How can we effectively restore species richness and natural composition of a Molinia-invaded fen?
Calcareous fens are among the most threatened and declining ecosystems throughout Europe and act as refugia for many endangered species. Traditionally, calcareous fens were mown but many of them have been abandoned and invaded by Molinia. This has led to changes in species composition, heavy loss in species diversity and a decrease in bryophyte cover. Fen restoration is complicated by an effective nutrient uptake and utilization of Molinia. Experimental tests of the effects of litter removal and different mowing regimes on restoration success are needed together with studies that compare the responses of different taxonomical groups. We applied three different types of restoration measures (once- and twice-a-year mowing, litter removal) in a block experiment over 6 years to test the effects on species density and composition of vascular plants and bryophytes. We used repeated measures ANOVA, RDA with the Monte Carlo test and principal response curves. Species density and composition of both taxonomic groups were significantly influenced by mowing, but not by pure litter removal. Contrary to bryophytes, vascular plant species composition was more strongly influenced by twice-a-year mowing than by annual mowing. Vascular plant species density sharply increased after 2 years in both annual and bi-annually mown plots, to a greater extent in the latter. Late-spring nutrient concentrations in Molinia varied inconsistently across the years, but showed a trend of lower K-concentrations in mown plots. The tissue N:P ratio indicated P-limitation. Using nine sub-plots within each experimental plot, we found that Bryum pseudotriquetrum was able to spread quickly over the mown plots, while some other typical fen bryophytes (Fissidens adianthoides, Palustriella commutata, Scorpidium cossonii) were unable to increase in frequency, perhaps due to a low generative reproduction and weak competitive ability. Synthesis and applications. Molinia-invaded fen vegetation can be restored through intensive mowing. We recommend twice-a-year mowing instead of traditional late annual mowing as an initial restoration measure. Since the effect of the standing Molinia crop in our long-abandoned fen turned out to be more detrimental to other plants than accumulated litter, we do not recommend any management that is based on litter removal rather than mowing.