. M. Mol, unpubl. ). Brabant (Fig. 1) The size of a typical moorland pool is between 0. 5 and 1. 5 ha, but many smaller and larger pools After the formation of the pools the open water exist (Fig. 2). The maximum size is 75 ha. The was colonized by aquatic macrophytes and in rel- maximum depth is rarely > 3 m, the mean water atively deep pools peat was formed. In the shal- depth at mean water level is typically between 0. 5 lowest pools the humic material was desiccated, pulverized and removed by the wind in extremely and 1 m (Fig. 3). In the smallest pools the mean depth is only 1 dm at mean water level. The shore dry years.