Long-term influence of hay-cutting on plant species richness, biodiversity and soil fertility in a Danish fen

•Three-fold increase of species richness accompanying bi-annual hay-cutting over 21 years.•Decline of vegetation height and reduction of former tall grasses to pygmy size.•Initial increase of oligotrophic and eutrophic species richness and evenness in the community.•Continued increase of oligotrophic species accompanying 50% reduction of soil phosphorus. We present a rare long-term management project of annual cutting, hay removal and plant species survey on a small Danish freshwater meadow (0.6 ha) for an extended period of 21 years. We evaluate whether disturbance by hay-cutting could reduce the dominance of tall grasses, increase species richness and change the representation of growth strategies and Ellenberg indicator values among species. We also evaluate whether hay removal could reduce soil fertility and increase the proportion of oligotrophic species. We found that species richness increased 3-fold over time accompanying a marked decline of vegetation height and proportion of tall, clonal grasses. Comparison with an adjacent un-cut area confirmed that species richness and evenness increased extensively in the plant community by hay-cutting. During the first ten years of the restoration period, species richness increased for both oligotrophic and eutrophic species, while during the latter 11 years only small oligotrophic species continued to increase. The proportion of Grime’s competitive C-strategies and Ellenberg’s N indices in the community declined over time in accordance with the 50% decrease of organic matter, total and exchangeable phosphorus in the soils. We conclude that long-term hay-cutting by disturbance and nutrient removal can markedly increase plant biodiversity as well as the representation of orchids and small species by breaking the dominance of tall grasses. Focused effort over decades can ensure a remarkable high biodiversity even in a small area, which will go unnoticed in common short-term experiments.