Restoration attempts aiming at the recovery of species-rich alluvial grasslands along the northern Upper Rhine were evaluated with regard to the effects of flooding and management on plant dispersal and seedling recruitment. The overall aim of the studies was to assess the limiting abiotic and biotic constraints to the restoration of flood-plain communities.
As a starting point for further studies large-scaled restoration measures on former arable fields were evaluated by analysing floristic composition and soil nutrient status of old and newly established meadows in the functional and fossil flood-plain. Whereas the functional flood-plain is exposed to direct flooding which allows a free input of diaspores, the fossil flood-plain is protected from direct flooding by a dyke. After 20 years of restoration a low restoration success was found and target species largely failed to establish. This can not be explained by unsuitable edaphic site conditions since old and new meadows differed only slightly in environmental variables. Consequently, limited dispersal and recruitment were revealed to be the main constraints for restoration. No evidence was found that species enrichment is enhanced via the input of diaspores by flooding in the functional flood-plain.
A study of the seedbanks of arable fields in the study area highlights the low potential of the soil seedbank for restoration of species-rich alluvial grasslands. However, for the persistence of endangered ephemeral mudflat vegetation in irregularly flooded arable fields, the seedbank proved to be of high significance. Most species found in ephemeral communities build up large persistent seedbanks and are characterized by short life cycles. At these sites, not only improved drainage and landfill threatens the occurrence of mudflat vegetation but also the replenishment of the seedbank is often prevented by early re-ploughing the fields.
Besides the availability of seed sources, the lack of suitable microsites for seedling recruitment is another limiting factor in restoration of semi-natural grasslands. In a seed addition experiment we studied the effects of different mowing regimes on seedling recruitment of six flood-meadow species. Generally, we found low germination rates suggesting a lack of suitable microsites in the closed swards and a weak effect of the applied management regimes. Competitive release by mowing twice was only found in the first study year, when biomass production was much higher than in the second year. Moreover physical hazards such as flooding and drought seemed to control seedling germination and establishment to a large extent.
The evaluation of management schemes is of high practical relevance for a successful conservation of endangered grassland species. We studied the population stage structure of the threatened Serratula tinctoria at 24 meadow sites which differed in vegetation and management. Generally, we found only slight differences between the population stage structure of S. tinctoria in early and late mown meadows. Both management types had high densities of seedlings. In contrast, the meadow pastures had significantly lower densities of seedlings but relatively high densities of juveniles and vegetative adults, indicating a higher importance of clonal propagation under grazing pressure. The results showed that viable populations of S. tinctoria may occur in all management regimes. Although S. tinctoria was considered to be highly sensitive to early mowing, our results suggest a facilitation of the species at more nutrient-rich sites by mowing in June.
The results of the present study documented that in the study area mainly biotic constraints, i.e. the lack of seed sources in the vicinity of restoration sites and in the seedbank, hamper successful restoration of alluvial grasslands. In existing species-poor grassland swards microsite limitation is another limiting factor. Finally we confirmed that the existing management regimes in the study area are appropriate for the maintenance of target species.
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