Simulated effects of floodplain restoration on plant community types

Aims: Channelization and artificial embankments have altered the natural flood regime of many rivers, impacting the hydrological characteristics of floodplain ecosystems and their biological communities. This study was undertaken on a floodplain meadow to assess spatial patterns of plant communities in relation to soil physical and chemical conditions, and the impacts of floodplain restoration that involved embankment-removal. Location: River Glaven, Hunworth, Norfolk, UK. Methods: Fine-scale plant and soil chemistry sampling was conducted prior to embankment removal, and hydrological and climatological conditions were monitored prior to and after embankment removal. Hydrological/hydraulic modelling simulated groundwater levels for a 10-year period to assess changes in soil aeration stresses and plant community composition following embankment-removal. Results: Hydrology was identified as the primary driver of plant community composition. Soil fertility was also important. Unique continuous measurements of vadose dissolved oxygen concentrations using oxygen optodes indicated strong coupling between water table depth and root zone dissolved oxygen concentrations. Reinstatement of overbank flows did not substantially affect aeration stress across most of the meadow because of pre-existing wet conditions. However, along the riverfloodplain ecotone, aeration stress increased substantially from conditions normally associated with dry grassland to those characteristic of fen communities (p<0.05). Conclusions: This restored water table regime may be suitable for more diverse plant assemblages. Benefits of flooding for increased species richness and transport of propagules may, however, be over-ridden without accompanying water level management during the growing season, or hay removal to balance additional supply of nutrients from river floodwater and sediment. Our results show that hydrological/hydraulic modelling combined with quantitative measures of plant water-requirements can provide practical and adaptive management tools to estimate the response of floodplain communities to changing water regimes.