Restoration of Koelerion glaucae vegetation in the Upper Rhine valley (Hesse, Germany): an update on the development of a model site after 19 years

We present an update on our studies on the restoration of the highly endangered Koelerion glaucae vegetation in the Upper Rhine valley (Hesse, Germany), a habitat type protected by the European Union Fauna-Flora-Habitat directive. Our three-step restoration approach (deep-sand deposition, inoculation with plant material, non-intensive donkey grazing) was previously introduced after 10 years in this journal. Subsequently, the experiment was expanded, enabling the analysis of the permanent plots of the older restoration site (19 years) but also those of a younger “neodune” (15 years). Phytosociological relevés were sampled for nine permanent plots in a nested approach, with two sizes (25 and 79 m²; both circular), and the data were analyzed using a constancy table, ordination, classification, and the calcu- lation of structural and diversity variables, including the target-species ratio (= TSR, qualitative and quantitative). Our results show that the development commenced with a Stellarietea stage which was replaced after 3 years by a stage dominated by target species (Koelerio-Corynephoretea, Festuco-Brometea). The phytodiversity reached its peak after 5–11 years, while the vegetation cover steadily increased. In recent years, the rate of development has slowed and reached a relatively stable state; slight changes are indicated by the occurrence of Festuco-Brometea species and an increase in bryophytes. A comparison of the older (R) and younger (Y) restoration plots revealed striking similarities, reinforcing the broad applicability of the approach. Remarkably, the initial floristic composition (modulated by diaspores from the donor sites) has not been fully equalized even after 19 years. The applied plot sizes revealed that the smaller plots captured fewer species (later years: ca. 80–90%) and that the larger plots reduced the effects of stochastic processes. As expected, the TSR was scarcely affected by plot size. The impact of heat/drought years on the annuals was found to be very limited, in contrast to the more exposed “Griesheimer Sand” near Darmstadt. The results demonstrate that after 19 years, the Koelerion glaucae vegetation exhibited excellent development. The TSRqual reached its peak at the end of the study period, whereas TSR quant and the number of red-listed/near-threatened species achieved a high level earlier. All values attained the benchmark established by the reference sites. In conclusion, the success of the restoration project was exceptionally high, and the project could serve as a model for the restoration of this vanishing habitat type. The management by donkey grazing is necessary to maintain the diverse pattern of mainly Koelerion glaucae vegetation and secure the restoration.