Abstract
Many lowland floodplain habitats have been disconnected from their rivers by flood defence banks. Removing or lowering these banks can reinstate regular flooding and thus restore these important wetland plant communities. In this study we analyse changes in wetland hydrology and plant community composition following the lowering of flood defence banks at a floodplain of the River Don in the United Kingdom (UK). The aim of the restoration project was to improve the quality of “floodplain grazing marsh” habitat, which is a group of wetland communities that are of conservation interest in the UK. We analyse changes in species richness and community composition over a period of 6 years, and compare the presence of indicator species from the target floodplain grazing marsh plant communities. The lowering of the flood banks increased the frequency of flood events, from an estimated average of 1.7 floods per year to 571 floods per year. The increased flooding significantly increased the proportion of time that the wetland was submerged, and the heterogeneity in hydrological conditions within the floodplain. There were significant differences in composition between the pre-restoration and restored plant communities. Plants with traits for moisture tolerance became more abundant, although the communities did not contain significantly more ‘target’ floodplain grazing marsh species at the end of the study period than prior to restoration. Colonisation by floodplain grazing marsh species may have been limited because environmental conditions were not yet suitable, or because of a shortage of colonising propagules. While the desired target plant community has not been achieved after 5 years, it is encouraging that the community has changed dynamically as a result of hydrological changes, and that moisture-tolerant species have increased in occurrence. Over the next few decades, the restored flood regime may cause further environmental change or colonisation events, thus helping increase the occurrence of desired floodplain grazing marsh indicator species.
Highlights
Natural floodplains are a threatened habitat in Europe; it has been estimated that up to 90% of the historical floodplain area has been converted to agricultural or urban use (Tockner and Stanford 2002)
Flooding is important in structuring plant community composition at multiple spatial scales; within habitat patches, across larger mosaics, and between floodplain wetlands at the landscape scale
The objectives of this study were (1) analyse the impacts of floodplain restoration on hydrology and plant community composition within the floodplain, over a period of 6 years, (2) analyse the response of plant species to floodplain restoration as a function of their functional traits, and (3) analyse the degree to which the plant communities became more similar to the desired floodplain grazing marsh communities
Summary
Natural floodplains are a threatened habitat in Europe; it has been estimated that up to 90% of the historical floodplain area has been converted to agricultural or urban use (Tockner and Stanford 2002). Flood water can transport plant matter and seeds from habitats elsewhere in the river network (Holzel and Otte 2001; Gerard et al 2007), adding novel species to the local pool (Moggridge and Gurnell 2010). This process may be important in maintaining the presence of species across a landscape, for example by allowing recolonisation of Wetlands Ecol Manage (2020) 28:403–417 habitat patches that have been cleared by disturbances (Gurnell et al 2006)
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