Restoration ecology of aquatic and terrestrial vegetation on non-calcareous sandy soils in The Netherlands

Plant and animal diversity of the Dutch landscape is mostly restricted to (semi-)natural ecosystems in nature reserves. Many of these plant and animal species are at this moment seriously threatened by environmental stresses, such as air pollution, eutrophication, desiccation or lowering of the water table and habitat fragmentation (e.g. Westhoff 1979; Vos & Zonneveld 1993). The preservation of biodiversity in existing nature reserves and natural areas is one of the most important aims in European nature conservation, besides the creation of new habitats (e.g. Marrs 1993; Anderson 1995). Many endangered plant and animal species, however, have been declining in the last decades in The Netherlands, although they are growing in managed nature reserves. One of the main threats to these species is the increase in airborne sulphur (SOy) and nitrogen (NHX and NOy) deposition. S and N pollutants have been shown to acidify ecosystems, and may cause damage to vegetation especially by soil-mediated processes, such as decreases in buffer capacity, base saturation and increases in heavy metals (Al) (e.g. Van Breemen et al. 1992; Ulrich 1983). Changes in N species are also associated with these processes (Roelofs et al. 1985). Deposition of airborne N compounds may, furthermore, cause eutrophication, because nitrogen is limiting for plant growth in many of these (semi-)natural, nutrient-poor ecosystems (e.g. Bobbink & Roelofs 1995a). The effects of desiccation of wet ecosytems are added to the just-mentioned acidifying and eutrophying consequences of atmospheric deposition. The negative effects are especially observed in oligotrophic, weakly buffered ecosystems in the areas with sandy Pleistocene deposits in The Netherlands (e.g. De Vries 1994; Bobbink & Roelofs 1995b).