Long-term vegetation dynamics after land-use change

After the establishment of the National Park "Schleswig-Holstein Wadden Sea " in Northern Ger-many, sheep grazing was strongly reduced, or abandoned and the maintenance of the artificial drainage system in the abandoned areas neglected. We studied the long-term development of plant species density and evenness in salt marshes of the "Hamburger Hallig". In this area, a grazing ex-periment including intensively grazed, moderately grazed and abandoned plots, was established in 1991 and modified in 1995 and 1999. Species density increased on all permanent plots except for the high salt-marsh plots ungrazed since 1992, which had the highest species density of all plots in 1992 and showed a slight but non-significant decrease in species density between 1995 and 2004. In the intensively grazed salt marsh, the increase in species density was delayed until 1998. Vege-tation evenness E1/D showed high inter-annual fluctuations with a strong decrease during the early years of the experiment but hardly any significant trend from 1995 to 2004. The decreasing species density between 1999 and 2004 on plots dominated by Elymus athericus indicated that this species was able to outcompete other species at high salt-marsh sites with well-aerated soils. In contrast, no decrease in species density could be observed on sites dominated by Atriplex portula-coides. In the low salt marsh and in less drained parts of the high salt marsh, soil water-logging and low sedimentation rates may impede large-scale species dominance and consequentially pre-vent a decrease in species density.