Nutrient limitation and vegetation changes in a coastal dune slack

Basiphilous pioneer species are among the most endangered plant species in The Netherlands. They find most of their refuges in young coastal dune slacks, especially on the Wadden Sea islands. For the purpose of nature management it is important to know which processes control the presence of basiphilous pioneer communities, and to learn about the nature of slacks harbouring the concerning successional sequences. In a large dune slack on the Island of Terschelling, we assessed soil nutrient status and tested for nutrient limitation in four chronosequential stages: 2, 6, 37 and ca. 80 yr of age. Stage 2 harboured a basiphilous pioneer vegetation; in the stages 3 and 4 a dense vegetation of dwarf shrubs and grasses occurred. Soil organic matter and nutrient concentrations in each stage were measured in 1991. In 1992 and 1993 fertilizers were applied to all stages to detect nutrient limitation. Rates of accumulation of organic matter, nutrients and above-ground biomass were estimated. When interpreted as successional stages, the different stages represent a sequence as expected on the basis of general successional theory. There was a peak in yearly nutrient accumulation between the 6- and 37-yr old stage and a steady state after ca. 80 yr. Between the first two and the latter two stages a shift occurred from allogenic to autogenic succession which correlated with a shift in emphasis from available nutrients to light availability as limiting resources. Basiphilous pioneer species suffered only deficiency of nitrogen, probably because of their low phosphorus requirements. It is concluded that in dune slack habitats, in addition to a low nutrient availability in general, a very low phosphorus availability favours basiphilous pioneer species to species showing co-limitation of nitrogen and phosphorus as found in some grasses and dwarf shrubs. A comparison between the effects of lime addition and the effects of nitrogen and phosphorus additions suggests that, in the early stages, soil buffering increases the availability of nitrogen and inhibits the availability of phosphorus. Sod cutting is an effective technique for restoring basiphilous pioneer vegetation, when slacks are acidified only superficially and buffering-mechanisms can be reactivated. Yearly mowing and removing of standing crop may prolong the lifespan of basiphilous pioneer vegetation, when soil acidification has not yet dropped below pH 6.