The Effects of Legacy Sediment Removal and Floodplain Reconnection on Riparian Plant Communities

Abstract

Stream restoration includes a number of different approaches intended to reduce sediment and nutrient export. Legacy sediment removal (LSR) and floodplain reconnection (FR) involve removing anthropogenically derived sediment accumulated in valley bottoms to reconnect incised streams to their floodplains. These projects also present an opportunity to create high-quality riparian and wetland plant communities and provide information about the early stages of wetland vegetation development and succession. We surveyed vegetation immediately after restoration at three sites and at three additional sites 1–3 years post-restoration to determine how LSR/FR affects riparian plant communities. Restoration increased the prevalence of hydrophytic herbaceous species at all sites, suggesting these projects successfully reconnected the stream to the floodplain. Pronounced decreases in woody basal area and stem density likely also influenced an increase in native and graminoid species after restoration. Only 16% of the indicator species identified for restored reaches were planted as part of the restoration, suggesting the local seed bank and other seed sources may be important for vegetation recovery and preservation of regional beta diversity. Although vegetation quality increased after restoration in reaches with initially low-quality herbaceous vegetation, vegetation quality did not improve or decreased after restoration in reaches with higher-quality vegetation before restoration. The practice of LSR/FR has the potential to improve the quality of some riparian vegetation communities, but the preservation of high-quality forested areas, even if they are atop legacy sediment terraces, should be considered, particularly if reductions in nutrient export do not offset losses in tree canopy.