Effects of environmental variables and cutting regime on species diversity and ecological niches in a degraded fen grassland ecosystem

The diversity and composition of species communities in grasslands are influenced by both environmental and management drivers, yet the extent of their combined impacts on species richness and species niche in fen grasslands remained unclear. This study investigated how cutting intensity and environmental fluctuations, particularly air temperature and groundwater level, influenced plant diversity and the composition of different species in a temperate, degraded fen grassland at Paulinenaue, northeastern Germany. Over three years (2021–2023), we monitored plant species composition in 108 fixed plots under three different cutting regimes (2, 3, and 4-cuts per growing season). The diversity indices were analyzed based on species richness, Shannon-Wiener, and Simpson diversity indices. Our results showed that the values of species richness were lowest in 2022 as the warmest and driest year, while the wetter years (2021 and 2023) supported higher species richness. Cutting intensity influenced richness patterns, with the 4-cut system generally supporting higher richness, but often favoring ruderal and disturbance-tolerant species over characteristic fen species. Plant diversity showed spatial autocorrelation, with similar diversity values clustering in space partially in relation to groundwater conditions. Species differed in ecological strategy, ranging from broad-niched generalists (e.g., Festuca arundinacea and Elymus repens ) to narrow-niched specialists depending on cutting intensity and hydrological conditions. Higher cutting intensity increased niche overlap and competition overall. The results showed that groundwater level, soil temperature, and soil nitrogen content were the most consistent predictors of plant diversity, significantly influencing species richness as well as Shannon and Simpson indices. While species richness was additionally affected by the 4-cut system, Shannon and Simpson diversity were mainly driven by the interaction between groundwater level and cutting regime. soil texture showed weaker or inconsistent effect. These findings indicated that cutting intensity alone was insufficient to restore fen species. Maintaining favorable hydrological conditions via rewetting might be essential for sustaining biodiversity in fen grasslands under increasing climatic variability. However, substantiating such robust conclusion requires long-term measurement, extended beyond three years of our study.