The Effects of Hydrological Fluctuation on Grassland Plant Communities in England and Ireland

Grassland communities that are subjected to seasonal flooding are highly valuable to society, as they support a wide variety of wildlife and provide numerous other ecosystem services including floodwater and carbon storage. These important habitats are threatened by several anthropogenic pressures and climate change arguably poses the greatest risk to their functioning because of the greater temperature and rainfall variability predicted to occur in the future. The effects of some aspects of water-regime are well studied (e.g. mean water-table depth), but the effect of fluctuating water tables on grassland plant communities is not clearly understood. This study aims to investigate the effects of hydrological fluctuation on wet grassland plant communities. Fine-scale hydrological and botanical data were gathered from a selection of sites representing two habitats which experience differing levels of water-level fluctuation: English floodplain meadows and Irish turloughs. A selection of grassland plant species (Ranunculus spp.) were also subjected to fluctuating water levels in a controlled experiment. This study presents a substantial body of evidence showing that high levels of hydrological fluctuation can result in a decline in plant species richness. Hydrological fluctuation was quantified as the degree of between-year (inter-annual) and within-year (intra-annual) variation in soil waterlogging across the preceding five growing seasons. Both increasing inter- and intra-annual variability correlate with a decline in plant species richness in English floodplain meadows and Irish turloughs. The experimental results suggest that overall wetness could be more important than hydrological variability, and that Ranunculus acris may be a better competitor under conditions of stress than R. repens. The results of this study have implications for grassland management under a more variable climate; it is proposed that a flexible management approach is required, which takes into account the role of fluctuating hydrology as an important driver of plant communities in seasonally flooded grassland habitats