Riparian vegetation and the soil seed bank five years after dam removal on the Elwha River, Washington

Damming of rivers is widespread and can profoundly impact riparian areas by altering the fluvial processes that drive riparian vegetation communities. Dam removal may reverse these effects; however, very few studies have examined the response of riparian vegetation to large dam removal and associated disturbances, such as the release of sediment. Understanding how dam removal impacts downstream riparian vegetation is crucial as dam removal becomes more common. The Elwha River, Washington, is the location of the largest dam removals to date and provides an unprecedented opportunity to explore questions related to dam removal and riparian vegetation. The objectives of this study were to 1) look at how riparian vegetation species richness and community composition changed five years after the removal of two large dams on the Elwha, and 2) examine how the soil seed bank relates to riparian landforms and location above and below the former dam sites. To do this I surveyed plant species richness, community composition, and soil seed bank species richness and seed abundance on three riparian landforms (bars, floodplains, and terraces) located above, between, and below the dams. I surveyed the above ground vegetation in 2016 and 2017 and compared it to data collected before dam removal (2005 and 2010) and immediately after removal (2012, 2013, and 2014). The soil seed bank was collected in 2017. Native species richness increased five years after removal on certain landforms, and sediment deposition following dam removal does not negatively impact species richness downstream. Community composition differed above and below the dams five years after removal. The soil seed bank had more species and was more abundant above the dams on floodplains and bars but was sparse below the dams. I expect that native species richness will continue to increase, as sediment continues to work its way through the system and perturbations begin to fall within natural levels. This study represents the largest dataset collected on riparian vegetation following dam removal and provides evidence that removal may increase native species richness, while sediment deposition may limit the soil seed bank