Siltation and hydrologic regime determine species composition in herbaceous floodplain communities

Abstract

Interest in the restoration of riparian habitat is increasing. However, little is known about factors responsible for riparian communities, especially grasslands. In order to construct plant communities for a restoration project in the floodplain of a large river in the Midwestern United States, we sampled four floodplains with various disturbance regimes located in Illinois and Missouri. They were chosen to be representative of different plant communities of floodplains, with a focus on herbaceous communities. The areas included backwater lakes, alluvial fans, groundwater seep marshes, oxbow marshes, seasonally inundated grassland, and non-inundated grassland. Vegetation, soils and groundwater or standing water depth were measured at various intervals along transects. Communities were produced using TWINSPAN and tested for differences in environmental factors. The soil morphology, taxonomic classification, and fertility parameters were similar among sites. Environmental factors influencing community composition were the presence of permanent water and silt deposition. We conclude that water depth determines species composition in permanently wet areas. Silt deposition determines composition in seasonally inundated grassland. Where silt deposition is high enough to inhibit seedling emergence, dominance is attained by plants able to reproduce vegetatively by rhizomes. Such a reproductive process leads to nearly monotypic stands produced by large clones. Results are discussed in relation to models of riparian processes and succession.