Vegetation dynamics in a wet dune slack I: rare species decline on the Waddenisland of Schiermonnikoog in The Netherlands

Vegetation changes in a wet dune slack complex have been studied over a period of 23 years, including some dry (1973-1977) and wet (1983-1987) periods. The vegetation was recorded in 1964, 1977, 1983 and 1987 in permanent quadrats. Additional information was derived from vegetation maps and distribution maps of rare species. It was observed that several species shifted up and down a height gradient in response to wet and dry periods. The recent surveys show a dramatic decline in the occurrence of calciphilous species, which is thought to be related to a process of accumulation of organic matter in the wet sites and decalcification in the dryer parts of the dune slack. The causes of the rapid vegetation change are discussed and measures to preserve the calciphilous species recommended.