Het gedrag van Juncus mutabilis en J. capitatus in enkele duingebieden van het Waddendistrict

At the moment the occurrence of Juncus mutabilis and J. capitatus is mainly determined by activities of man (excavations) and by extreme fluctuations in climatic factors (the occurrence of very low water levels in summer). In the summer of 1972 and 1973 the water levels on the island of Terschelling were very low. As a consequence some small dune lakes were left dry and J. mutabilis was growing abundantly in two of them, where previously it was absent or of sporadic occurrence. In 1972 and 1973 J. mutabilis was also recorded from two other places, where it occurs in small numbers each year. In the nature reserve Koegelwieck on Terschelling excavation, followed by a lowering of the water level, the land remaining dry, resulted in a new ephemeral habitat of Juncus mutabilis and J. capitatus. In the Netherlands this is the last known locality of J. capitatus; in the former century this species was growing in two other dune slacks on Terschelling. In the dune area on the mainland near Schoorl, province of North-Holland (dunes poor in lime, as is the case at Terschelling), J. mutabilis was growing in a recent excavation. The species has only been found during years in which extremely low water levels occurred. In years with a normal or high water level not a single plant was growing there.