Alternative stable states in a wet calcareous dune slack in The Netherlands

Abstract

Evidence is presented for the occurrence of alterna- tive stable states in a wet calcareous dune slack on the Frisian island of Texel, The Netherlands. An early pioneer stage (0.5 kg m-2 total standing crop) and a more productive later succes- sional stage (2.9 kg m-2) occur side by side, with sharp bounda- ries between them. The pioneer vegetation has been recorded at the site for more than 62 yr. These features indicate the occurrence of a positive-feedback mechanism that has led to alternative stable states. Analyses of ground and surface water composition, and decalcification depths, indicated that hydrologically the study site can be characterized as a flow-through slack, with exfiltration of calcareous groundwater on one side and infiltration of sur- face water on the other side of the slack. These differences in hydrological conditions have led to distinct differences in envi- ronmental conditions within the dune slack. The occurrence of the two successional stages can, however, not be explained by differences in hydrological conditions since both stages occur side by side in the centre of the dune slack. It is, therefore, more likely that biotic interactions are the cause of the vegeta- tion pattern. Three possible mechanisms for feedback proc- esses are discussed: (1) enhanced nitrogen loss; (2) sulfide toxicity and (3) nutrient accumulation in internal cycle.